Master of Engineering, Tokyo Institute of Technology

Doctor of Engineering, Tokyo Institute of Technology

thermal-hydraulics

Energy and Environment

boiling heat transfer

気液二相流

Nuclear Engineering

Manufacturing technology (mechanical, electrical/electronic, chemical engineering), Thermal engineering

Manufacturing technology (mechanical, electrical/electronic, chemical engineering), Fluid engineering

Energy, Nuclear engineering

Oct. 2011 - Present
The University of Electro-Communications, 教授

Oct. 2011 - Mar. 2013
Osaka University, 招聘教授

Apr. 2007 - Sep. 2011
Osaka University, Associate Professor

Jun. 2002 - Mar. 2007
Osaka University, Associate Professor

Mar. 1999 - May 2002
Osaka University, Lecturer

Jan. 1997 - Feb. 1999
Central Research Institute of Electric Power Industry, 主任研究員

Aug. 1992 - Dec. 1996
Central Research Institute of Electric Power Industry, 担当研究員

Apr. 1990 - Jul. 1992
Central Research Institute of Electric Power Industry, Researcher

Apr. 1988 - Mar. 1990
Tokyo Institute of Technology, Graduate School of Science and Engineering, Department of Nuclear Engineering

Apr. 1984 - Mar. 1988
Tokyo Institute of Technology, School of Science, 応用物理学科, Japan

Apr. 1981 - Mar. 1984
千葉県立船橋高等学校

2023 - Present
国際企画委員, 日本機械学会動力システム部門, Society

2022 - Present
委員, 日本原子力学会標準委員会システム安全専門部会炉心燃料分科会

2022 - Present
理事・情報部会長, 日本混相流学会

2022 - Present
第30回原子力工学国際会議組織委員長, 日本機械学会, Society

2021 - Present
専門委員, 独立行政法人大学改革支援・学位授与機構 学位審査会

2020 - Present
委員長, 日本ボイラ協会ボイラー構造委員会

2020 - Present
委員, 日本原子力研究開発機構炉心安全・熱流動分科会

2020 - Present
委員, 日本原子力研究開発機構安全研究専門委員会

2020 - Present
部門賞委員, 日本機械学会動力システム部門, Society

2019 - Present
運営委員, 日本機械学会動力システム部門, Society

Apr. 2024 - Mar. 2025
部門長, 日本機械学会動力・エネルギーシステム部門, Society

Aug. 2022 - Jul. 2024
編集委員長, 日本混相流学会

Apr. 2023 - Mar. 2024
部会長, 日本原子力学会熱流動部会, Society

Apr. 2023 - Mar. 2024
副部門長, 日本機械学会動力エネルギーシステム部門, Society

Apr. 2023 - Mar. 2024
総務委員長, 日本機械学会動力エネルギーシステム部門, Society

2022 - 2023
副部会長, 日本原子力学会熱流動部会

2022 - 2023
計算力学技術者資格認定委員会1級混相流WG主査, 日本機械学会, Society

2019 - 2023
国際企画委員長, 日本機械学会動力システム部門, Society

2021 - 2022
第12回原子炉熱流動と安全に関する日韓シンポジウム組織委員長, 日本原子力学会

2018 - 2022
委員長, 日本原子力学会原子炉における機構論的限界熱流束評価技術研究専門委員会, Society

2019 - 2020
フェロー賞選考委員, 日本機械学会

2020
熱水力安全評価基盤技術高度化戦略マップ検討ワーキンググループ委員, 日本原子力学会熱流動部会

2004
広報委員長, 日本原子力学会熱流動部会, Society, 日本原子力学会熱流動部会広報委員会

1997 - 1999
編集委員, 日本混相流学会, Society, 日本混相流学会

Mar. 2024
日本原子力学会
フェロー賞（指導学生による受賞：名手海人）

Mar. 2024
日本原子力学会熱流動部会
優秀講演賞賞（指導学生による受賞）辻村玲摩

Sep. 2023
日本原子力学会熱流動部会
優秀講演賞賞（日本原子力学会2023年春の年会）（指導学生による受賞）Raka Firman Baskara Permana

Mar. 2023
日本原子力学会
フェロー賞（指導学生による受賞） 柳田大智

Mar. 2022
日本原子力学会
フェロー賞（指導学生による受賞）田渕純平

Mar. 2022
日本原子力学会
フェロー賞（指導学生による受賞）辻村玲摩

Mar. 2022
日本原子力学会
Experimental Study on Onset Condition of Boiling Entrainment from Falling Liquid Film with Gas Sheared Flow
「学生ポスターセッション博士学生優秀賞」（指導学生による受賞）Raka Firman Baskara Permana

Mar. 2022
日本原子力学会
熱流動部会奨励賞（指導学生による受賞）梅原裕太郎

Mar. 2022
日本原子力学会熱流動部会
核沸騰による流下液膜からの液滴飛散開始条件に関する実験的検討
優秀講演賞賞（日本原子力学会2021年度秋の大会）（指導学生による受賞）田渕純平

1996
日本機械学会動力エネルギーシステム部門優秀講演論文賞
Japan

1996
日本混相流学会論文賞
Japan

Interference effect of nearby nucleation sites on bubble release frequency in subcooled flow boiling
Ryoma Tsujimura; Tomio Okawa
Last, Applied Thermal Engineering, Elsevier BV, 247, 123078-123078, Jun. 2024, Peer-reviwed
Scientific journal, English

Bubble feature extraction in subcooled flow boiling using AI-based object detection and tracking techniques
Wen Zhou; Shuichiro Miwa; Ryom Tsujimura; Thanh-Binh Nguyen; Tomio Okawa; Koji Okamoto
International Journal of Heat and Mass Transfer, 222, 125188, 01 May 2024, Peer-reviwed
Scientific journal, English

Experiments on the onset of boiling entrainment from a falling liquid film with gas sheared flow
Raka Firman Baskara Permana; Junpei Tabuchi; Yuki Narushima; Hajime Furuichi; Tomio Okawa; Kenichi Katono
Corresponding, International Journal of Heat and Mass Transfer, 220, 125006, Mar. 2024, Peer-reviwed, The process of droplet entrainment caused by nucleate boiling within a falling liquid film was visualized using a high-speed camera to investigate the mechanism and the onset conditions of the boiling entrainment. In the experiments, air was flowed along the liquid film to explore the potential effects of vapor core flow on the droplet generation process in diabatic annular two-phase flow. The two types of boiling entrainment mechanisms called the jet-type and the filament-type were observed in the present work. The jet-type boiling entrainment commenced at relatively low wall heat flux and produced small droplets. The filament-type was observed at higher wall heat flux and it produced larger droplets. Hence, the filament-type was supposed to have a greater impact on the liquid film flow rate in diabatic annular two-phase flow. The experimental data accumulated in this work were used to develop correlations for the onset conditions of the jet-type and filament-type boiling entrainments.
Scientific journal, English

Experimental validation of the mechanism and condition for the onset of significant void in subcooled flow boiling
Thanh Binh Nguyen; Tomio Okawa
Last, International Journal of Heat and Mass Transfer, 219, 124881, Feb. 2024, Peer-reviwed, In evaluating the axial void fraction profile in the subcooled boiling region in heated channels, accurate determination of the point of onset of significant void (POSV) is of critical importance. Recently, it has been reported that the POSV can be predicted with high accuracy if the onset of significant void (OSV) is assumed to be caused by a sudden decrease in the interfacial area due to global bubble coalescence (GBC) leading to the formation of large bubbles. Therefore, in this work, experimental investigations were carried out to measure the void fraction, bubble size and bubble number density. The experimental results were utilized to validate the mechanism causing the OSV in subcooled flow boiling. The experimental results were in qualitative and quantitative agreement with the model based on the GBC concept. Thus, it was validated that the sudden decrease in condensation rate due to large bubble formation is the main cause of OSV. It was also confirmed that the GBC occurs when the local void fraction near the heated wall reaches about 0.3, analogous to the flow regime transition from bubbly flow to slug flow in adiabatic gas-liquid two-phase flows.
Scientific journal, English

Effects of nozzle orifice shape on jet breakup and splashing during liquid jet impact onto a horizontal plate
Guofu Sun; Yi Zhan; Tomio Okawa; Mitsuhiro Aoyagi; Akihiro Uchibori; Yasushi Okano
Corresponding, Experimental Thermal and Fluid Science, 151, 111095, 01 Feb. 2024, Peer-reviwed, In some accident situations in nuclear power plants, liquid coolant is discharged from a non-circular rupture hole. Thus, to improve the understanding on the effects of nozzle orifice shape on the jet behavior, experiments were carried out on the liquid jet ejected from oval nozzles. In the experiments, water was used as the test liquid, the area-equivalent nozzle diameter was within 1.8-2.9 mm, the aspect ratio of the nozzle hole was within 2.6-4.8, and the jet velocity was varied within 0.25-11.3 m/s. At low and high liquid flow rates, the liquid jet behaved similarly to the circular jet; the jet breakup lengths and the size of the droplets formed after the jet breakup were expressed by similar dimensionless correlations as those for the circular jet. At the intermediate liquid flow rate, a bamboo leaf-like structure formed on the liquid jet dominated the jet breakup. The jet breakup length was therefore correlated using a theory for the surface tension-induced shape oscillation of elliptical fluid. These correlations enabled to estimate the liquid jet state at any distance from the nozzle. It was also confirmed that if the state of the liquid jet at the impact point is known, the splash rate and the size of the splashed droplets can be predicted satisfactorily using the available correlations based on the experimental data for the circular jet. Therefore, the present work extended the available knowledge on the liquid jet discharged from a circular nozzle to enable the prediction of the behavior of the liquid jet and the characteristics of the secondary droplets produced during liquid jet impact onto a solid surface for the oval nozzles.
Scientific journal, English

Semi-mechanistic prediction of DNB heat flux based on global bubble coalescence concept
Thanh-Binh Nguyen; Tomio Okawa
Last, International Journal of Thermal Sciences, Elsevier BV, 195, 108646-108646, Jan. 2024, Peer-reviwed, It is widely accepted that in subcooled flow boiling, the departure from nucleate boiling (DNB) is incepted when the liquid-sublayer formed underneath a large bubble is evaporated to disappear by the heat input from the wall. This implies that large bubbles appear near the wall as a precursor of DNB. A strong relationship is therefore expected to exist between the heat flux at the large bubbles formation (qLBF) and that at the onset of DNB (qDNB). Recently, a mechanistic model was developed to predict the occurrence of “global bubble coalescence” that is defined as the bubble coalescence in which a sufficient number of bubbles merge to develop into a distinctly large bubble. In this work, the values of qLBF calculated based on the global bubble coalescence concept were compared with the experimental values of qDNB. It was found that qDNB is commonly higher than qLBF as expected and the heat flux ratio qDNB/qLBF approaches asymptotically to nearly one with an increase in qLBF, implying that the DNB is incepted as soon as large bubbles are formed when qLBF is high enough. Thus, the heat flux ratio was expressed as a function of several dimensionless quantities. The predictive performance of the resulting semi-mechanistic correlation for the DNB heat flux was assessed using the critical heat flux (CHF) data in subcooled flow boiling. It was demonstrated that the present correlation achieved substantially good performance in comparison with existing ones. This indicated that the large bubble formation is the key phenomenon to cause the DNB in subcooled flow boiling and consequently the use of qLBF as the scaling parameter is a promising way to predict the value of qDNB accurately.
Scientific journal, English

Applications of nano particle sol for capillary-driven heat pipe
Yifan Liu; Menglei Wang; Yutaro Umehara; Tomio Okawa; Daisuke Shimizu
Proceedings of 33rd International Symposium on Transport Phenomena (ISTP-33), No. 150, Sep. 2023, Peer-reviwed
International conference proceedings, English

A study on boiling entrainment from a liquid film on a heated surface
Raka Firman B; Tomio Okawa; Yuki Narushima; Hajime Furuichi; Kenichi Katono
Proceeding of 20th International Topical Meeting on Nuclear Reactor Thermal Hydraulics, Aug. 2023, Peer-reviwed
International conference proceedings, English

A Simple Analytical Model to Predict the Freeze Plug Opening Time in Molten Salt Reactors
Muhammad Ilham; Tomio Okawa
Last, Journal of Nuclear Engineering and Radiation Science, ASME International, 9, 4, 1-11, 30 Jun. 2023, Peer-reviwed, Abstract

Freeze plug is an important passive safety system used in the molten salt reactors (MSRs). It enables automatic drainage of the liquid fuel from the core to the storage tanks in an emergency to stop nuclear fission chain reaction without any operator's action and electric power supply. The opening time, that is the time taken for the freeze plug to open, is therefore of considerable importance to ensure passive safety of the MSRs. In our previous studies, systematic numerical simulations were carried out to understand how the fundamental design parameters such as the tube diameter and wall thickness of the freeze plug affected the opening time. In this work, a simple analytical model was developed for rough estimation of the opening time. It was shown that the opening time calculated by the present simple model was in fairly good agreement with that by the full simulation using the mass, momentum and energy conservation equations for the salt and the heat conduction equation within the wall material. The present simple model was hence shown to be useful particularly for the schematic design of the improved MSR freeze plugs.
Scientific journal, English

Experiments on thermal performance of heat pipes using nanoparticle layer as the wick
Menglei Wang; Yifan Liu; Tomio Okawa
Last, Applied Thermal Engineering, 228, 120518, 25 Jun. 2023, Peer-reviwed, To enhance the usability of heat pipes and vapor chambers as the cooling device for portable electronic systems, necessity of thin wick is rapidly increasing. In the present work, the layer of silica nanoparticles of about 10 μm in thickness was formed on the copper tube inner wall to explore experimentally if it works as the thin wick. It was demonstrated that in comparison with the ordinary heat pipe using a wire mesh as the wick, the thermal resistance was even improved (0.62 to 0.23 K/W) and the deterioration of the maximum heat transfer rate was not significant (120 to 78 W). This indicated that the present nanoparticle layer can be used as a very thin wick of the heat pipes and vapor chambers. If the mass of nanoparticles deposited on the heat pipe inner wall was sufficient, the thermal performance was not deteriorated even after the maximum heat transfer rate condition was experienced. It was also demonstrated that the maximum heat transfer rate can be enhanced up to 100 W when the nanoparticles were suspended in the working fluid enclosed in the container.
Scientific journal, English

Phenomenological modeling of quenching during falling liquid film cooling of high-temperature wall
Yutaro Umehara; Tomio Okawa
Last, Applied Thermal Engineering, 225, 120210, 05 May 2023, Peer-reviwed, The wetting velocity that is the propagation velocity of a liquid film falling along a high temperature wall is of considerable importance in a variety of technical applications such as emergency core cooling in nuclear power plant, heat exchangers, and metallurgical processes. To predict the wetting velocity, the quenching model that consists of the heat transfer coefficient (HTC) distribution and the quenching temperature is needed. Since direct measurements of these quantities were technically difficult, most of the existing quenching models were developed so as to match the calculated wetting velocity with the experimental data. It was therefore unknown if the existing models can be applied to the conditions under which their predictive performance was not tested. In this work, to develop a new quenching model that is applicable in wide range of experimental conditions, transient of the wall temperature profile during quenching was measured using a high-speed infrared camera. A silicon wafer that was transparent against infrared rays was utilized as the wall. In addition, a normal high-speed camera was used to understand the hydrodynamic phenomena encountered during quenching. As a result, it was found that nucleate boiling in the liquid film was the main heat transfer mechanism near the wetting front and the width of the area where significant heat transfer occurred was in the same order of magnitude as the size of nucleation bubbles. Based on these findings, a phenomenological quenching model was developed. It was shown that the present model predicts the wetting velocity more accurately than the existing ones not only for the present data but also for those accumulated in different conditions. It was hence considered that the present model well describes the thermal-hydraulic phenomena encountered near the wetting front during quenching.
Scientific journal, English

Experimental study on the breakup of liquid jet discharged from a nozzle with sudden contraction
Guofu Sun; Tomio Okawa; Mitsuhiro Aoyagi; Akihiro Uchibori; Yasushi Okano
Proceedigns of 30th International Conference on Nuclear Engineering, ICONE30-1177, May 2023, Peer-reviwed, When liquid sodium leaks as a liquid jet from piping in a sodium-cooled fast reactor, the jet impinges with structures to produce splashing droplets. As a result of the large contact area with the surrounding air, the generation of splashing droplets can cause significant combustion. According to previous studies on circular nozzles, the amount of splash is affected by the state of the jet at the moment of impingement. However, those nozzles were designed with an entrance length formed before the outlet helping the jet to develop, which is non-existent in actual pipe leakage. In view of this, in the present work, a nozzle with a sudden contraction in the cross-sectional area was designed to reproduce a supposed pipe leakage, and the breakup behavior of jet discharged from this nozzle was observed using a high-speed camera. The result shows that due to the disturbance generated by the sudden contraction, the breakup of jet was accelerated until the jet transformed into a particularly stable state when the jet velocity exceeded a certain value. Once the jet has transformed, it will not turn back unless turning down the flow rate to a very low value. The stable jet after the transformation has a longer breakup length than that before the transformation in the same flow rate, which means the hysteresis of the breakup length occurs.
International conference proceedings, English

Role of bubble coalescence in causing OSV in subcooled flow boiling
Binh Thanh Nguyen; Ryoma Tsujimura; Tomio Okawa
Last, Proceedigns of 30th International Conference on Nuclear Engineering, ICONE30-1054, May 2023, Peer-reviwed, Despite available correlations that can indicate the conditions for the point of onset of significant void (POSV) with reasonably high accuracy, the mechanism to cause OSV is still not understood sufficiently. The bubble coalescence-based mechanistic model proposed by Okawa (2021) yields a good agreement with the Saha-Zuber empirical correlation on the prediction of subcooling conditions at OSV. Therefore, experiments were carried out to investigate the possible role of bubble coalescence in OSV for subcooled flow boiling. The high-speed camera (HSC) was used to observe the bubble dynamics before and during the OSV occurrence. In contrast, a gamma densitometer and a void probe are used to measure the global and local void fraction at POSV.
International conference proceedings, English

Early and late splashes during drop impact onto a quiescent liquid film
Tomio Okawa; Katsuyuki Kawai; Kohei Kubo
Lead, Proceedigns of 11th International Conference on Multiphase Flow, #772, Apr. 2023
International conference proceedings, English

Numerical investigation on the effects of fundamental design parameters on freeze plug performance in molten salt reactors
Muhammad Ilham; Indarta Kuncoro Aji; Tomio Okawa
Last, Nuclear Engineering and Design, 403, 112144, Mar. 2023, Peer-reviwed, Systematic numerical simulations were performed to explore the effects of the wall thickness and inner diameter of the freeze plug utilized in the molten salt reactors (MSRs). The opening time that is one of the most important factors to ensure the passive safety of the MSRs is dependent obviously on the initial shape of the frozen salt formed in the freeze plug tube. In view of this, a simulation of the solidification process was carried out first and it was followed by the melting process simulation. In the simulation of solidification, it was found that the heating and cooling powers required to close the freeze plug appropriately tend to increase with increased values of the wall thickness and tube diameter. The simulation results of melting showed that the opening time can be shortened by utilizing a thick tube wall since the heat conveyed within the tube wall by thermal conduction increases to accelerate the melting of the frozen salt. As for the effect of the tube diameter, small tube was advantageous to shorten the opening time but required a long time to drain the liquid salt from the reactor core to the drain tank after the opening. The present simulation results indicated that all the three processes of solidification, melting and drainage should be understood sufficiently to utilize the freeze plug as an effective passive safety system in the MSRs.
Scientific journal, English

Validation of a Phenomenological Quenching Velocity Model in Extended Experimental Conditions
Hiroyuki Umebayashi; Yutaro Umehara; Tomio Okawa
Last, 12th Japan-Korea Symposium on Nuclear Thermal Hydraulics and Safety, N12E1098, 02 Nov. 2022, False
International conference proceedings, English

Improving Freeze Plug Performance in Molten Salt Reactors by Changing the Tube Inclination
Muhammad Ilham; Tomio Okawa
Last, 12th Japan-Korea Symposium on Nuclear Thermal Hydraulics and Safety, N12E1082, 02 Nov. 2022, False
International conference proceedings, English

Critical Heat Flux and Large Bubble Formation in Subcooled Flow Boiling
Nguyen Thanh Binh, Ryoma Tsujimura, Tomio Okawa
Last, 12th Japan-Korea Symposium on Nuclear Thermal Hydraulics and Safety, N12E1113, 01 Nov. 2022, False
International conference proceedings, English

Experiment on boiling entrainment from a falling liquid film
T. Okawa, J. Tabuchi, R. Firman, Y. Narushima, H. Furuichi, and K. Katono
Lead, 1st European-American-Japanese Two-Phase Flow Group Meeting, 25 Oct. 2022, Invited, True
International conference proceedings, English

Quenching Heat Transfer Characteristics of Copper Rod in Saturated and Various Subcooled Condition
H. Zeol; M. Z. Sulaiman; H. Z. Hui; H. Ismail; T. Okawa
Last, Lecture Notes in Mechanical Engineering, 361-377, 09 Aug. 2022, Peer-reviwed, This study investigated the quenching performance of a copper rod with 50 mm length and diameter of 20 mm. The specimen was heated to 600 °C as the initial temperature and immersed in a quenching pool of pure water (distilled water) followed with a subsequent quench seven times. Under atmospheric pressure, the experiments are conducted in saturated and various subcooled conditions (90, 80 and 60 °C). The cooling curves (temperature vs time) and the cooling rate curves (°C/s) of the copper cylinder are obtained from the experiment. Results show that the cooling performance for 1st quench and the subsequent quench for saturated and 90 °C subcooled condition shows a different performance related to the formation of the oxide layer at the copper surface that changes the surface characteristic. Vice versa, the cooling performance in 80 °C and 60 °C subcooled conditions has consistent performance for all quench, which is believed to be the domination of the subcooling effect, even though the physical surface appearance shows the same. Overall, the cooling curve of the copper rod was enhanced with the increase of subcooled temperature, especially for 60 °C subcooled conditions. The cooling curves for the subcooled of 90 and 80 °C still maintain the slope with the three-section shape, which is similar for the saturated case, but for the 60 °C subcooled conditions, the cooling curve slope suddenly increased and shifted to the left, showing the drastic decrease of centre temperature and the impact on the highly subcooled condition. The cooling rate curve shows the increasing peak value of cooling rate with increasing the subcooled temperature, which is the highest value during quench in 60 °C conditions. The minimum heat flux (MHF) point temperature rises and occurs faster, and the Critical Heat Flux (CHF) point is achieved early with the increasing subcooled temperature. The highly subcooled condition 60 °C shows no film boiling regime formation and the MHF point location is not visible.
International conference proceedings

EXPERIMENT STUDY ON THE EFFECT OF NOZZLE SHAPE ON LIQUID JET BREAKUP
Guofu Sun, Yi Zhan, Tomio Okawa, Mitsuhiro Aoyagi, Akihiro Uchibori, Yasushi Okano
Proceedings of 29th International Conference on Nuclear Engineering, 7-B, ICONE29-93074-V07BT07A032, 08 Aug. 2022, Peer-reviwed, True, When a liquid sodium leakage accident occurs in a sodium-cooled fast reactor, the injected sodium collides with structures to produce splashing droplets, which can result in a violent combustion. According to previous studies on circular nozzles, the amount of splash is affected by the state of the jet at the moment of impact. However, the outlet shape of damaged area is hardly to be circular; and meanwhile it influences the flow pattern of jet a lot. Considering about this, in the present work, high-speed cameras were used to observe the jet discharged from oval nozzles vertically downward to investigate the falling process of the jet. The result shows that surface wave appears on the jet and within a certain range of flow velocity it can be observed obviously, meanwhile accelerate the breakup of jet.
International conference proceedings, English

NUMERICAL ANALYSIS OF SIMPLE HEAT EQUATION TO PREDICT THE OPENING TIME OF THE FREEZE PLUG IN MOLTEN SALT REACTORS
Muhammad Ilham; Tomio Okawa
Last, Proceedings of 29th International Conference on Nuclear Engineering, 4, ICONE29-92284-V004T04A026, 08 Aug. 2022, Peer-reviwed, True, Freeze plug is an important safety system that can be used in Molten Salt Reactors (MSRs). Capable of removing the decay heat of the fuel in the core by draining the liquid fuel in the core to the safety tank located underneath the core by gravity, with no use of a mechanical valve or operator and electrical supply. The better understanding of designing the freeze plug system, the more efficient the MSRs safety system will be. The opening time of the freeze plug is one of the major importance in ensuring the passive safety of the MSRs. In this study, systematic numerical simulations are carried out to understand the solidification and melting processes of salt in MSRs freeze plugs. The simulation results from a previous study showed that the steady-state form of the frozen salt in the solidification process has a big impact on the opening time of the plug in the melting process. Based on these results, a simple method to estimate the opening time using the thermal conduction theory was developed. It was found that the opening time of the simulation results has a good agreement with the opening time from the analytical equation. Thus, it was demonstrated that this method is useful to estimate the performance of well-designed freeze plugs.
International conference proceedings, English

Experimental Study on Measurement of Boiling Entrainment Rate From Falling Liquid Film With Gas Sheared Flow
B. P. Raka Firman; Junpei Tabuchi; Tomio Okawa; Yuki Narushima; Hajime Furuichi; Kenichi Katono
Proceedings of 29th International Conference on Nuclear Engineering, 15, ICONE29-90420-V015T16A022, 08 Aug. 2022, Peer-reviwed, True, Study on the droplet entrainment in annular two-phase flow has attracted many researchers in connection with the liquid film dryout in the annular two-phase flow. However, the knowledge on the entrainment due to nucleate boiling is still insufficient to predict the droplet entrainment rate accurately. In this study, measurements were made of the droplet entrainment rate with falling liquid films on the locally heated test section. The visualization using a high-speed camera to clarify the accuracy of the measurement results also carried out in this experiment. For the first step in this study, the measurement of the droplet entrainment rate was performed after getting a stable or steady state conditions. Then, we investigated the influence of the separator position on the reproducibility of the droplet entrainment rate to get the droplet entrainment rate - separator position map. As a result, the reproducibility of the droplet entrainment rate increases with decreasing separation distance from the liquid film. In additions, we also investigated the influence of the heat flux and liquid film flow rate on the reproducibility of the droplet entrainment rate. As a result, the reproducibility of the droplet entrainment rate increases with increasing the heat flux and liquid film flow rate. In the future, we expect to investigate the influence of gas flow rate, and degree of subcooling on the reproducibility of the droplet entrainment rate and some correlation models are developed.
International conference proceedings, English

POSSIBLE ROLE OF BUBBLE COALESCENCE IN CAUSING OSV AND DNB IN SUBCOOLED FLOW BOILING
Tomio Okawa; Nguyen Thanh Binh
Lead, Japan-U.S. Seminar on Two-Phase Flow Dynamics, Keynote Lecture, 11 May 2022, Invited, True
International conference proceedings, English

Developing a semi-mechanistic correlation for the onset of significant void in subcooled flow boiling
Tomio Okawa
Lead, International Communications in Heat and Mass Transfer, Elsevier, 134, 106047, May 2022, Peer-reviwed, True, A new correlation was developed for the point of the onset of significant void (OSV) or the net vapor generation (NVG) in subcooled flow boiling. The correlation was developed based on a mechanistic model that assumes that the OSV occurs when the void fraction in the bubble layer formed near the heated wall reaches the critical value to cause global bubble coalescence. To achieve a good agreement with the experimental data reported in the literature, the model constant associated with the effect of the micro-convection induced by bubble growth was expressed as a function of several dimensionless numbers. For 232 experimental data points for the thermodynamic equilibrium quality at the point of OSV (POSV) reported in 22 experimental datasets, the mean absolute error was 47.8% for the original mechanistic model and it was reduced to 21.6% after the above-mentioned modification. The semi-mechanistic correlation developed in this work would hence be useful in technical applications for which the accurate prediction of the POSV is required, as in the nuclear reactor safety analysis.
Scientific journal, English

Spatial distribution of heat transfer coefficient in the vicinity of wetting front during falling liquid film cooling of a vertical hot wall
Yutaro Umehara; Keisuke Yamagata; Tomio Okawa
Last, International Journal of Heat and Mass Transfer, Elsevier, 185, 122422, 01 Apr. 2022, Peer-reviwed, True, Spatial distribution of the heat transfer coefficient (HTC) in the vicinity of the wetting front is of crucial importance in predicting the wetting velocity during liquid film cooling of a high-temperature solid surface. However, mainly due to the lack of experimental data, no sufficiently reliable model has been developed so far for the HTC distribution in this fundamental thermal-hydraulic situation. In the present work, experiments using a high-speed infra-red camera were carried out to measure the transient of wall temperature distribution during wetting of a high-temperature vertical wall with a falling liquid film. Based on the HTC distributions calculated from the measured temperature data, dimensionless correlations were developed for the HTC distribution near the wetting front. It was confirmed that the propagation velocities of the wetting front (wetting velocity) calculated using the proposed correlations agree with the experimental data accumulated in this work well.
Scientific journal, English

Heat Transfer Characteristics of Metallic Body during Quenching in Saturated Nanofluids
H. Ismail; M. Z. Sulaiman; T. Okawa
International Journal of Automotive and Mechanical Engineering, 19, 1, 9403-9411, 24 Mar. 2022, Peer-reviwed, True, Heat transfer characteristics (HTCs) of a metallic rod during quenching were investigated in saturated water-based nanofluids and the results were compared to distilled water. In this study, a 50 mm length of cylindrical copper rod with a diameter of 15 mm was rapidly quenched at an initial temperature of 600°C in saturated water-based nanofluids under atmospheric pressure. Three different types of nanoparticles (Al2O3, SiO2and TiO2) were used to obtain water-based nanofluids with 0.001% particle volume fraction. In this experiment, heat transfer rates were evaluated using the cooling curves (temperature vs time) of the copper rod quenched in different quenching media. Results showed that the quenching heat transfer in nanofluids was stochastic during the first quenching, with SiO2 showing a deterioration in HTC, enhancement was observed in Al2O3 and TiO2 compared to distilled water in the present work. However, after successive quenching, from second to seventh, all nanofluids demonstrated enhancement in the cooling time, with quenching in TiO2 nanofluid showing the most significant enhancement compared to distilled water. Some deposition of nanoparticles on the surface of the rod was noticed, due to oxidation in distilled water quench, and mixed oxidation and nanoparticles deposition in the nanofluid quench. The present work suggested oxidation lead to deterioration of HTC, but a mixed effect of oxidation and nanoparticles deposition during the quenching process, altered the surface roughness of the rod surface and improved wettability. Hence, it was possible that the effect of surface structure vapour during film boiling influenced the dynamics of the bubbles in nucleate boiling and therefore enhanced the rapid cooling of the rod
Scientific journal, English

Effect of wall material on boiling heat transfer of nanofluid
Tomio OKAWA, Koki NAKANO, Ren NING, Yutaro UMEHARA and Yasuo KOIZUMI
Lead, 32nd International Symposium on Transport Phenomena, No.23, 20 Mar. 2022, Peer-reviwed, True
International conference proceedings, English

VISUALIZATION STUDY FOR THE MECHANISMS TO CAUSE OSV AND DNB IN SUBCCOLED FLOW BOILING
Tomio Okawa, Yuya Endo, and Ryoma Tsujimura
Lead, 19th International Topical Meeting on Nuclear Reactor Thermal Hydraulics (NURETH-19), Paper No. 36360, 09 Mar. 2022, Peer-reviwed, True
International conference proceedings, English

EXPERIMENTS FOR THE MECHANISMS DETERMINING CHF AND HTC VALUES IN NANOFLUID POOL BOILING
Ren Ning, Koki Nakano, Yutaro Umehara, Yasuo Koizumi, and Tomio Okawa
19th International Topical Meeting on Nuclear Reactor Thermal Hydraulics (NURETH-19), Paper No. 36362, 09 Mar. 2022, Peer-reviwed, True
International conference proceedings, English

PHENOMENOLOGICAL INTERPRETATION OF HEAT TRANSFER COEFFICIENT DISTRIBUTION NEAR THE REWETTING FRONT
Yutaro Umehara; Tomio Okawa
Last, 19th International Topical Meeting on Nuclear Reactor Thermal Hydraulics (NURETH-19), Paper No. 34055, 09 Mar. 2022, Peer-reviwed, True
International conference proceedings, English

EXPERIMENTAL AND NUMERICAL INVESTIGATIONS OF SPLASHING DURING SINGLE DROP IMPACT ONTO A LIQUID FILM
Tomio Okawa, Katsuyuki Kawai, Kohei Kubo, Susumu Yamashita and Hiroyuki Yoshida
Lead, 19th International Topical Meeting on Nuclear Reactor Thermal Hydraulics (NURETH-19), Paper No. 34082, 08 Mar. 2022, Peer-reviwed, True
International conference proceedings, English

Experimental observation of droplet entrainment by nucleate boiling in liquid film
Yuki Narushima, Kenichi Katono, Junpei Tabuchi; Tomio Okawa
Last, 19th International Topical Meeting on Nuclear Reactor Thermal Hydraulics (NURETH-19), Paper No.34130, 07 Mar. 2022, Peer-reviwed, True
International conference proceedings, English

Fundamental characteristics of secondary drops produced by early splash during single-drop impingement onto a thick liquid film
Tomio Okawa; Katsuyuki Kawai; Kohei Kubo; Sota Kitabayashi
Lead, Experimental Thermal and Fluid Science, Elsevier, 131, 110533, 01 Feb. 2022, Peer-reviwed, True, Basic features of early-splash droplets produced during single-drop impact onto a quiescent liquid film were explored experimentally using water and three aqueous solutions of glycerol at different concentrations as the test liquids. It was found that the time from the contact of primary drop with a liquid film is a crucial parameter in determining the size, ejection velocity, and ejection angle of secondary drops. It was also shown that the time period for the early splash to occur, the total number of secondary drops, and the number of secondary drops produced per unit time are dependent primarily on the impact Weber number. From the experimental data accumulated in this work, dimensionless correlations were developed for these quantities. These correlations can be used to calculate various quantities such as the total mass, total surface area, and total kinetic energy of the secondary drops. As a typical example, the total mass of the secondary drops was calculated to explore its dependence on the Weber and Ohnesorge numbers.
Scientific journal, English

Variations of nanoparticle layer properties during nucleate pool boiling
Tomio Okawa; Koki Nakano; Yutaro Umehara
Lead, Journal of Physics: Conference Series, Journal of Physics: Conference Series, 2116, 1, 012002, 08 Dec. 2021, Peer-reviwed, False, The nanoparticle layer detachment during nucleate pool boiling and its influences on heat transfer surface properties were explored experimentally. The material of the heat transfer surface was copper and the nanoparticle layer was formed on the heat transfer surface by nucleate boiling in the water-based TiO2 nanofluid. It was found that the detachment of the nanoparticle layer during nucleate boiling in pure water is significant. In the present experiment, more than half of nanoparticles deposited on the heated surface were detached before the CHF condition was reached. The thickness and roughness decreased accordingly. However, the wettability and wickability that are the influential parameters on the CHF value were maintained even after the occurrence of nanoparticle layer detachment and deteriorated only after the CHF condition was reached. It is therefore considered that the onset of CHF brings qualitative change to the capillary suction performance of the layer of nanoparticles. In exploring the effect of the nanoparticle layer properties on the nucleate boiling heat transfer, sufficient attention should be paid to the variation of the nanoparticle layer properties during nucleate boiling.
International conference proceedings, English

On the mechanism of onset of significant void in subcooled flow boiling
Tomio Okawa
International Journal of Heat and Mass Transfer, 181, Dec. 2021, Peer-reviwed, True, Theoretical investigation was carried out for a possible mechanism of the onset of significant void (OSV) or net vapor generation (NVG) in subcooled flow boiling. Based on available knowledge on the two-phase flow regime transition from bubbly to slug flows, it was postulated that bubble coalescence is intensified substantially when the local void fraction near the heated wall exceeds a critical value. It was discussed that this can be a main cause of a rapid increase in the cross-sectional area-averaged void faction since the condensation rate decreases suddenly due to a sudden decrease in the interfacial area of vapor bubbles. It was demonstrated that if the vaporization rate on the heat transfer surface is evaluated using a reliable wall heat transfer correlation, the OSV conditions calculated by the present model agree with those by a widely-used empirical correlation well.
Scientific journal, English

Effect of Nanoparticle Layer Coating on Heat Transfer Performance of Heat Pipe
Menglei Wang; Tomio Okawa
Heat Transfer Engineering, 42, 19-20, 1748-1754, 13 Nov. 2021, Peer-reviwed, Invited, False, In order to enhance the heat transfer performance, the screen mesh wick and the inside wall of the container of a heat pipe were pre-coated with a silica nanoparticle layer. When the screen mesh was coated, the thermal resistance decreased by 47% on average. This would be attributed to enhanced wicking performance of screen mesh by the nanoparticle layer. When the container inside wall was coated, reduction of the thermal resistance was less significant but the heat pipe worked well even when the wire mesh was eliminated. This demonstrates that the nanoparticle layer on the wall acted as a very thin wick. It is expected that nanoparticle coating on the screen mesh contributes to the improvement of heat transfer performance of the heat pipe and that on the container inner wall contributes to the downsizing.
Scientific journal, English

A mechanistic modeling of OSV in subcooled flow boiling
Tomio Okawa
Proceedings of 2nd Asian Conference on Thermal Sciences, Paper No. 50221, 07 Oct. 2021, Peer-reviwed, False
International conference proceedings, English

Improvements of thermal resistance and CHF by nanoparticle coating in heat pipes
Menglei Wang, Tomio Okawa
Proceedings of 2nd Asian Conference on Thermal Sciences, Paper No. 50611, 07 Oct. 2021, Peer-reviwed, False
International conference proceedings, English

Microscopic heat transfer characteristics during cooling of high temperature surface by a falling liquid film
Yutaro Umehara; Tomio Okawa
International Conference on Nuclear Engineering, Proceedings, ICONE, 2, 04 Aug. 2021, Peer-reviwed, False, When the dry region on the high-temperature wall is cooled down by liquid, the dry region is transformed into wet region as the temperature shifts in a short time. Vigorous boiling occurs during the temperature transition. Especially, it is called the rewetting phenomenon that liquid contacts dry region on the high-temperature wall in the first time. In nuclear plants, it is essential for nuclear power plant safety to understand this phenomenon. Under severe accidents, a liquid film formed by an emergency core cooling system (ECCS) removes the decay heat of fuel rods. To determine the start time of ECCS, the time required for the liquid film to cover the fuel rods needs to be clarified. Working time is calculated by using rewetting velocity which means the boundary movement speed between liquid and dry region. Previous research reported rewetting velocity was calculated by heat conduction equation using the boundary condition (= heat transfer coefficient (HTC) distribution). However, proposed HTC distributions were not defined directly from experimental data but defined to match experimental results. This is because the temperature was measured by thermocouples which were not enough to obtain temperature distribution. In this study, two kinds of experiments were done to define HTC distribution from experimental results and to elucidate the rewetting phenomenon. First, as liquid film fell along high temperature wall, the transition of temperature distribution of the wall was measured by an infrared camera. Copper plate was used to heat transfer surface. From the results of temperature distribution, HTC distribution was calculated. And then, four critical parameters that consisted of HTC distribution were investigated against experimental parameters. Second, a similar experiment was conducted using silicon wafer which is transparent for infrared rays as the heat transfer surface to observe the rewetting phenomenon, especially the precursory cooling region. The precursory cooling region is considered as one of the important regions to calculate exact rewetting velocity. An infrared camera and high speed camera was used in this experiment. An infrared camera took temperature distribution from the backside which is the opposite side of the liquid film. A high-speed camera took liquid film condition from the front side. Since both cameras were synchronized, this experimental apparatus was able to observe both HTC distribution and liquid film condition at the same time. As a result of observation, the precursory cooling region is the area where drying and wetting are repeated by boiling.
International conference proceedings, English

Numerical investigation of safety system parameters in molten salt reactor: Wall effect on freeze valve opening time
Muhammad Ilham; Indarta Kuncoro Aji; Tomio Okawa
International Conference on Nuclear Engineering, Proceedings, ICONE, 1, 04 Aug. 2021, Peer-reviwed, False, Molten Salt Reactor (MSRs) is one of the fourth generation Nuclear Power Plants with better capabilities and potentialities compared to previous generation, the enthusiasm for molten fuel reactor has been increasing around the world. MSRs has passive safety where if the core is overheating cause by accident event, the liquid salt fuel was required to be moved to the safety drain tank underneath the core vessel by gravity force. During this occasion, the freeze valve (FV) that formed in the pipe located between the core and drain tank must be melt out promptly to prevent the vessel to reach it is melting point. In this paper, we conduct on thermal analysis of the freeze valve at the solidification and melting process based on finite elements methods. The enthalpy-porosity method adopted by ANSYS Fluent was used to simulated the designed system at specified condition. The Oak Ridge National Laboratory of Molten Salt Reactor Experiment Freeze valve system was used as a references for parameters investigation. Using pipe wall thickness of 5 mm, 10 mm, and 15 mm to examined the wall effect to thermal properties of the designed freeze valve. The wall pipe for FV systems material was also investigate in order to examine its effect to the opening time. Further, the temperature distributions of the valve system were obtained and analyzed. It was found that the wall effect has significant impact to the solidification and melting process.
International conference proceedings, English

Droplet generation during spray impact onto a downward-facing solid surface
Yi Zhan; Guofu Sun; Tomio Okawa; Mitsuhiro Aoyagi; Takashi Takata
Experimental Thermal and Fluid Science, Elsevier, 126, 110402, 01 Aug. 2021, Peer-reviwed, True, Experiments were conducted to explore the droplet generation process when an ascending liquid spray impacts on a downward-facing solid surface. The droplets generated in the present experiments were classified into the two types: one is the splashing droplets generated by the impacts of droplets in spray and the other is the falling droplets produced from a liquid film formed on the solid surface. The falling droplets were further classified to the first falling droplets and the satellite droplets. It was found that the ratio of the splashing droplets increases with an increase in the impact Weber number. As for the droplet size, the splashing droplets were much smaller than the falling droplets. Hence, the oil immersion method and the image analysis were used to measure the sizes of the splashing droplets and the falling droplets, respectively. The size of the splashing droplets was in the same order with the impacting droplets in spray. On the other hand, the size of the falling droplets was proportional to the wavelength of the most unstable wave in the Rayleigh-Taylor instability. Interestingly, the diameters of the falling droplets (first falling and satellite droplets) were inversely proportional to their order. Using the experimental data accumulated in this work, dimensionless correlations were developed for the rates and sizes of the splashing and falling droplets.
Scientific journal, English

Role of nanoparticle layer in determining minimum heat flux temperature during quenching of high-temperature body
Yutaro UMEHARA, Tomio OKAWA
Mechanical Engineering Journal, JAPAN SOC MECHANICAL ENGINEERS, 20, 00527, 18 Mar. 2021, Peer-reviwed, False
Scientific journal, English

Experimental Investigation of the Deformation of Gas-Liquid Two-Phase Jet
Guofu Sun, Yi Zhan, Takuji Kobayashi, Tomio Okawa, Motoki Irikura
Japanese Journal of Multiphase Flow, 35, 1, 77-84, 15 Mar. 2021, Peer-reviwed, False
Scientific journal, Japanese

Experiments on splashing thresholds during single-drop impact onto a quiescent liquid film
Tomio Okawa; Kohei Kubo; Katsuyuki Kawai; Sota Kitabayashi
Experimental Thermal and Fluid Science, 121, 01 Feb. 2021, Invited, True, The process of normal impingement of a single drop onto a quiescent liquid film was investigated experimentally to determine the thresholds of the early and late splashes. Seven liquids of different hydraulic properties were used and the liquid film thickness was varied within about 0.2–3 times the impacting drop diameter. It was found that the critical Weber number for the early splash to occur is dependent on the liquid properties (more specifically, the Ohnesorge number) but not on the liquid film thickness. On the other hand, the critical Weber number of the late splash was influenced by both the liquid properties and the liquid film thickness. It increased gradually with an increase in the liquid film thickness in a thin film range and jumped to a constant value when the critical film thickness was reached. The critical film thickness was 0.7 times the impacting drop diameter in average in the present experiments. Using the present experimental data, dimensionless correlations were developed for the thresholds of the early and late splashes. The predictive performances of the proposed correlations were tested against the experimental databases available in the literature. Due to the complexity of the drop impact process, noticeable disagreements remained between the experimental data and the predictions even in the simple situation explored in this work. Nonetheless, the present dimensionless correlations succeeded to express the overall trends of the splashing thresholds fairly well.
Scientific journal, English

Experimental observation of nucleate boiling entrainment in a liquid film
Junpei Tabuchi; Yuki Narushima; Kenichi Katono; Tomio Okawa
International Conference on Nuclear Engineering, Proceedings, ICONE, 4, 2021, Many studies have been conducted on droplet entrainment in an annular flow regime, but little is known about droplet entrainment caused by nucleate boiling. In this report, visualization results of droplet entrainment caused by nucleate boiling are described. We observed two processes of droplet entrainment. The first one causes bubble bursting at a water surface. The second one causes filament breakup which occurs when the vapor bubble reaches and collapses at the interface between air and liquid. From comparison of the phenomena for the two processes, we found that the diameters of the droplets and vapor bubbles were considerably different. Using the results of this research allows the effect of forced convection to be taken into account. In the future, we plan to expand the amount of data and develop a boiling entrainment model under forced convection conditions.
International conference proceedings

Fundamentals for power engineering
Tomio Okawa; Hitoshi Asano; Kei Ito; Shoji Mori; Hisashi Umekawa; Ryosuke Matsumoto; Cheol Ho Pyeon; Daisuke Ito
Fundamentals of Thermal and Nuclear Power Generation, 77-226, 01 Jan. 2021, This chapter describes the academic knowledge needed in the design and operation of power plants. In both thermal and nuclear power plants, thermal energy is transferred to mechanical energy. Efficiency to convert the thermal energy to the mechanical energy is hence very important in power plants. Basic laws of energy conversion are discussed in Section 3.1: Thermodynamics. In the power plants, the heat is transferred to working fluid such as water, helium, and liquid sodium. After receiving the heat, the working fluid is conveyed to turbine to generate electricity. The fluid flow and heat transfer are hence important fundamental processes encountered in the power plants. These are discussed in Section 3.2: Fluid Dynamics and Section 3.3: Heat Transfer, respectively. The academic knowledge discussed in Sections 3.1-3.3 is commonly important in the thermal and nuclear power plants. The most essential difference between the thermal and nuclear power plants is the heat generation method. The heat is generated by fuel combustion in the thermal power plants and by nuclear reaction in the nuclear power plants. The basic principles of combustion and nuclear reaction are discussed in Section 3.4: Combustion and Section 3.5: Nuclear Physics, respectively.
In book

Experimental study on secondary droplets produced during liquid jet impingement onto a horizon solid surface
Yi Zhan, Yusuke Kuwata, Tomio Okawa, Mitsuhiro Aoyagi, Takashi Takata
Experimental Thermal and Fluid Science, Elsevier, 120, 110249, 01 Jan. 2021, Peer-reviwed, True, Experiments were preformed to measure the mass and size of secondary droplets splashed during liquid jet impingement onto a horizontal solid surface. To explore the effects of liquid hydraulic properties such as viscosity and surface tension, pure water, two aqueous solutions of glycerin, and two aqueous solutions of ethanol were used as the test solutions. Using the experimental data accumulated in this work, dimensionless correlation was developed for the splash ratio (ratio of the droplet splash rate to the jet flow rate). Here, it was assumed that the mass of secondary droplets splashed per impact can be expressed as a function of the impact Weber number and the Ohnesorge number. The correlations for the Sauter mean diameter and the size distribution of secondary droplets were also proposed. It was shown that the size distribution of the secondary droplets can be fitted with the log-normal distribution. Two parameters determining the log-normal distribution profile were correlated by the impact Weber number.
Scientific journal, English

Study on melting process of solid salt contained in metal vessel
Tomio Okawa, Tatsuya Tokushima, Indarta Kuncoro Aji, and Mahammad Ilham
Advances in Thermal Hydraulics (ATH '20), 308-319, 20 Oct. 2020, Peer-reviwed, True, To elucidate the role of pipe wall in determining the opening time of the freeze plug used in molten salt reactors, melting process of salt contained in metal vessel was investigated experimentally. Heat transfer salt (HTS) was used as the test salt because of its relatively low melting point (142°C under the atmospheric pressure). It was found that the melting time decreases noticeably with an increase in the thermal diffusivity of the wall material and a decrease in the wall thickness. This indicates that the heat transfer within the metal wall has a great impact on the opening time of the freeze plug in real molten salt reactors. It is expected that the experimental data accumulated in this work can be used as the validation data of the numerical models to predict the freeze plug opening time. It was also shown experimentally that the melting process of HTS can be accelerated if high-thermal-conductivity plate is attached on the outer wall of the vessel as a melting accelerator.
International conference proceedings, English

Special section: Selected papers from ICONE-27
Jovica Riznic; Tomio Okawa; Asif Arastu; Leon Cizelj; Shripad Revankar; Richard Schultz
Journal of Nuclear Engineering and Radiation Science, 6, 4, 01 Oct. 2020
Scientific journal

Two-phase flow regimes of refrigerant R134a in an oscillating horizontal rectangular minichannel conduit
Koji Enoki, Masaharu Ono, Tomio Okawa, Atsushi Akisawa, Budi Kristiawan, Agung Tri Wijayanta
International Journal of Refrigeration, Elsevier, 118, 261-268, 01 Oct. 2020, Peer-reviwed, True, In this study, refrigerant R134a experiments were carried out to observe two-phase flow in an oscillating horizontal rectangular minichannel conduit at refrigerant mass flux of approximately 30 kg•m −2•s −1 to 300 kg•m −2•s −1, saturation temperature of approximately 30 °C, and average vapor quality in the test section inlet of approximately 0.05 to 0.9. The test section was made from quartz and consists of a visible rectangular minichannel with a hydraulic diameter of 0.84 mm. The test section was oscillated vertically using a reciprocating motor. The effects of oscillation on slug and stratified flow patterns were investigated. The experimental observation was compared with an existing flow pattern map. The thin liquid film around the vapor of slug flow mostly moved up and down when the test section oscillated. As a result, a dry surface was not observed around the vapor due to the lower heat transfer coefficient. When the test section oscillated, the stratified flow changed to annular flow, and a dry surface was also not observed on the upper side of the minichannel conduit.
Scientific journal, English

Photographic Study on the Onset of Significant Void in Water Subcooled Flow Boiling at Low Pressure
Tomio Okawa, Yuya Endo, Hongji Li, and Hideki Takahashi
3rd International E-Conference on Advances in Engineering Technology and Management (ICETM 2020), ICETM-20-116, 27 Sep. 2020, Peer-reviwed, False
International conference proceedings, English

Water flow boiling heat transfer in vertical minichannel
Koji Enoki, Masaharu Ono, Tomio Okawa, Budi Kristiawan, Agung Tri Wijayanta
Experimental Thermal and Fluid Science, Elsevier, 117, 110147, 01 Sep. 2020, Peer-reviwed, True, In recent years, microchannel water flow boiling has demonstrated excellent heat removal capabilities, particular for nuclear reactor safety utilities, ranging from mini-scale cooling devices to chemical industry plant purposes. Of particular interest is cooling technology using water as the primary refrigerant. This paper presents the results of an experimental investigation into the water boiling heat transfer in a minichannel conduit vertically oriented with the upward flow. The test section was a circular minichannel tube made of stainless steel with a 2.12 mm inner diameter. To observe the flow regime, the test section was equipped with sight tubes made of transparent perfluoroalkoxy alkane resin and Pyrex glass at the inlet and outlet, respectively. Water flow boiling experiments were conducted at a saturation temperature of approximately 100 °C, with the mass flux of water as the working fluid from 20 to 120 kg/m2·s, and the average vapor quality in the test section inlet of approximately 0.05 to 0.9. The predicted two-phase flow regimes at the test section were classified as three patterns, namely slug, annular, and churn flows, using a flow pattern map with the vapor quality as the abscissa versus the mass velocity as the ordinate. The flow patterns were confirmed and visualized by observations at the test section outlet sight tube. The characteristics of the heat transfer and fluid flow were analyzed. Moreover, the forced convective heat transfer coefficient and pressure drop in the test section were determined based on the experimental data. The highest mass flux yielded the highest convective heat transfer coefficient. Despite this significant heat transfer coefficient improvement, the pressure loss was higher than that for a higher mass flux, indicating that the pressure loss was more pronounced owing to the presence of the minichannel at a higher mass flux.
Scientific journal, English

Prediction of Extremely High Minimum Heat Flux Point During Quenching in Nanofluid
Yutaro Umehara, Tomio Okawa
ASME Nuclear Engineering Conference powered by ICONE, Paper No: ICONE2020-16032, 03 Aug. 2020, Peer-reviwed, True, In nanofluid that is a liquid containing colloidal dispersion of nanometer-sized particles, it is known that the quenching heat transfer characteristics such as TMHF (minimum heat flux temperature) is improved mainly due to modification of the surface properties caused by nanoparticle deposition during boiling. In this study, the water-based silica (SiO2) nanofluid was used to clarify the mechanisms of the quenching characteristics variation in nanofluid. The Inconel 718 rodlet (φ16×30mm) and SUS304 rodlet (φ15×30mm) were used as the test pieces. The four properties of the nanoparticle layer were measured: Roughness, wettability, wickability, and thickness. Then, the temperature transient in the nanoparticle layer was calculated using the one-dimensional heat conduction equation. It was shown that the surface temperature of the nanoparticle layer should be maintained low enough for a sufficiently long time after immersing the high-temperature test piece in the nanofluid for remarkable increase in TMHF to occur.
International conference proceedings, English

Measurement of heat transfer coefficient profile during quenching of a vertical hot wall with a falling liquid film
Tomio Okawa, Keisuke Yamagata, Yutaro Umehara
Nuclear Engineering and Design, Elsevier, 363, 110629, 01 Jul. 2020, Peer-reviwed, Invited, True, In the safety analysis of LWRs, spatial distribution of the heat transfer coefficient (HTC) in the vicinity of the rewetting front is of importance in order to predict the rewetting velocity of high-temperature fuel rods after the onset of reflooding during a loss-of-coolant accident. Many models are available for the HTC profile in literature, but they have not been validated sufficiently mainly due to the lack of experimental data. In the present study, transient of the wall temperature distribution during the wetting process of a high-temperature vertical plate with a falling liquid film was measured using a high-speed infra-red camera. The measured temperature data were substituted to the heat conduction equation to derive the HTC profile. It was confirmed that the wetting velocity measured in this work can be reproduced within the error of 10% if the calculated HTC profile is used. This indicates that the experimental data obtained in this work can be used as the base to develop a high-performance correlation for the HTC profile near the wetting front.
Scientific journal, English

Quasi steady state of nucleate boiling heat transfer in low concentration of single and hybrid Al2O3-SiO2 water-based nanofluids
M. A.H. Aizzat; M. Z. Sulaiman; T. Okawa
IOP Conference Series: Materials Science and Engineering, 788, 1, 05 Jun. 2020, Experiment were carried out to clarify the steady state condition of heat transfer coefficient (HTC) for single and hybrid Al2O3-SiO2 water-based nanofluids in low concentration of nucleate pool boiling. Al2O3 and SiO2 were chosen and ultra-sonification for 1 hour after being diluted in distilled water. The total of 0.001 vol.% concentration was divided into composition ratio of 0:100, 25:72, 50:50, 75:25, 100:0 for hybrid nanofluids. Successively, all the HTC values were collected through experimental works. In the present work, it was found that the steady stated for single and hybrid nanofluids were achieved except for single SiO2 nanofluid where even after 5 hours, the value of its ΔTW keep on decreasing. The nanoparticles deposited on the surface heater were suspected as the main factor for the current experimental results.
International conference proceedings

Effects of surface tension and viscosity on liquid jet breakup
Yi Zhan, Yusuke Kuwata, Kiyotaka Maruyama, Tomio Okawa, Koji Enoki, Mitsuhiro Aoyagi, Takashi Takata
Experimental Thermal and Fluid Science, Elsevier, 112, 109953, 01 Apr. 2020, Peer-reviwed, True, The breakup process of liquid jet was explored through visualization using a high-speed camera to develop a prediction model for the impact frequency (the number of droplet passing per unit time). The experiments were conducted for the three jet regimes of Rayleigh, first wind-induced and second wind-induced. The five liquids (water, two ethanol aqueous solutions, and two glycerin aqueous solutions) were used as the test liquids to explore the effects of surface tension and viscosity. Since the droplets were produced due to jet breakup, the impact frequency was zero just downstream of the nozzle and increased asymptotically with an increase in the distance from the nozzle. Thus, auxiliary correlations were developed for the minimum breakup length where the jet breakup is initiated, the maximum breakup length where the jet breakup is completed, and the mean droplet size and the impact frequency in the equilibrium region downstream of the maximum breakup length. Since the correlations were dependent on the flow regime, the boundaries between the three jet regimes were also determined using the present observation results. It was demonstrated that the impact frequencies calculated by the proposed model agree with the experimental data accumulated in this work fairly well in the Rayleigh and second wind-induced regimes, while agreement was deteriorated to some extent in the transition (first wind-induced) regime. For the application to the prediction of splashing rate during coolant leakage in sodium-cooled fast reactors, a simple method to ensure conservative estimation of the impact frequency was also proposed.
Scientific journal, English

気水分離器における液滴キャリーオーバーに関する研究（気液二相噴流の分裂開始長さに関する実験的検討）
詹翼、孫国富、大川富雄、入倉基樹
混相流, 日本混相流学会, 34, 1, 75-81, 15 Mar. 2020, Peer-reviwed, Invited, False
Scientific journal, Japanese

焼結製法による高気孔率金属繊維多孔質伝熱面におけるサブクール沸騰熱伝達による高熱流束除熱
KUMATORI Kousuke; OTOMO Yusuke; SANTIAGO-GALICIA Edgar; OKAWA Tomio; ORITO Kenji; TAKITA Kenji; SAIWAI Toshihiko; ENOKI Koji
JAPANESE JOURNAL OF MULTIPHASE FLOW, THE JAPANESE SOCIETY FOR MULTIPHASE FLOW, 34, 1, 64-74, 15 Mar. 2020, Peer-reviwed, False, Nowadays high performance cooling equipment are required for high heat dissipation electric devices. In this research, the experiments were carried out with high porosity sintered fiber on the surface under the forced convection subcooled boiling heat transfer. The experimental parameter of the porous is the filling height relative to the channel height, and the porosity of porous material was 86 %. According to the experiments, all of porous surfaces, almost 2 times higher performance on boiling heat transfer compared with the bare surface under the same wall superheat ⊿T_sat (K) conditions. However, the high height porous condition, the CHF tend to low compared with lower height porous condition and bare surface. In addition, the relations between the bubble behavior and the heat transfer were analyzed from experimental results and the videos acquired by high speed camera.
Scientific journal, Japanese

Measurement of Capillary Wicking Height for the Packed Bed of Nanoparticles
Tomio Okawa and Yutaro Umehara
International Conference on Engineering & Technology (ICET-20), 53-56, 28 Feb. 2020, Peer-reviwed, False
International conference proceedings, English

Greetings from JSME and the report on ICone27
Tomio Okawa
Journal of Nuclear Engineering and Radiation Science, 6, 1, 2020
Scientific journal

An experimental and numerical study of wall effect on freeze valve performance in a molten salt reactor
Indarta Kuncoro Aji, Tatsuya Tatsuya, Motoyasu Kinoshita, Tomio Okawa
Journal of Nuclear Engineering and Radiation Science, ASME, 6, 2, 21105, 2020, Peer-reviwed, True, Freeze valve is an important passive safety system used in the molten salt reactor (MSR). In the freeze valve, salt is frozen by external cooling in the pipe connecting the reactor core and drain tanks. On the occasion of station blackout, the cooling system stops and the frozen salt melts automatically to open the freeze valve. Then, the molten salt containing fuel is drained out from the reactor core to the drain tanks and the nuclear reactions stop passively. Obviously, the melting time of frozen salt is important from the viewpoint of MSR safety. In this study, experiments and numerical simulations were conducted to investigate the effects of important parameters on the opening time of freeze valve. A particular attention was paid to the effect of pipe wall since its thermal conductivity is much higher than that of salt. The calculated opening time agreed with the experimental results fairly well, indicating that the present numerical model is useful to investigate the opening time of freeze valve in accidental condition of MSR.
Scientific journal, English

Convection effect of the freeze valve melting process on molten salt reactor
Indarta Kuncoro Aji; Tatsuya Tokushima; Koji Enoki; Motoyasu Kinoshita; Tomio Okawa
International Conference on Nuclear Engineering, Proceedings, ICONE, 2019-May, 18 May 2019, Freeze valve is one of the major safety systems used in the Molten Salt Reactor (MSR), that automatically opens in case of accident. The freeze valve was first developed in the Oak Ridge National Laboratory (ORNL) to be utilized in the Molten Salt Reactor Experiment (MSRE). Currently, many organizations have developed different types of MSR. Thus, appropriate freeze valve designs are needed. Fundamental studies to understand the basic features of freeze valve are important to develop new designs. The purpose of this study is to analyze the effect of convection in the molten salt on the opening time of the freeze valve. Numerical simulations were conducted using the ANSYS Fluent 18.1 and the results were compared with the experimental data obtained under similar conditions. From the calculation results, it was found that the convection in molten salt is often neglected but it noticeably accelerate the opening time of the freeze valve. It was concluded that the convection effect should be taken into consideration in the design of high-performance freeze valve.
International conference proceedings

Experimental study on splash ratio and size of secondary droplets during liquid jet collision onto a solid surface
Yi Zhan; Yusuke Kuwata; Kiyotaka Maruyama; Koji Enoki; Tomio Okawa; Mitsuhiro Aoyagi; Takashi Takata
International Conference on Nuclear Engineering, Proceedings, ICONE, 2019-May, 18 May 2019, If liquid sodium leaks as a liquid jet from pipe in sodium-cooled fast reactors, sodium droplets are produced during liquid jet impingement on the structures. Due to large contact area with surrounding air, generation of secondary droplets may lead to violent fire combustion. In the present work, high speed camera was first used to observe the liquid jet before the impingement. The splash ratio was then measured experimentally. It was shown that a phenomenological model using the impact frequency and the impact Weber number as the important variables can predict the splashing rate well. Distribution of the secondary droplet size was also measured by image analysis. It was indicated that the Sauter mean diameter of the secondary droplets was fairly proportional to the size of primary droplets.
International conference proceedings

An experimental study of the freeze valve opening time with considering wall effect
Tatsuya Tokushima; Indarta Kuncoro Aji; Koji Enoki; Motoyasu Kinoshita; Tomio Okawa
International Conference on Nuclear Engineering, Proceedings, ICONE, 2019-May, 18 May 2019, Nuclear power has a lot of merits including low carbon-dioxide emission and economic advantage. However, it possesses fear factors such as severe accident and radioactive waste. Currently, several new-type reactors are developed to overcome those issues. Molten Salt Reactor (MSR) is one of the generation IV nuclear reactors. To enhance safety, it adopts a passive safety system called freeze valve. Freeze valve is located between the reactor and drain tanks. The salt in the freeze valve is kept in the solid state by forced cooling during normal operation, and melts passively in the occasion of station blackout to discharge the liquid fuel in the reactor vessel to the drain tanks. Obviously, accurate prediction of opening time of freeze valve is important to assure the safety of MSR. In the freeze valve, salt is contained in a metal tube. Since thermal conductivity of the tube wall material is much higher than that of salt, heat conduction within the tube wall may have great impact on the opening time of the freeze valve. Thus, in this study, the wall effect on the opening time of the freeze valve was experimentally investigated. To investigate the wall effect, a metal stick was placed vertically in the frozen HTS (heat transfer salt), and liquid HTS was poured on the frozen HTS. The heat in the liquid HTS was mainly conveyed in the stick by thermal conduction to transferred to the solid HTS. In consequence, melting of solid HTS started around the metal stick rather than the solid-liquid interface. Since tension was applied to the stick in the upward direction, the stick moved upward when the melt interface reached the bottom end of stick. This result suggested that opening of the freeze valve is established along the tube wall in the real MSR plant. To provide experimental data to validate the numerical simulation results, parametric trends of the onset time of stick movement on the several important parameters were investigated. These data can also be used as the guideline to determine the wall material, wall thickness, length of freezing region, and the initial temperature of the frozen salt.
International conference proceedings

Temperature distribution measurement during quenching of high-temperature wall with a falling liquid film
Tomio Okawa; Keisuke Yamagata; Koji Enoki
18th International Topical Meeting on Nuclear Reactor Thermal Hydraulics, NURETH 2019, 300-306, 2019, The spatial distribution of heat transfer coefficient (HTC) in the vicinity of the rewet front is most important information in determining the rewet velocity during cooling of high-temperature fuel rods with falling liquid film. Thus, many models are available for the HTC distribution in literature, but they have not been validated sufficiently mainly due to the lack of experimental data. In this study, transient of temperature distribution during rewetting of high-temperature vertical plate was measured using a high-speed infra-red camera. The measured temperature data were substituted to the heat conduction equation to derive the HTC distribution. It was confirmed that the rewet velocity measured in this work can be reproduced within the error of 10% if the calculated HTC distribution is used. This indicates that the experimental data obtained in this work can be used as the base data to develop a high-performance correlation for the HTC distribution near the rewet front.
International conference proceedings

Experiments on Lift-off Bubble Size Distribution in Subcooled Flow Boiling
OKAWA Tomio; KAIHO Kazuhiro; LI Hongji; OHORI Koki; SAKAMOTO Shintaro; ENOKI Koji
The Proceedings of the National Symposium on Power and Energy Systems, The Japan Society of Mechanical Engineers, 2019.24, A215, 2019, Bubble size is an important fundamental parameter in subcooled flow boiling. In this study, bubble behavior in subcooled flow boiling was visualized to confirm that mean bubble size is considerably different between different nucleation sites. Empirical correlations were developed using the present experimental data for the distribution of mean bubble size and the distribution of the size of bubbles produced in each site. Observation of the nucleation sites using a microscope revealed that tower-shaped structure is present at the nucleation sites where small bubbles are produced whilst no such structure was found at the sites where large bubbles are produced. This result suggest that bubble size is dependent significantly on the nucleation site geometry.
Japanese

Evaluation of the Performance of Nanofluid as Quenching Coolant
Umehara Yutaro; Okawa Tomio; Enoki Koji
Tetsu-to-Hagane, The Iron and Steel Institute of Japan, 105, 11, 1050-1058, 2019, Nanofluid is a liquid in which nanometer-sized particles are dispersed in base liquid. It is known that the critical heat flux and the wall superheat at the minimum heat flux in pool boiling are improved in nanofluids. In this research, performance of silica-water nanofluid as quenching coolant is explored experimentally since the above-mentioned parameters play important roles in this application. First, we investigated the immersion cooling of high-temperature test piece in the nanofluid; here, the test piece was cylindrical in shape and made of Inconel 718 or SUS304. It was confirmed that the test piece is cooled faster in the nanofluid than in distilled water. It was also found that non-uniformity of temperature in the test piece during quenching is mitigated in the nanofluid. This indicates that the silica nanofluid is considered a promising coolant to avoid the occurrence of hardening crack during quenching. Finally, Vickers hardness test was done for the Inconel 718 test piece. It was shown that the hardness tends to increase with an increase in the cooling rate even under the high cooling rate of about 1000 K/min although the difference of hardness was not noticeable between the experiments using the distilled water and the silica nanofluid as the quenching coolant.
Japanese

Droplet generation during liquid jet impingement onto a horizontal plate
Yi Zhan; Naoki Oya; Koji Enoki; Tomio Okawa; Mitsuhiro Aoyagi; Takashi Takata
Experimental Thermal and Fluid Science, 98, 86-94, Nov. 2018, In sodium-cooled fast reactors, liquid sodium leakage from piping may lead to fire accident. In the case that the liquid sodium is discharged as a liquid jet, a number of droplets are produced during liquid jet impingement on the structures; the surfaces of splashed droplets serve as a main reaction field of sodium combustion. In the present work, a liquid jet was emanated vertically downward from a circular nozzle hole onto a horizontal disk to measure the total amount and the maximum size of splashed droplets. It was found that the splashing rate was negligibly small when the liquid jet impinged as a continuous jet whilst a significant amount of liquid was splashed when the liquid jet impinged as a broken jet. Thus, a prediction method was first developed for the impact frequency of the primary droplets produced due to the jet breakup. It was then shown that a phenomenological model using the impact frequency and the impact Weber number as the important variables can predict the splashing rate well. It was also indicated that the size of the maximum splashed droplets was fairly proportional to the size of primary droplets.
Scientific journal

Droplet generation during liquid jet impingement onto a horizontal plate
Yi Zhan; Naoki Oya; Koji Enoki; Tomio Okawa; Mitsuhiro Aoyagi; Takashi Takata
EXPERIMENTAL THERMAL AND FLUID SCIENCE, ELSEVIER SCIENCE INC, 98, 86-94, Nov. 2018, In sodium-cooled fast reactors, liquid sodium leakage from piping may lead to fire accident. In the case that the liquid sodium is discharged as a liquid jet, a number of droplets are produced during liquid jet impingement on the structures; the surfaces of splashed droplets serve as a main reaction field of sodium combustion. In the present work, a liquid jet was emanated vertically downward from a circular nozzle hole onto a horizontal disk to measure the total amount and the maximum size of splashed droplets. It was found that the splashing rate was negligibly small when the liquid jet impinged as a continuous jet whilst a significant amount of liquid was splashed when the liquid jet impinged as a broken jet. Thus, a prediction method was first developed for the impact frequency of the primary droplets produced due to the jet breakup. It was then shown that a phenom-enological model using the impact frequency and the impact Weber number as the important variables can predict the splashing rate well. It was also indicated that the size of the maximum splashed droplets was fairly proportional to the size of primary droplets.
Scientific journal, English

Heat transfer coefficient of nucleate boiling in low concentration level of single and hybrid Al2O3-SiO2 water-based nanofluids
M A H Aizzat; Muhamad Zuhairi Sulaiman; Koji Enoki; Tomio Okawa
AIP Conference Proceedings, Materials Science and Engineering, 469, 1, 012109, Oct. 2018, Peer-reviwed, Experiments were conducted to identify the Heat Transfer Coefficient (HTC) in saturated pool boiling of single and hybrid water-based nanofluids. In these experiments, Al 2 O 3 and SiO 2 nanoparticles were selected and diluted into two separate single nanofluids, and they were mixed in a different ratio from 0:100, 25:75, 50:50, 75:25 and 100:0 percent to achieve a final total concentration of 0.001 vol. %. Successively, the mixtures were used to obtain the HTC values through experimental works. In the present work, it was found that in the lowest concentration (0.00025 vol.%) of Al 2 O 3 nanofluid, the HTC enhanced considerably but deteriorated for SiO 2 nanofluid. Separately, as for the hybrid nanofluids, the HTCs were dramatically enhanced at the initial stage but slowly deteriorated once the time variation increased, especially in a higher ratio of SiO 2 nanofluid. The deposition of the nanoparticles onto the surface heater suggested being the main factor, where in the present case, the significant coexisting effect of the deposited hybrid nanoparticles (Al 2 O 3 and SiO 2 ) on the heated surface to the changes of ΔT W due to different nanoparticles properties.
Scientific journal, English

Nanoparticle layer detachment and its influence on the heat transfer characteristics in saturated pool boiling of nanofluids
Yosuke Watanabe; Koji Enoki; Tomio Okawa
International Journal of Heat and Mass Transfer, Elsevier BV, 125, 171-178, Oct. 2018

Nanoparticle layer detachment and its influence on the heat transfer characteristics in saturated pool boiling of nanofluids
Yosuke Watanabe; Koji Enoki; Tomio Okawa
International Journal of Heat and Mass Transfer, 125, 171-178, Oct. 2018, In nucleate pool boiling of nanofluid, nanoparticles suspended in liquid are deposited to form nanoparticle layer on the heated surface. As a result, surface properties are changed and the critical heat flux (CHF) is usually enhanced. However, since adhesion of the nanoparticle layer to the heated surface is not necessarily strong, partial detachment of the nanoparticle layer frequently occurs during nucleate boiling. In this study, peeling test was conducted for the nanoparticle layer formed during nucleate boiling of the water-based nanofluids to measure the adhesion force. The material of the heated surface was copper, and TiO2, Al2O3 and SiO2 were used as the nanoparticle material. It was found that the adhesion force is highly dependent on the nanoparticle material; it was greatest for SiO2 and weakest for TiO2 in the present experiments. Then, saturated pool boiling curves were obtained using the damaged heated surfaces after the peeling. For the nanoparticle-layer coated surfaces without peeling, the heat transfer coefficient (HTC) was lower and CHF was higher than those for the bare surface. It was shown that with an increase in the mass of the nanoparticles removed by peeling, the decrease of HTC and the increase of CHF were mitigated. Even higher HTC values and a lower CHF value than those for the bare heated surface were measured for significantly damaged surfaces.
Scientific journal

Nanoparticle layer detachment and its influence on the heat transfer characteristics in saturated pool boiling of nanofluids
Yosuke Watanabe; Koji Enoki; Tomio Okawa
INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER, PERGAMON-ELSEVIER SCIENCE LTD, 125, 171-178, Oct. 2018, In nucleate pool boiling of nanofluid, nanoparticles suspended in liquid are deposited to form nanoparticle layer on the heated surface. As a result, surface properties are changed and the critical heat flux (CHF) is usually enhanced. However, since adhesion of the nanoparticle layer to the heated surface is not necessarily strong, partial detachment of the nanoparticle layer frequently occurs during nucleate boiling. In this study, peeling test was conducted for the nanoparticle layer formed during nucleate boiling of the water-based nanofluids to measure the adhesion force. The material of the heated surface was copper, and TiO2, Al2O3 and SiO2 were used as the nanoparticle material. It was found that the adhesion force is highly dependent on the nanoparticle material; it was greatest for SiO2 and weakest for TiO2 in the present experiments. Then, saturated pool boiling curves were obtained using the damaged heated surfaces after the peeling. For the nanoparticle-layer coated surfaces without peeling, the heat transfer coefficient (HTC) was lower and CHF was higher than those for the bare surface. It was shown that with an increase in the mass of the nanoparticles removed by peeling, the decrease of HTC and the increase of CHF were mitigated. Even higher HTC values and a lower CHF value than those for the bare heated surface were measured for significantly damaged surfaces. (C) 2018 Elsevier Ltd. All rights reserved.
Scientific journal, English

Observation and modelling of bubble dynamics in isolated bubble regime in subcooled flow boiling
Tomio Okawa; Kazuhiro Kaiho; Shintaro Sakamoto; Koji Enoki
Nuclear Engineering and Design, 335, 400-408, 15 Aug. 2018, Experiments of subcooled flow boiling in isolated bubble regime were conducted to understand the dynamics of individual bubbles. The test fluid was water, the flow direction was vertical upward, and the pressure was slightly higher than the atmospheric pressure. In the present experiments, the bubbles produced at nucleation sites were commonly lifted off the vertical heated surface. They were then propelled into the subcooled bulk liquid to disappear due to heat transfer with subcooled liquid. It was frequently observed that the bubbles were suddenly accelerated in the lateral direction before the disappearance. Using the present observation results, new correlations were developed for the bubble lift-off velocity and the bubble acceleration phenomenon after the lift-off. It was shown that the bubble trajectories and the time-variation of bubble size calculated using the present correlations are in fairly good agreement with the experimental results.
Scientific journal

Observation and modelling of bubble dynamics in isolated bubble regime in subcooled flow boiling
Tomio Okawa; Kazuhiro Kaiho; Shintaro Sakamoto; Koji Enoki
NUCLEAR ENGINEERING AND DESIGN, ELSEVIER SCIENCE SA, 335, 400-408, Aug. 2018, Experiments of subcooled flow boiling in isolated bubble regime were conducted to understand the dynamics of individual bubbles. The test fluid was water, the flow direction was vertical upward, and the pressure was slightly higher than the atmospheric pressure. In the present experiments, the bubbles produced at nucleation sites were commonly lifted off the vertical heated surface. They were then propelled into the subcooled bulk liquid to disappear due to heat transfer with subcooled liquid. It was frequently observed that the bubbles were suddenly accelerated in the lateral direction before the disappearance. Using the present observation results, new correlations were developed for the bubble lift-off velocity and the bubble acceleration phenomenon after the lift-off. It was shown that the bubble trajectories and the time-variation of bubble size calculated using the present correlations are in fairly good agreement with the experimental results.
Scientific journal, English

Modeling of bubble behavior in low void fraction subcooled flow boiling
Shintaro Sakamoto; Koji Enoki; Hiroki Ohori; Tomio Okawa
International Conference on Nuclear Engineering, Proceedings, ICONE, 9, 2018, In predicting the void fraction in subcooled flow boiling, accurate evaluation of single bubble behaviors is of considerable importance. In particular, bubble lift-off velocity affects the void fraction significantly since the bubble disappear quickly due to heat transfer with subcooled liquid if the lift-off velocity is high. In this study, the process of bubble lift-off was experimentally investigated to develop mechanistic correlations for the bubble lift-off velocity. In the development of the correlations, it was assumed that the bubble lift-off velocity in the horizontal direction is proportional to the bubble growth rate and that in the vertical direction is determined primarily by the local liquid velocity evaluated at the bubble center position. Then, impact of the bubble lift-off velocity on the void fraction was explored through numerical simulations. In the simulations, the bubble lift-off velocity in the lateral direction was parametrically changed. It was shown that the mean void fraction decreases with an increase in the lateral bubble lift-off velocity since the bubble condensation is enhanced. It was therefore confirmed that accurate evaluation of the bubble lift-off velocity is important for high accuracy prediction of the void fraction.
International conference proceedings

MODELING OF BUBBLE BEHAVIOR IN LOW VOID FRACTION SUBCOOLED FLOW BOILING
Shintaro Sakamoto; Hiroki Ohori; Koji Enoki; Tomio Okawa
PROCEEDINGS OF THE 26TH INTERNATIONAL CONFERENCE ON NUCLEAR ENGINEERING, 2018, VOL 9, AMER SOC MECHANICAL ENGINEERS, 2018, In predicting the void fraction in subcooled flow boiling, accurate evaluation of single bubble behaviors is of considerable importance. In particular, bubble lift-off velocity affects the void fraction significantly since the bubble disappear quickly due to heat transfer with subcooled liquid if the lift-off velocity is high. In this study, the process of bubble lift-off was experimentally investigated to develop mechanistic correlations for the bubble lift-off velocity. In the development of the correlations, it was assumed that the bubble lift-off velocity in the horizontal direction is proportional to the bubble growth rate and that in the vertical direction is determined primarily by the local liquid velocity evaluated at the bubble center position. Then, impact of the bubble lift-off velocity on the void fraction was explored through numerical simulations. In the simulations, the bubble lift-off velocity in the lateral direction was parametrically changed. It was shown that the mean void fraction decreases with an increase in the lateral bubble lift-off velocity since the bubble condensation is enhanced. It was therefore confirmed that accurate evaluation of the bubble lift-off velocity is important for high accuracy prediction of the void fraction.
International conference proceedings, English

Direct Measurement of Heat Transfer Coefficient Distribution around the Rewetting Front during High Temperature Surface Liquid Cooling
Yamagata Keisuke; Enoki Koji; Okawa Tomio
The Proceedings of the Thermal Engineering Conference, The Japan Society of Mechanical Engineers, 2018, 0039, 2018, In order to elucidate the rewetting phenomenon in detail, it is important to know the distribution of the heat transfer coefficient in the vicinity of the rewetting front. However, the distribution of the heat transfer coefficient is not yet known in detail, and there are few data that directly measured its distribution. In this study, the temperature distribution of the wall surface temperature when the liquid film flows down was measured using a high-speed infrared camera, and the distribution of heat transfer coefficient was directly measured from the temperature data. We tried to experimentally determine the dominant factors influencing the heat transfer coefficient by changing experiment parameters.
Japanese

Effect of Various Parameters on Boiling Heat Transfer of Nanofluids
Watanabe Yosuke; Enoki Koji; Okawa Tomio
The Proceedings of the Thermal Engineering Conference, The Japan Society of Mechanical Engineers, 2018, 0037, 2018, In nucleate boiling of nanofluids, heat transfer coefficient and CHF change due to nanoparticle layer formation on the heated surface. It is considered that the nanoparticle layer causes the changes of wettability and roughness of the heated surface. In this study, influences of the total mass of nanoparticle layer and properties of the heated surface were experimentally investigated parametrically changing the nanoparticle concentration and the heat flux during nanoparticle layer formation. The heated surface material was copper and the material of nanoparticles was titanium-dioxide, alumina, and silica. The influencing parameter for the total mass of the nanoparticle layer was elucidated experimentally.
Japanese

Study on Thermal Performance of Nanofluid in Heat Pipe
Menglei Wang; Enoki Koji; Okawa Tomio
The Proceedings of the Thermal Engineering Conference, The Japan Society of Mechanical Engineers, 2018, 0029, 2018, In this research, silicon dioxide/DI water nanofluid was used as the working fluid in a cylindrical heat pipe to experimentally investigate its effect to enhance the heat transfer performance. In the experiment, the one end of the heat pipe was heated using a nichrome wire heater and the other end was cooled using a fan. In addition, effects of pre-coating of the screen mesh and the inside wall of the heat pipe with nanoparticles on heat transfer performance were explored. The experimental results showed that in comparison with the normal heat pipe BBW (bare tube / bare mesh / pure water), the thermal resistance of BNW (bare tube / nanoparticle-coated mesh / pure water) is reduced by 2-8% for the input power of 3-9W. When the power was increased to 9-25W, however, the thermal resistances of NXW (nanoparticle-coated tube / no mesh / pure water) and NBW (nanoparticle-coated tube / bare mesh / pure water) decreased 13-32%. This corresponded to the heat transfer enhancement of 15-40%.
Japanese

Elucidation of Flow Distribution of Two-phase Flow in the Parallel Mini-channel Evaporator
ONO Masahaharu; ENOKI Koji; KANJA Keisuke; NAKAMURA Taichi; OKAWA Tomio; NISHIDA Kosaku; KATO Masashi
Transactions of the Japan Society of Refrigerating and Air Conditioning Engineers, Japan Society of Refrigerating and Air Conditioning Engineers, 34, 4, 413, 31 Dec. 2017, In this study, the experiments were performed to observe vertically upward vapor-liquid two-phase flow of HFE-7000 in the parallel microchannel whose hydraulic diameter was 0.93 mm per a path by using a high-speed camera. The experiments were conducted by heating the microchannel portion for use as evaporator. The saturation temperature was 30 °C. In order to investigate the influence of the inlet qualities of the test section on the flow pattern, the experiments were conducted to keep exit quality at xout = 0.9 and mass flow rate W = 0.0022 kg･s^-1, set inlet qualities at xin = 0, 0.2 0.7. From the results of the observation, it was clear that the cause of drift flow was unevenness of the vapor and liquid distribution in the inlet header. Moreover, back flow which is thought to degrade the heat exchanger performance was observed as well, and it was found that the cause was rapid expansion of the vapor plug. Therefore, when the inlet header part was made a modified shape narrowing the flow path area as it goes away from the test section inlet part, drift flow and back flow were suppressed. Furthermore, in order to investigate the influence on the heat exchanging performance by drift flow and back flow, the downstream side of the test section was photographed and measured using an IR camera. In the case of a vapor-liquid mal-distribution in the inlet header section, temperature rise accompanying dryout had been constantly occurring on the downstream side in the flow path near the test section inlet. However, in the case of the modified inlet header shape, it was found that the mal-distribution of the heat distribution on the downstream side due to the drift was reduced.
Japanese

Measurement of the maximum bubble size distribution in water subcooled flow boiling at low pressure
Kazuhiro Kaiho; Tomio Okawa; Koji Enoki
INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER, PERGAMON-ELSEVIER SCIENCE LTD, 108, 2365-2380, May 2017, Peer-reviwed, Bubbles of various Sizes are produced at nucleation sites on a heated surface in subcooled flow boiling. However, although many correlations have been developed so far for the mean bubble size, systematic information is scarce for the bubble size distribution. In the present work, bubble nucleation process in water subcooled flow boiling at low pressure was observed in detail using an ITO-deposited transparent glass plate as the heated surface. It was found that bubbles of different sizes are produced at each nucleation site and moreover the mean bubble size differs considerably between different sites. It was hence considered that the reliability of subcooled flow boiling analysis codes can be improved if the effect of the bubble size distribution is included. In view of this, quantitative investigation was done for the bubble size distribution. It was shown that under the experimental conditions tested in this work, the distribution of the maximum size of individual bubbles produced at nucleation sites are fitted well with the gamma distribution. The dependences of the measured maximum bubble size distribution on important dimensionless numbers were explored to develop new correlations for the bubble size distribution. (C) 2017 Elsevier Ltd. All rights reserved.
Scientific journal, English

Experiments on the splashing limit during drop impact onto a thin liquid film
Sota Kitabayashi; Koji Enoki; Tomio Okawa
International Conference on Nuclear Engineering, Proceedings, ICONE, 9, 2017, The phenomenon of secondary droplet production during single drop impingement onto a liquid film is encountered in many industrial situations. Typical examples in the field of nuclear engineering are the spray cooling of hot surface and the atomization of radioactive liquids in severe accident. Therefore, the prediction of the onset of secondary droplet production is very important. It is known that the two types of droplet splashing mechanisms are present: the prompt splash and the late splash. The main purpose of this research is to determine the splashing limit separately for the prompt splash and the late splash. It is expected that the splashing limits are expressed using the three dimensionless numbers: the Weber number, the Ohnesorge number, and the dimensionless film thickness. Experiments were hence carried out using pure water and silicone oil as the working liquid. The experimental ranges were 129-606 for the Weber number, 0.00183-0.00300 for the Ohnesorge number, and 0.13-3.0 for the dimensionless film thickness. It was found that the occurrence of splashing can be predicted more accurately if the splashing limit is evaluated separately for the prompt splash and the late splash.
International conference proceedings

Splash during liquid jet impingement onto a horizontal plate
Yi Zhan; Naoki Oya; Koji Enoki; Tomio Okawa; Shuji Ohno; Mitsuhiro Aoyagi; Takashi Takata
International Conference on Nuclear Engineering, Proceedings, ICONE, 9, 2017, In the numerical analysis of sodium fire event following coolant leakage in a sodium-cooled fast reactor, sufficient understanding is needed for the liquid jet atomization to estimate the combustion rate accurately. In the present work, a liquid jet was emanated vertically downward from a circular nozzle onto a liquid film formed on a horizontal plate. The droplets produced at the impact point were investigated. The splash ratio (the ratio of the mass of the splashed droplets to the jet flow rate) was measured under varied experimental conditions of nozzle diameter, fall height and flow rate of liquid jet. The experimental result depended significantly on the morphology of liquid jet upon impact. The splashing rate was negligibly small when the liquid jet impinged as the continuous jet but a significant amount of liquid was splashed when the liquid jet impinged as the broken jet. Thus, we developed a method to estimate the impact frequency. It was shown that the splash ratio can be correlated well if the impact frequency is included in the correlation.
International conference proceedings

Characteristics of bubbles produced at nucleation sites in subcooled flow boiling
Tomio Okawa; Kazuhiro Kaiho; Shintaro Sakamoto; Koji Enoki
17th International Topical Meeting on Nuclear Reactor Thermal Hydraulics, NURETH 2017, 2017-September, 2017, In subcooled flow boiling region, wall heat flux is spent for vaporization and temperature rise of liquid phase. Thus, accurate evaluation of vaporization rate is of considerable importance in estimating the void fraction in this region that influences the natural circulation flow rate, two-phase flow stability, fuel burnup, and transient response in RIA (Reactivity-Initiated Accident) event. In the present work, bubble production process in water subcooled flow boiling at low pressure was visualized in detail to measure the nucleation site density, the bubble size, the bubble release frequency, and the vaporization rate. Appropriate correlations were selected for the nucleation site density and the bubble release frequency. New dimensionless correlations were developed for the bubble size distribution since no such correlation was available in literature. It was found that the bubbles of various sizes are produced in subcooled flow boiling and the effect of bubble size distribution should be taken into consideration for accurate prediction of the vaporization rate.
International conference proceedings

Peeling of nanoparticle layer on a heated surface and its influence on pool boiling CHF
Tomio Okawa; Yosuke Watanabe; Koji Enoki
17th International Topical Meeting on Nuclear Reactor Thermal Hydraulics, NURETH 2017, 2017-September, 2017, Nanofluid is paid particular attention as the promising method to enhance critical heat flux in boiling heat transfer. It is considered that the CHF enhancement is mainly caused by the modification of surface properties such as the wettability and roughness due to the formation of nanoparticle layer on the heated surface. However, since adhesion of the nanoparticle layer is not firm, peeling of the layer often occurs during boiling. In the present work, peeling test was conducted to measure quantitatively the adhesion strength of the nanoparticle layer formed on the heated surface. The material of the heated surface was copper and titanium-dioxide, alumina, and silica were selected as the materials of nanoparticles. It was found that the total mass and the adhesion strength of the nanoparticle layer were dependent strongly on the particle material. The CHF enhancement ratio by nanoparticle layer tended to decrease with an increase in the peeling ratio. It may be interesting to note that the boiling heat transfer was improved whilst CHF was deteriorated with the increase of the loss of nanoparticles.
International conference proceedings

The Characteristics of Vapor-liquid Two-phase Flow in Mini-Channels
NAKAMURA Taichi; ONO Masaharu; ENOKI Koji; OKAWA Tomio; KATO Masashi; NISHIDA Kousaku
The Proceedings of Conference of Kanto Branch, The Japan Society of Mechanical Engineers, 2017.23, 413, 2017
Japanese

Bubble lift-off conditions on numerical analysis of subcooled flow boiling
SAKAMOTO Shintaro; KAIHO Kazuhiro; ENOKI Koji; OKAWA Tomio
The Proceedings of Conference of Kanto Branch, The Japan Society of Mechanical Engineers, 2017.23, 501, 2017
Japanese

EXPERIMENTS ON THE SPLASHING LIMIT DURING DROP IMPACT ONTO A THIN LIQUID FILM
Sota Kitabayashi; Koji Enoki; Tomio Okawa
PROCEEDINGS OF THE 25TH INTERNATIONAL CONFERENCE ON NUCLEAR ENGINEERING, 2017, VOL 9, AMER SOC MECHANICAL ENGINEERS, 2017, The phenomenon of secondary droplet production during single drop impingement onto a liquid film is encountered in many industrial situations. Typical examples in the field of nuclear engineering are the spray cooling of hot surface and the atomization of radioactive liquids in severe accident. Therefore, the prediction of the onset of secondary droplet production is very important. It is known that the two types of droplet splashing mechanisms are present: the prompt splash and the late splash. The main purpose of this research is to determine the splashing limit separately for the prompt splash and the late splash. It is expected that the splashing limits are expressed using the three dimensionless numbers: the Weber number, the Ohnesorge number, and the dimensionless film thickness. Experiments were hence carried out using pure water and silicone oil as the working liquid. The experimental ranges were 129-606 for the Weber number, 0.00183-0.00300 for the Ohnesorge number, and 0.13-3.0 for the dimensionless film thickness. It was found that the occurrence of splashing can be predicted more accurately if the splashing limit is evaluated separately for the prompt splash and the late splash.
International conference proceedings, English

SPLASH DURING LIQUID JET IMPINGEMENT ONTO A HORIZONTAL PLATE
Yi Zhan; Naoki Oya; Koji Enoki; Tomio Okawa; Shuji Ohno; Mitsuhiro Aoyagi; Takashi Takata
PROCEEDINGS OF THE 25TH INTERNATIONAL CONFERENCE ON NUCLEAR ENGINEERING, 2017, VOL 9, AMER SOC MECHANICAL ENGINEERS, 2017, In the numerical analysis of sodium fire event following coolant leakage in a sodium-cooled fast reactor, sufficient understanding is needed for the liquid jet atomization to estimate the combustion rate accurately. In the present work, a liquid jet was emanated vertically downward from a circular nozzle onto a liquid film formed on a horizontal plate. The droplets produced at the impact point were investigated. The splash ratio (the ratio of the mass of the splashed droplets to the jet flow rate) was measured under varied experimental conditions of nozzle diameter, fall height and flow rate of liquid jet. The experimental result depended significantly on the morphology of liquid jet upon impact. The splashing rate was negligibly small when the liquid jet impinged as the continuous jet but a significant amount of liquid was splashed when the liquid jet impinged as the broken jet. Thus, we developed a method to estimate the impact frequency. It was shown that the splash ratio can be correlated well if the impact frequency is included in the correlation.
International conference proceedings, English

Relationship with heat flux and noise and pressure fluctuation in porous-micro-channel
HIRATA Ryo; OHASHI Junki; ENOKI Koji; OKAWA Tomio
The Proceedings of Conference of Kanto Branch, The Japan Society of Mechanical Engineers, 2017.23, 410, 2017
Japanese

The detachment of nanoparticle layer on boiling heat-transfer of nanofluids
WATANABE Yosuke; ENOKI Koji; OKAWA Tomio
The Proceedings of Conference of Kanto Branch, The Japan Society of Mechanical Engineers, 2017.23, 405, 2017
Japanese

A study on secondary droplet production during drop impingement onto a thin liquid film
KITABAYASHI Sota; ENOKI Koji; OKAWA Tomio
The Proceedings of Conference of Kanto Branch, The Japan Society of Mechanical Engineers, 2017.23, 302, 2017
Japanese

Systematic measurements of heat transfer characteristics in saturated pool boiling of water-based nanofluids
Muhamad Zuhairi Sulaiman; Daisuke Matsuo; Koji Enoki; Tomio Okawa
International Journal of Heat and Mass Transfer, Elsevier BV, 102, 264-276, Nov. 2016

Systematic measurements of heat transfer characteristics in saturated pool boiling of water-based nanofluids
Muhamad Zuhairi Sulaiman; Daisuke Matsuo; Koji Enoki; Tomio Okawa
International Journal of Heat and Mass Transfer, 102, 264-276, 01 Nov. 2016, Experiments were carried out to investigate the heat transfer characteristics in saturated pool boiling of water-based nanofluids. An upward-facing copper surface of 20 mm in diameter was used as the heated surface. Main experimental parameters in this work were nanoparticles' material (TiO2, Al2O3 and SiO2), mass concentration (0.04, 0.4 and 1 kg/m3) and dispersion condition (fine and coarse dispersions). Effects of these parameters on the time-variation of wall superheat under constant heat flux, the heat transfer coefficient (boiling curve) and the critical heat flux (CHF) were explored. It was found that the particle dispersion condition has no noticeable influence on the heat transfer characteristics within the range tested in this work. Whilst, the material and concentration of nanoparticles greatly affected the time-variation of wall superheat and the boiling curve. In particular, it was found that the wall superheat likely to increase significantly when the nanoparticle layer formed on the heated surface is partially detached. The CHF in nanofluid was 2.5-3 times higher than that for pure water in all the experimental conditions.
Scientific journal

Systematic measurements of heat transfer characteristics in saturated pool boiling of water-based nanofluids
Muhamad Zuhairi Sulaiman; Daisuke Matsuo; Koji Enoki; Tomio Okawa
INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER, PERGAMON-ELSEVIER SCIENCE LTD, 102, 264-276, Nov. 2016, Experiments were carried out to investigate the heat transfer characteristics in saturated pool boiling of water-based nanofluids. An upward-facing copper surface of 20 mm in diameter was used as the heated surface. Main experimental parameters in this work were nanoparticles' material (TiO2, Al2O3 and SiO2), mass concentration (0.04, 0.4 and 1 kg/m(3)) and dispersion condition (fine and coarse dispersions). Effects of these parameters on the time-variation of wall superheat under constant heat flux, the heat transfer coefficient (boiling curve) and the critical heat flux (CHF) were explored. It was found that the particle dispersion condition has no noticeable influence on the heat transfer characteristics within the range tested in this work. Whilst, the material and concentration of nanoparticles greatly affected the time variation of wall superheat and the boiling curve. In particular, it was found that the wall superheat likely to increase significantly when the nanoparticle layer formed on the heated surface is partially detached. The CHF in nanofluid was 2.5-3 times higher than that for pure water in all the experimental conditions. (C) 2016 Elsevier Ltd. All rights reserved.
Scientific journal, English

Accurate estimation of vaporization rate in subcooled flow boiling based on the results of visualization experiment
Kazuhiro Kaiho; Koji Enoki; Tomio Okawa
International Conference on Nuclear Engineering, Proceedings, ICONE, 5, 2016, This research presents experimental approaches to accumulate the data for mechanistic model in subcooled flow boiling. A number of photographic studies have been provided to investigate phenomena of bubble nucleation and condensation process for accurate prediction of void fraction such as bubble detachment diameter, bubble detachment frequency and nucleation site density in a subcooled flow boiling. In this work, a transparent heated surface was used to obtain the data from back side of heated surface to avoid overlapping bubbles by using high speed video camera. It enabled to observe bubble nucleation process and active nucleation sites. The experiment was performed in a vertical rectangular channel at atmospheric pressure and the water was used as test fluid. In generally, the computational analysis for a subcooled flow boing deal with mean bubble size as the size of bubbles produced on heated surface. Although, it was found that mean bubble size can represent bubbles produced at same site because they are almost uniform size. Even though the size of bubbles at same site are almost uniform, the difference of the size of bubbles between other sites are considerable value. Therefore, mean bubble size on the surface should not represent bubbles for all site otherwise the serious error may be caused. It seems that bubble detachment diameter should not be given by correlations of mean bubble detachment diameter for accurate prediction of vaporization rate. Some researchers proposed that bubble size distribution should be considered by Gaussian distribution [1-3]. However, it found that bubble size distribution data accumulated in this work cannot be fitted by Gaussian distribution and there are probability that larger bubbles are neglected due to the configuration features of Gaussian distribution. So, Gamma distribution was used to predict the bubble size distribution and it was evaluated in terms of heat flux, wall superheat, mass flux and liquid subcooling. And then, by the experimental approaches, the important dimensionless parameters are identified such as Nusselt number, Jakob number, Reynolds number and dimensionless subcooling. Furthermore, vaporization rate was calculated by correlations of bubble detachment diameter, bubble detachment diameter and nucleation site density and compared with the data. Finally, the effect of using mean bubble size or bubble size distribution on vaporization rate was investigated.
International conference proceedings

Experimental study on splashing during liquid jet impingement onto a liquid film
Zhan Yi; Naoki Oya; Koji Enoki; Tomio Okawa; Shuji Ohno; Mitsuhiro Aoyagi
International Conference on Nuclear Engineering, Proceedings, ICONE, 5, 2016, A liquid jet is of considerable importance in many industrial fields including jet cleaning, jet engine and combustion. As an important example, the Monju nuclear power plant in Japan experienced a sodium leak in 1995. This led to a fire accident because the sodium reacted with oxygen in the air. To predict the significance of the fire accident, accurate evaluation of the amount of splashed droplets caused by the sodium jet impingement is of great importance. In this work, the relationship between the condition of a liquid jet and the amount of splashed droplets is explored experimentally. In the experiments, a liquid jet was emanated vertically downward from a circular nozzle onto a liquid film formed on a horizontal plate. Visualization using a high speed camera was performed to observe the condition of the liquid jet. From the nozzle, the mode of the liquid jet changed jet, lump and drop. Here, the jet mode means the continuous jet with smooth surface, the lump mode the continuous jet with disturbed surface and the drop mode the broken jet. Dependences of the transition length to each mode on the important parameters such as the jet velocity and the nozzle diameter were investigated. Measurement was also conducted for the splash ratio that is defined as the ratio of the amount of splashed droplets to the jet flow rate. It was found that the splash ratio is high when the liquid jet is in the drop mode at the impact point. It was shown that the splash ratio can be correlated well as a function of the impact Weber number and the Strouhal number of the droplets impinging the liquid film.
International conference proceedings

Vapor - liquid Two - phase Flow Patterns and the Effects of Oscillation of a Refrigerant Flowing in Small Horizontal Rectangular Tube
OHNO Masaharu; ENOKI Koji; NAGAYAMA Kunihiro; AKISAWA Atsushi; Okawa Tomio; MIYATA Kazushi; MORI Hideo
The Proceedings of the National Symposium on Power and Energy Systems, The Japan Society of Mechanical Engineers, 2016.21, A212, 2016, In this study, we performed to observe horizontal vapor-liquid two phase flow of refrigerant R 134a in the rectangular glass channel which hydraulic diameter is 0.84 mm. The test glass tube was added the oscillation to the gravity direction for slug and stratified flow under low mass flux and heat flux. When the channel was oscillated of certain frequency and amplitude, the liquid of the corner part of rectangular channel waved up and down. In slug flow, dry surface of central was supplied the liquid by the oscillation and was formed the thin liquid. In stratified flow, dry surface of top was supplied the liquid and flow pattern changed to annular from stratified flow. The oscillation influences were investigated in a quantitative way.
Japanese

Effects of bubble lift-off conditions on numerical analysis of subcooled flow boiling
Sakamoto Shintaro; Kaiho Kazuhiro; Enoki Koji; Okawa Tomio
The Proceedings of the Thermal Engineering Conference, The Japan Society of Mechanical Engineers, 2016, C221, 2016, Numerical simulation of subcooled flow boiling in a vertical rectangular duct was carried out using a one-way bubble tracking method; this method treats each bubble as an individual particle. Although a bubble behavior after lifting off the heated surface depends on the bubble size and velocity at lift-off, these lift-off conditions are not taken into consideration in existing subcooled flow boiling models. In the present simulation, the distributions of lift-off velocity and bubble size were considered. It was demonstrated that the calculated void fraction in subcooled flow boiling is influenced significantly by the lift-off conditions.
Japanese

High-Heat-Flux Heat Removal Using A Porous-Micro-Channel
OHASHI Junki; TSUKAMOTO Akihiro; ENOKI Koji; OKAWA Tomio
The Proceedings of the National Symposium on Power and Energy Systems, The Japan Society of Mechanical Engineers, 2016.21, A223, 2016, Experiments were carried out to investigate the characteristics of high-heat-flux heat removal using a porous-micro-channel (a small rectangular channel filled with a porous-metal plate). The working fluid was water and the main experimental parameters were the inlet liquid subcooling, mass flux, and channel size. It was shown that nucleate boiling heat transfer is enhanced in the porous-micro-channel in comparison with the normal rectangular channel. Another important finding was that as in the normal channel, the micro-bubble emission boiling (MEB) occurs in the porous-micro-channel if the mass flux and liquid subcooling are high enough. In addition, the flow oscillation and noise production that often accompany with the MEB in the normal channel were mitigated significantly in the porous-micro-channel. This indicates that the porous-micro-channel enables the stable and high-heat-flux heat removal by the MEB.
Japanese

ACCURATE ESTIMATION OF VAPORIZATION RATE IN SUBCOOLED FLOW BOILING BASED ON THE RESULTS OF VISUALIZATION EXPERIMENT
Kazuhiro Kaiho; Koji Enoki; Tomio Okawa
PROCEEDINGS OF THE 24TH INTERNATIONAL CONFERENCE ON NUCLEAR ENGINEERING, 2016, VOL 5, AMER SOC MECHANICAL ENGINEERS, 2016, This research presents experimental approaches to accumulate the data for mechanistic model in subcooled flow boiling. A number of photographic studies have been provided to investigate phenomena of bubble nucleation and condensation process for accurate prediction of void fraction such as bubble detachment diameter, bubble detachment frequency and nucleation site density in a subcooled flow boiling. In this work, a transparent heated surface was used to obtain the data from back side of heated surface to avoid overlapping bubbles by using high speed video camera. It enabled to observe bubble nucleation process and active nucleation sites. The experiment was performed in a vertical rectangular channel at atmospheric pressure and the water was used as test fluid. In generally, the computational analysis for a subcooled flow boing deal with mean bubble size as the size of bubbles produced on heated surface. Although, it was found that mean bubble size can represent bubbles produced at same site because they are almost uniform size. Even though the size of bubbles at same site are almost uniform, the difference of the size of bubbles between other sites are considerable value. Therefore, mean bubble size on the surface should not represent bubbles for all site otherwise the serious error may be caused. It seems that bubble detachment diameter should not be given by correlations of mean bubble detachment diameter for accurate prediction of vaporization rate. Some researchers proposed that bubble size distribution should be considered by Gaussian distribution [1-3]. However, it found that bubble size distribution data accumulated in this work cannot be fitted by Gaussian distribution and there are probability that larger bubbles are neglected due to the configuration features of Gaussian distribution. So, Gamma distribution was used to predict the bubble size distribution and it was evaluated in terms of heat flux, wall superheat, mass flux and liquid subcooling. And then, by the experimental approaches, the important dimensionless parameters are identified such as Nusselt number, Jakob number, Reynolds number and dimensionless subcooling. Furthermore, vaporization rate was calculated by correlations of bubble detachment diameter, bubble detachment diameter and nucleation site density and compared with the data. Finally, the effect of using mean bubble size or bubble size distribution on vaporization rate was investigated.
International conference proceedings, English

Measurements of pressure fluctuation and sound production in flow boiling in a porous-micro-channel
Ohashi Junki; Hirata Ryo; Enoki Koji; Okawa Tomio
The Proceedings of the Thermal Engineering Conference, The Japan Society of Mechanical Engineers, 2016, I112, 2016, Experiments were carried out to investigate the characteristics of high-heat-flux heat removal using a porousmicro-channel under the condition of high liquid subcooling and high mass flux. The working fluid was water and the main experimental parameters were insertion of porous plate and channel size. It was shown that fluctuation of heating surface temperature and flow oscillation are mitigated if the porous plate is placed in the flow channel. Another finding was that the sound production in the flow channel under the high heat flux condition is also mitigated in the porous-micro-channel. These results indicate that the porous-micro-channel is advantageous for the high-heat-flux heat removal from high-power-density devices.
Japanese

An experimental study on the secondary droplets production during the drop impingement onto a thin liquid film
Kitabayashi Sota; Enoki Koji; Okawa Tomio
The Proceedings of the Thermal Engineering Conference, The Japan Society of Mechanical Engineers, 2016, C135, 2016, The secondary droplets production during the drop impingement onto a liquid film has been studied extensively by many researchers. It is known that the two types of splashing mechanisms are present: the prompt splash and the late splash. In the prompt splash, small droplets are scattered with high velocity immediately after the impact. Whilst, in the late splash, larger droplets of lower velocity are produced after the development of liquid crown. In the present experiments, the empirical correlations of splashing limit for pure water are proposed separately for the prompt and late splashes. Since the splashing limit was dependent on the impact velocity and liquid film thickness, the two dimensionless numbers (the Weber number and the dimensionless film thickness) were used in the correlations.
Japanese

An experimental study on the secondary droplets production during the drop impingement onto a liquid film
ENOKI Koji; Matsuse Kohei; Okawa Tomio
The Proceedings of the National Symposium on Power and Energy Systems, The Japan Society of Mechanical Engineers, 2016.21, A213, 2016, Experiments were performed to explore the conditions for the onset of secondary droplet formation during single drop impact onto a liquid film(splashing). It was found that there were two types of splashing: the prompt splash and the late splash. In the prompt splash, small droplets were scattered with high velocity immediately after the impact. Whilst, in the late splash, larger droplets of lower velocity were produced after the development of liquid crown. The condition for the onset of splashing was expressed using the two dimensionless numbers: the Weber number, the Ohnesorge number. The correlations were given separately for the prompt and late splashes. Moreover new correlations were developed about prediction of the secondary droplet diameter and velocity.
Japanese

NUMERICAL SIMULATION OF SUBCOOLED FLOW BOILING USING A BUBBLE TRACKING METHOD
Tomio Okawa; Naoki Miyano; Kazuhiro Kaiho; Koji Enoki
PROCEEDINGS OF THE 24TH INTERNATIONAL CONFERENCE ON NUCLEAR ENGINEERING, 2016, VOL 3, AMER SOC MECHANICAL ENGINEERS, 2016, Peer-reviwed, The process of bubble nucleation in subcooled flow boiling was visualized using a high speed camera to show that the bubble size can be significantly different between the nucleation sites. However, the bubble size is usually assumed constant in the numerical simulation of subcooled flow boiling. To explore the effect of the bubble size distribution on the void fraction in subcooled flow boiling, numerical simulations were performed using a bubble tracking method in which the size and position of each bubble are calculated individually using a Lagrangian coordinates. In the present simulation, the void fraction was greater when the bubble size distribution was taken into consideration. Since the bubble tracking method requires many correlations, further improvement is necessary. The present numerical results however indicate that the bubble size distribution should be taken in to consideration to evaluate the void fraction in subcooled flow boiling accurately.
International conference proceedings, English

Influence of surface wettability on bubble behavior and void evolution in subcooled flow boiling
Rouhollah Ahmadi; Tomio Okawa
INTERNATIONAL JOURNAL OF THERMAL SCIENCES, ELSEVIER FRANCE-EDITIONS SCIENTIFIQUES MEDICALES ELSEVIER, 97, 114-125, Nov. 2015, Peer-reviwed, Influence of heated surface wettability on bubble behavior and the mechanisms to cause net vapor generation (NVG) were explored experimentally in a vertical upward subcooled flow boiling. The cross-section of the heated channel was rectangular in shape and a part of one side of the flow channel was heated electrically using two cartridge heaters embedded in a copper block. Experiments were performed under various conditions of surface wettability and system pressure. On a hydrophilic heated surface, at boiling incipience all the bubbles departed from nucleation sites immediately after nucleation; and then, the bubbles were lifted off the heated surface at atmospheric pressure, whilst slid on the surface at elevated pressures. On the contrary, when the surface was hydrophobic, bubbles mostly stuck on the nucleation sites under the conditions close to boiling incipience at all the pressures tested in this work (100-400 kPa). It was hence confirmed that wettability of the heated surface has significant impact on the bubble behavior under low void fraction conditions. Using rather hydrophilic heated surface, two mechanisms causing a rapid increase in the net vapor generation rate were observed in two ranges of pressure. At atmospheric pressure, reattachment of the lift-off bubbles, and at moderate pressure, wake-effect of preceding sliding bubbles are triggering mechanisms of the significant increase of net vapor generation rate at the point of onset of significant void (OSV). An important result revealed in this work was that on a hydrophobic heated surface with high contact angle around 90, bubble departure from all the nucleation sites which is a necessary condition to cause NVG, occurs in proximity to OSV. The bubble behavior after the departure was similar to those observed for the hydrophilic surface. The direct cause of OSV was hence not significantly different between the hydrophilic and hydrophobic heated surfaces, but the bubble departure from nucleation sites was a good indication of OSV when the heated surface was hydrophobic. (C) 2015 Elsevier Masson SAS. All rights reserved.
Scientific journal, English

B213 Visualization Analysis of the Boiling Characteristics during the Water Subcooled Flow Boiling in Low Pressure
Kaiho Kazuhiro; Tuchie Kenta; Miyano Naoki; Enoki Koji; Okawa Tomio
The Proceedings of the Thermal Engineering Conference, The Japan Society of Mechanical Engineers, 2015, _B213-1_-_B213-2_, 2015, In the subcooled flow boiling, bubble size is important in calculating the vaporization rate accurately. On the surface, a number of bubbles that are different size are generated. If the bubble size is assumed to be uniform, a serious error may be caused in the numerical simulation. In this study, considering bubble size distribution, the bubble size are predicted by the gamma distribution. It was demonstrated that the gamma distribution satisfactorily describes the bubble distribution data. Furthermore, correlations were developed for the dimensionless bubble size and shape parameter to enable accurate prediction of them in the subcooled flow boiling.
Japanese

Visualization study on the mechanisms of net vapor generation in water subcooled flow boiling under moderate pressure conditions
Rouhollah Ahmadi; Tatsuya Ueno; Tomio Okawa
INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER, PERGAMON-ELSEVIER SCIENCE LTD, 70, 137-151, Mar. 2014, Peer-reviwed, Visualization study using a high speed camera was carried out to explore the mechanisms of net vapor generation (NVG) in subcooled flow boiling. The flow direction was vertical upward, the working fluid was water, and the experiments were conducted under moderate pressure conditions; a rather hydrophilic surface was used as a heated surface. In all the experimental conditions tested in this work, the bubbles nucleated on the heated surface typically departed from the nucleation sites and then slid upward on the vertical heated surface. When the NVG condition was exceeded, the number of bubbles increased noticeably and the range of bubble size became wider with an increase in the thermal-equilibrium quality. It was however found that the rapid increase of the void fraction after the NVG condition was reached could mainly be attributed to the formation of a small number of large bubbles. It was also observed that the large bubbles were mostly produced at the nucleation sites immediately after other sliding bubbles passed over the sites. It was therefore supposed that the production of large bubbles in the wake region that was formed behind the preceding sliding bubbles was a key phenomenon in causing the NVG in the present experiments. (C) 2013 Elsevier Ltd. All rights reserved.
Scientific journal, English

Experimental identification of the phenomenon triggering the net vapor generation in upward subcooled flow boiling of water at low pressure
Rouhollah Ahmadi; Tatsuya Ueno; Tomio Okawa
INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER, PERGAMON-ELSEVIER SCIENCE LTD, 55, 21-22, 6067-6076, Oct. 2012, Peer-reviwed, Observation of the bubble behavior was made using a high-speed camera to investigate the mechanisms to cause the net vapor generation in subcooled flow boiling. In the experiments, water was used as the test fluid, the flow direction was vertical upward, and the pressure was kept close to the atmospheric pressure. At high liquid subcooling close to the condition of the onset of nucleate boiling, all the bubbles were lifted off the heated surface immediately after the nucleation to disappear quickly in the subcooled bulk liquid due to condensation. It was found that the void fraction did not increase significantly unless the liquid subcooling became low enough for some bubbles to be reattached to the heated surface after the lift-off. When the reattachment took place, the bubble lifetime was substantially elongated since the bubbles slid up the vertical heated surface for a long distance after the reattachment. The reattachment therefore contributed to an increase in the void fraction. It was concluded that in the experimental conditions tested in this work, the bubble reattachment to the heated surface was a key phenomenon to cause the sharp increase of the void fraction at the point of net vapor generation. (C) 2012 Elsevier Ltd. All rights reserved.
Scientific journal, English

Boiling time effect on CHF enhancement in pool boiling of nanofluids
Tomio Okawa; Masahiro Takamura; Takahito Kamiya
INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER, PERGAMON-ELSEVIER SCIENCE LTD, 55, 9-10, 2719-2725, Apr. 2012, Peer-reviwed, Using TiO2-water nanofluids as the test liquid, pool boiling experiments were carried out to investigate the dependence of the nucleate boiling heat transfer, surface wettability and critical heat flux (CHF) on the boiling time in nanofluids. In the experiments performed at sufficiently high nanoparticle concentrations, the boiling heat transfer first degraded, then improved, and finally reached an equilibrium state. It was hence supposed that the present nanofluids had competing effects to deteriorate and enhance the nucleate boiling heat transfer. As for the surface wettability and CHF, the static contact angle asymptotically decreased whilst the CHF asymptotically increased with an increase in the boiling time. The maximum CHF enhancement measured in the present experiments was 91%, and strong correlation was found between the contact angle and the CHF. Although the boiling time needed to achieve the maximum CHF enhancement was less than a minute at high particle concentrations, a longer time of the order of 1 h was necessary at the lowest particle concentration tested in this work. This experimental result indicated that sufficient attention should be paid to the boiling time effect particularly in industrial applications of nanofluids to emergency cooling. (C) 2012 Elsevier Ltd. All rights reserved.
Scientific journal, English

Boiling heat transfer and critical heat flux enhancement of upward- and downward-facing heater in nanofluids
Muhamad Zuhairi Sulaiman; Masahiro Takamura; Kazuki Nakahashi; Tomio Okawa
International Conference on Nuclear Engineering, Proceedings, ICONE, American Society of Mechanical Engineers (ASME), 5, 1, 451-458, 2012, Peer-reviwed, Boiling heat transfer (BHT) and critical heat flux (CHF) performance were experimentally studied for saturated pool boiling of water-based nanofluids. In present experimental works, copper heaters of 20 mm diameter with titanium-oxide (TiO2) nanocoated surface were produced in pool boiling of nanofluid. Experiments were performed in both upward and downward facing nanofluid coated heater surface. TiO2nanoparticle was used with concentration ranging from 0.004 until 0.4 kg/m3 and boiling time of tb = 1, 3, 10, 20, 40 and 60 minutes. Distilled water was used to observed BHT and CHF performance of different nanofluids boiling time and concentration configurations. Nucleate boiling heat transfer observed to deteriorate in upward facing heater, however, in contrast effect of enhancement for downward. Maximum CHF for upward- and downward-facing heater are 2.1 and 1.9, respectively. Reduction of mean contact angle demonstrate enhancement on the critical heat flux for both upward-facing and downward-facing heater configuration. However, nucleate boiling heat transfer shows inconsistency in similar concentration with sequence of boiling time. For both downward- and upward-facing nanocoated heater's BHT and CHF, the optimum configuration denotes by C = 400 kg/m3 with tb = 1 minute which shows the best increment of boiling curve trend with lowest wall superheat ΔT = 25 K and critical heat flux enhancement of 2.02 times. Copyright © 2012 by ASME.
International conference proceedings, English

Influence of a flow obstacle on boiling two-phase flow
Takeyuki Ami; Hisashi Umekawa; Mamoru Ozawa; Tomio Okawa
Nihon Kikai Gakkai Ronbunshu, B Hen/Transactions of the Japan Society of Mechanical Engineers, Part B, 78, 788, 894-904, 2012, Peer-reviwed, In the view of the safety operation and the designing of boiling two-phase flow equipments, boiling two-phase flow characteristics, especially the critical heat flux is very important. In addition, the flow obstacle in a boiling channel, such as the spacer of boiling water reactor, influences the boiling heat transfer and flow characteristics. In this study, the experimental investigation was conducted by using a SUS304 tube (I.D. = 8 mm, L T =810, 840 and 900 mm) in which a rod-type flow obstacle (O.D. = 3.6 mm L = 20 mm) was equipped. Two kinds of detecting points of CHF, i.e. at the upstream of the flow obstacle and at the exit of the test section were observed. The difference of the detecting location of the CHF has been explained well by using the film flow model. On the basis of these results the influence length of the turbulent effect by the flow obstacle and the magnitude of the entrainment caused by the boiling were estimated. © 2012 The Japan Society of Mechanical Engineers.
Scientific journal, Japanese

Onset of nucleate boiling in mini and microchannels: A brief review
Tomio Okawa
Frontiers in Heat and Mass Transfer, 3, 1, 2012, Peer-reviwed, The present article summarizes the studies on the thermalhydraulic condition under which the onset of nucleate boiling (ONB) is triggered in subcooled flow boiling. Available correlations and experimental data show that the ONB is tended to be delayed in mini and microchannels. It is known that the ONB condition is significantly dependent on the surface condition even in conventional-sized channels. Accumulation of ONB data accompanied by the information on the surface condition is therefore considered of importance to elucidate the mechanisms of boiling incipience in microchannels. Discussion is also made for the bubble dynamics observed in mini and microchannels. It is indicated that the bubble behavior at ONB in mini and microchannels may significantly be different from that in conventional-sized channels and have greater impact on the system performance. Further studies on the bubble dynamics following nucleation at ONB are also requested to improve the design of heat transfer devices using mini and microchannels.
Scientific journal, English

Boiling heat transfer during single nanofluid drop impacts onto a hot wall
Tomio Okawa; Kenta Nagano; Takahiro Hirano
Experimental Thermal and Fluid Science, 36, 78-85, Jan. 2012, Peer-reviwed, Experiments were carried out to explore boiling heat transfer during successive impacts of single nanofluid drops onto a hot stainless steel plate. Nucleate boiling heat transfer and critical heat flux were improved significantly when nanometer-sized titanium-dioxide particles were dispersed in water drops. In contrast, colloidal dispersion of the nanoparticles degraded the heat transfer when the plate temperature was too high. A thin nanoparticle layer was formed on the plate during the nucleate boiling of nanofluid drops to improve the surface wettability. Observation of the impact process revealed that droplet spreading area at low plate temperatures was wider for the nanofluid drops. An increase in the liquid-solid contact area was expected to be a primary cause of the nucleate boiling heat transfer improvement. At high plate temperatures, phase change caused immediately after the drop impact appeared more significant for the nanofluid drops. It was considered that the significant vaporization in the initial stage inhibited the liquid-solid contact in the later stage to degrade the overall heat transfer. © 2011 Elsevier Inc.
Scientific journal, English

Bubble dynamics at boiling incipience in subcooled upward flow boiling
Rouhollah Ahmadi; Tatsuya Ueno; Tomio Okawa
INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER, PERGAMON-ELSEVIER SCIENCE LTD, 55, 1-3, 488-497, Jan. 2012, Peer-reviwed, Bubble dynamics in water subcooled flow boiling was investigated through visualization using a highspeed camera. The test section was a vertical rectangular channel, and a copper surface of low contact angle was used as a heated surface. Main experimental parameters were the pressure, mass flux and liquid subcooling. Although all the experiments were conducted under low void fraction conditions close to the onset of nucleate boiling, no bubbles stayed at the nucleation sites at which they were formed. Depending on the experimental conditions, the following two types of bubble behavior were observed after nucleation: (1) lift-off from the heated surface followed by collapsing rapidly in subcooled bulk liquid due to condensation, and (2) sliding along the vertical heated surface for a long distance. Since the bubble lift-off was observed only when the wall superheat was high, the boundary between the lift-off and the sliding could be determined in terms of the Jakob number. Based on the present experimental results, discussion was made for the possible mechanisms governing the bubble dynamics. (C) 2011 Elsevier Ltd. All rights reserved.
Scientific journal, English

Rate of droplet deposition in steam-water annular flow and effect of a flow obstacle
Tomio Okawa; Toshihiro Murakami; Rei Takei
NUCLEAR ENGINEERING AND DESIGN, ELSEVIER SCIENCE SA, 241, 11, 4497-4503, Nov. 2011, Peer-reviwed, Deposition rate of droplets in steam-water annular two-phase flow was measured using a 5 mm diameter vertical round tube as a test section. In the experimental conditions tested in this work, the droplet mass transfer coefficient decreased with an increase in the droplet concentration in the gas core flow and with an increase in the length of a deposition section. The dependence on these two parameters agreed fairly well with predictions by available correlations. Placing a small cylindrical tube concentrically in the test section round tube, the effect of a flow obstacle on the deposition rate of droplets was also experimentally investigated. It was found that the obstacle effect was significant and the deposition rate of droplets increased approximately three times in average. The obstacle effect measured in this work was compared with an empirical correlation and a simple mechanistic model that were developed using experimental results of air-water annular flows. Fairly good agreement was achieved in both cases, which would indicate that the mechanism of deposition enhancement induced by the flow obstacle is similar between air-water and steam-water flows. (C) 2011 Elsevier B.V. All rights reserved.
Scientific journal, English

Dependence of bubble behavior in subcooled boiling on surface wettability
Takahiro Harada; Hiroshi Nagakura; Tomio Okawa
NUCLEAR ENGINEERING AND DESIGN, ELSEVIER SCIENCE SA, 240, 12, 3949-3955, Dec. 2010, Peer-reviwed, This paper presents the results of visualization experiments that were carried out to investigate the dynamics of vapor bubbles generated in water pool boiling. In the experiments, vapor bubbles were generated on a vertical circular surface of a copper block containing nine cartridge heaters, and the contact angle of the heated surface was used as a main experimental parameter. The experiments were performed under subcooled as well as nearly saturated conditions. To enable clear observation of individual bubbles with a high speed camera, the heat flux was kept low enough to eliminate significant overlapping of bubbles. When the contact angle was small, the bubbles were lifted-off the vertical heated surface within a short period of time after the nucleation. On the other hand, when the contact angle was large, they slid up the vertical surface for a long distance. When bubbles were lifted-off the heated surface in subcooled liquid, bubble life-time was significantly shortened since bubbles collapsed rapidly due to condensation. It was shown that this distinct difference in bubble dynamics could be attributed to the effects of surface tension force. (c) 2010 Elsevier B.V. All rights reserved.
Scientific journal, English

Numerical verification of a simplified model for vapor bubble lift-off from a hydrophilic heated flat-wall
Tomio Okawa; Kosuke Hayashi; Akio Tomiyama
NUCLEAR ENGINEERING AND DESIGN, ELSEVIER SCIENCE SA, 240, 12, 3942-3948, Dec. 2010, Peer-reviwed, Three-dimensional interface tracking simulations were carried out to investigate the role of surface tension force in the process of vapor bubble lift-off from a hydrophilic heated surface in nucleate boiling. Since bubbles are frequently flattened along the heated surface in photographic experiments reported in literature, a bubble was assumed to be spheroidal in shape in the initial condition. The effect of phase change at the bubble interface was not taken into consideration for the sake of simplicity. In the present numerical simulations, the initially spheroidal bubble approached the spherical shape due to the surface tension force and was eventually lifted off the surface. The change in bubble shape induced local liquid flow directing toward the bubble base, that was the direct cause of the occurrence of the bubble lift-off. The dependence of the bubble migration velocity on several important parameters including the bubble size, surface tension coefficient and the density of surrounding liquid was also investigated. The change in bubble shape from flattened to more rounded causes the reduction of the surface energy, while the formation of local liquid flow leads to an increase in the kinetic energy. It was demonstrated that the bubble migration velocity after the lift-off can successfully be interpreted from the standpoint of energy conservation during the lift-off process. (c) 2010 Elsevier B.V. All rights reserved.
Scientific journal, English

Photographic Study on Bubble Motion in Subcooled Pool Boiling
Tomio Okawa; Takahiro Harada; Yuta Kotsusa
JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER-TRANSACTIONS OF THE ASME, ASME-AMER SOC MECHANICAL ENG, 132, 10, Oct. 2010, Peer-reviwed, Using a static contact angle of a vertical heated wall as a main experimental parameter, a photographic study was carried out to elucidate the mechanisms to determine the vapor bubble dynamics during subcooled pool boiling. The test fluid was distilled water and the experiments were performed under the atmospheric pressure; liquid subcooling was set to around 5 K. To enable clear observation of bubble behavior with a high speed camera, the experiments were conducted in an isolated bubble regime near the onset of nucleate boiling. Distinctly different bubble behaviors were observed on hydrophobic and hydrophilic surfaces: the bubbles were adhered to the surface for a long period of time when the contact angle was large while lifted-off the surface within a short period of time after the nucleation when the contact angle was small. Since buoyancy does not remove the bubble from the vertical surface, the mechanisms of bubble lift-off were investigated. It was indicated that the change in bubble shape induced by the surface tension force, unsteady growth force, and local liquid flow induced by heterogeneous condensation around the bubble are considered to promote the bubble lift-off while the surface tension force acting on the three-phase common line prevented the lift-off. Effects of the surface wettability on the lift-off bubble diameter, the elapsed time from the nucleation at the lift-off, and the condensation rate after the lift-off were also investigated. [DOI: 10.1115/1.4001064]
Scientific journal, English

Numerical Study of Particle Concentration Effect on Deposition Characteristics in Turbulent Pipe Flows
Yasushi Yamamoto; Tomio Okawa
JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY, TAYLOR & FRANCIS LTD, 47, 10, 945-952, Oct. 2010, Peer-reviwed, The present study focuses on the particle concentration effect on the particle deposition onto a channel wall in numerically simulated turbulent pipe flows. Large eddy simulation of the incompressible Navier-Stokes equations was performed to calculate the time-dependent turbulent flow field of continuous gas phase. Considering the technical application to the droplet motion in boiling water reactor subchannels, the flow direction was set to be vertically upward. The particles were placed at random initial locations in the pipe. The subsequent particle motion was tracked individually using a simple Lagrangian equation. To investigate the particle concentration effect on the rate of particle deposition, "two-way method" was applied, in which the two-way interactions between the continuous phase and the particles were taken into consideration. The calculated results showed that the mass transfer coefficient of particle deposition decreased noticeably with an increase in the particle concentration. This tendency was consistent with the experimental observations and empirical correlations. The present numerical results indicated that the turbulence modulation in the continuous gas phase is one of the primary causes of the reduction of the deposition mass transfer coefficient observed under high-droplet-concentration conditions.
Scientific journal, English

Liquid film behavior in annular two-phase flow under flow oscillation conditions
Tomio Okawa; Taisuke Goto; Yosuke Yamagoe
INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER, PERGAMON-ELSEVIER SCIENCE LTD, 53, 5-6, 962-971, Feb. 2010, Peer-reviwed, Experiments were carried out to investigate the effects of sinusoidal forced oscillation of the inlet flow rate on the time variations of local liquid film thickness and the frequencies of large wave's passing in steam-water annular two-phase flows. The liquid film thickness oscillated with the same period as the inlet flow rate. The mean film thickness in the thin film regions decreased and approached to an asymptotic value with an increase in the oscillation period of the inlet flow rate. This result was consistent with the experimental results of the occurrence of liquid film dryout under flow oscillation conditions reported in the literature. It was hence considered that the axial liquid transport from the thick to thin film regions mitigates the reduction of the critical heat flux caused by the flow oscillation. It was also found that the wave frequency in the thin film region increased with a decrease in the oscillation period. This observation suggested that the disturbance waves contribute to the enhancements of the liquid transport and consequently the critical heat flux associated with the liquid film dryout under flow oscillation conditions. (C) 2009 Elsevier Ltd. All rights reserved.
Scientific journal, English

Experimental studies on pool boiling characteristics of titanium dioxide-water nano-fluids
Tomio Okawa; Takahito Kamiya
International Conference on Nuclear Engineering, Proceedings, ICONE, 4, 1085-1089, 2010, Peer-reviwed, It is known that dispersion of a small amount of nanometer-sized particles in liquid can cause substantial improvement of the critical heat flux in pool boiling. Nanofluids (colloidal suspensions of nanoparticles in a base fluid) may therefore be used as the coolant in industrial applications in which high-heat-flux removal is needed. If it is supposed that the deposition of nanoparticles onto the heated surface during nucleate boiling is the main cause of the CHF enhancement in nanofluids, a certain time period is considered to be necessary for the CHF to be improved. In view of this, preliminary experiments were performed in the present work to investigate the time scale of CHF improvement
here, distilled water was used as a base fluid, and TiO2 and copper were selected as the materials of nanoparticles and heated surface, respectively. Under a particular experimental conditions of nanoparticle concentration and nucleate boiling heat flux (40 mg/l and 500 kW/m2), an approximate time scale of CHF improvement was 10 min
this value might not be negligibly short in some nanofluid applications. The measured time-variations of the wall superheat during the nucleate boiling in nanofluid suggested that longer time periods are required for the CHF enhancement at lower heat fluxes and lower nanoparticle concentrations. In particular, 40 min was not sufficient for the wall superheat to reach a steady-state value at the lowest nanoparticle concentration of tested in this work (9 mg/l). © 2010 by ASME.
International conference proceedings, English

Liquid film dryout in a boiling channel under flow oscillation conditions
Tomio Okawa; Taisuke Goto; Jun Minamitani; Yosuke Yamagoe
INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER, PERGAMON-ELSEVIER SCIENCE LTD, 52, 15-16, 3665-3675, Jul. 2009, Peer-reviwed, A simple theory was developed to elucidate the influence of sinusoidal oscillation of the inlet now rate on the occurrence of liquid film dryout in an annular two-phase flow regime in a boiling channel. The theory assumes that the critical heat flux (CHF) under an oscillatory condition can be calculated from values in steady states provided that the effect of axial mixing of the liquid film is appropriately considered. The trends of CHFs calculated using a one-dimensional three-fluid model and those experimentally measured under atmospheric pressure were in reasonable agreement with the proposed theory. However, the CHF values measured under oscillatory conditions were usually higher in the experiment than in the numerical simulation, which indicated that axial liquid transport induced by disturbance waves might enhance axial mixing of the liquid Him. (C) 2009 Elsevier Ltd. All rights reserved.
Scientific journal, English

Effects of Particle Relaxation Time and Continuous-Phase Reynolds Number on Particle Deposition in Vertical Turbulent Pipe Flows
Yasushi Yamamoto; Tomio Okawa
JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY, TAYLOR & FRANCIS LTD, 46, 4, 382-391, Apr. 2009, Peer-reviwed, The present study focuses on the mechanism of particle deposition onto a channel wall in numerically simulated turbulent pipe flows. Large eddy simulation of the incompressible Navier-Stokes equations was performed to calculate the time-dependent turbulent flow field of continuous gas phase. Considering the engineering application to the droplet motion in boiling water reactor subchannels, the flow direction was set to be vertically upward. The particles were placed at random initial locations in the pipe, assuming that the initial velocities in the lateral direction are equal to zero. The subsequent motion of particles was tracked individually using a simple Lagrangian equation. It was shown that small particles tended to accumulate in the viscous sublayer while large particles moved parallel to the pipe wall. The deposition coefficient was hence highest when the particles were intermediate in size or in relaxation time. The calculated results indicated that the effect of the continuous-phase Reynolds number on the deposition mass transfer coefficient was not negligible since it affected the drag and the lift coefficients and, consequently, the particle motion in the lateral direction.
Scientific journal, English

Analytical Study on Boiling Transition under Flow-Power Oscillating Condition for Hyper ABWR(Light Water Reactor・Advanced Reactor,Power and Energy System Symposium)
NOZAKI Kenichiro; OKAWA Tomio; HOTTA Akitoshi
Transactions of the Japan Society of Mechanical Engineers Series B, The Japan Society of Mechanical Engineers, 75, 751, 400-402, 2009, In the envisioned high-power density core plant, the degradation of stability is concerned that is attributed to the shortening of rod heat conduction time constant upon the dense fuel grid and the increment of pressure drop caused by the wall friction. To confirm the influence of such degradation on the fuel soundness, the bundle geometry critical power test was held under flow-power oscillation condition which envisages the unstable phenomenon occurring under in the flow transient event. This paper describes the analysis results of the cited test with sub-channel code NASCA, and the discussion of the mechanism of boiling transition under flow-power oscillating condition.
Japanese

URL
DOI URL

Variation of critical heat flux by flow oscillation in a small vertical channel
Toshihiro Murakami; Rei Takei; Tomio Okawa
International Conference on Nuclear Engineering, Proceedings, ICONE, 4, 303-309, 2009, Peer-reviwed, The effect of sinusoidal oscillation of inlet mass flux on the critical heat flux (CHF) in forced convective boiling was investigated in experiment and numerical calculation. In the experiment, the test section was a small stainless steel round tube of 5 mm in inside diameter, filtrated and deionized tap water was used as a test fluid, and the flow direction was set to vertical upward. The heated length was 1,600 mm. Electric power supplied to a circulation pump was varied periodically to oscillate the inlet mass flux sinusoidally. Direct current was passed through the test section tube to heat it ohmically. The occurrence of critical heat flux condition was detected using the signal from the thermocouples that were spot-welded on the outer wall of the test section tube. In the present experimental conditions, it was expected that the critical heat flux condition was triggered by the dryout of liquid film in annular two-phase flow regime. The main experimental parameters were the time-averaged inlet mass flux and the amplitude and period of flow oscillation. The system pressure was also used as an important experimental parameter since a boiling water reactor is operated under high pressure condition. If the oscillation period is long enough, it is expected that the critical heat flux under the flow oscillation condition is close to that for the steady state when the flow rate is equal to the minimum flow rate in the oscillatory condition. On the other hand, the decrease of the critical heat flux would be mitigated if the oscillation period is shortened, since interaction would take place between the thin and thick film regions within a boiling channel. In accordance with this expectation, the critical heat flux measured under the flow oscillation condition was reduced with an increase in the oscillation period. It was demonstrated that the reduction of critical heat flux under flow oscillation condition can be correlated fairly well using the concept of dimensionless heated length. Numerical calculations using a one-dimensional three-fluid model were also carried. The calculated critical heat fluxes for flow oscillation conditions increased with increased value of dimensionless heated length, as in the present experiment. Copyright © 2009 by ASME.
International conference proceedings, English

EXPERIMENTAL STUDY ON LIQUID FILM DRYOUT UNDER OSCILLATORY FLOW CONDITIONS
Yosuke Yamagoe; Taisuke Goto; Tomio Okawa
ICONE17, VOL 4, AMER SOC MECHANICAL ENGINEERS, 463-470, 2009, Peer-reviwed, The use of high power density core is one of the promising ways to improve economic efficiency of advanced boiling water reactors. It is however known that in boiling two-phase flows, an increase in power density commonly reduces the margin to the onset of unanticipated flow instability. Hence, in the development of a boiling water reactor of high power density core, ability to predict the occurrence of boiling transition is considered indispensable even when the coolant flow rate is not in the steady state. In the present work, sinusoidal oscillation was applied to the inlet mass flux and the experimental measurement of the critical heat flux was carried out under flow oscillation conditions. It was shown that the critical heat flux decreases monotonically with increased values of oscillation amplitude and oscillation period. These results are consistent with experimental data reported by previous investigators. A simple theory was then proposed to estimate the critical heat flux in oscillatory flow condition. Considering the application to the advanced boiling water reactors, the triggering mechanism of the critical heat flux condition is supposed to be the liquid film dryout in annular two-phase flow regime of high vapor quality. Under the flow oscillation condition, it is expected that long waves are formed on a liquid film due to the time variation of inlet mass flux. Assuming that the wave evolution within a boiling channel is influential in the occurrence of the local dryout of a liquid film, an available nonlinear wave theory was applied to the estimation of critical heat flux under the flow oscillation condition. It was demonstrated that the critical heat fluxes measured under the oscillatory conditions agree proposed theory fairly well.
International conference proceedings, English

Effect of impingement angle on the outcome of single water drop impact onto a plane water surface
Tomio Okawa; Takuya Shiraishi; Toshiaki Mori
Experiments in Fluids, 44, 2, 331-339, Feb. 2008, Peer-reviwed, Experiments of single water drop impact onto a plane water surface were carried out to investigate the effect of impingement angle on the total mass of secondary drops produced during the collision. When the impingement angle (the angle between the velocity vector of primary drop and the normal vector to water surface) was less than 50°, an increase in the impingement angle led to a remarkable increase in the total mass of secondary drops
this could be attributed to a significant increase in the secondary drop size. However, no secondary drop was observed within the experimental ranges tested when the impingement angle exceeded 70°. © 2007 Springer-Verlag.
Scientific journal, English

Measurement of the deposition rate of droplets in a vertical tube containing a flow obstacle
Tomio Okawa; Akio Kotani; Naoya Shimada; Isao Kataoka
NUCLEAR TECHNOLOGY, AMER NUCLEAR SOCIETY, 158, 2, 304-313, May 2007, Peer-reviwed, The critical heat flux in an annular two-phase flow regime is influenced significantly by an obstacle placed in a flow channel. Since the transition to critical heat flux condition in this flow regime is caused by the depletion of liquid film, it is probable that the flow obstacle has a notable influence on the rate of droplet deposition and, consequently, the film flow rate in the annular regime. Also, the obstacle's effect on the deposition rate would be important in predicting the critical heat flux in a boiling water reactor core because the grid spacer can be regarded as a flow obstacle placed in the subchannel. The obstacle effect was studied experimentally for vertical upward air-water annular flow; placing 12 small tubes of different cross sections concentrically in the test section tube one by one, the influence of obstacle geometry on the deposition rate was investigated. The rate of droplet deposition markedly increased if the present tubular obstacle was placed; the rate of increase was between similar to 30 and 200% and depended primarily on the obstacle shape. Using the experimental data, an empirical correlation to account for the obstacle's effect was proposed.
Scientific journal, English

Simultaneous measurement of void fraction and fundamental bubble parameters in subcooled flow boiling
Tomio Okawa; Hayato Kubota; Tatsuhiro Ishida
NUCLEAR ENGINEERING AND DESIGN, ELSEVIER SCIENCE SA, 237, 10, 1016-1024, May 2007, Peer-reviwed, Visualization was performed for the vapor bubbles in water subcooled flow boiling in a vertical heated tube to measure simultaneously the void fraction and the four fundamental bubble parameters: nucleation site density, bubble release frequency, bubble lifetime and bubble size. Using the mass flowrate and liquid subcooling as the experimental parameters, the changes of void fraction and bubble parameters with the wall heat flux were measured. The results of image analysis showed that the vapor void fraction could be approximated by the function of nucleation site density and bubble lift-off diameter; the bubble lift-off diameter was more influential than the nucleation site density. It was hence concluded that the bubble lift-off diameter could be regarded as the key parameter to determine the vapor void fraction under the present experimental conditions. The strong relation of bubble lift-off diameter to superheated liquid layer thickness was indicated for the future model development studies of bubble lift-off diameter. (C) 2007 Elsevier B.V. All rights reserved.
Scientific journal, English

On the deposition rate of droplets in annular flow around a flow obstacle
Tomio Okawa; Takayuki Fujita; Jun Minamitani; Isao Kataoka
Multiphase Science and Technology, 19, 4, 305-321, 2007, Peer-reviwed, Measurement of the deposition rate of droplets in vertical upward annular flow was carried out using air and water as test fluids. The test section was a 5-mm-diameter circular tube, in which a small tube was placed concentrically to test the influence of the presence of a flow obstacle on the deposition rate of droplets. The local deposition rate of droplets was measured at the inlet and downstream of the obstacle section to investigate the mechanism of deposition augmentation caused by the flow obstacle. In contrast to the hypothesis used in available models for the obstacle effect, significant deposition augmentation was detected not only downstream, but also at the inlet of the obstacle section. Simple models for the local deposition rates of droplets around the flow obstacle were developed based on the present experimental results.
Scientific journal, English

Single water drop impingement onto a falling water film
Tomio Okawa; Takuya Shiraishi; Takeshi Tada; Isao Kataoka
FEDSM 2007: PROCEEDINGS OF THE 5TH JOINT ASME/JSME FLUIDS ENGINEERING SUMMER CONFERENCE, VOL 2, PTS A AND B, AMER SOC MECHANICAL ENGINEERS, 317-322, 2007, Peer-reviwed, The process of oblique impact of single water drops onto a water film failing down on an inclined wall was observed using a high-speed camera. The angle between the velocity vector of the impacting drop and the normal vector to the inclined wall was set to the values greater than 70 degrees. In Our previous experiments for the single water drop impact onto a quiescent water surface, no secondary drop was observed in this large impingement angle range. In the present experiments, however, secondary drops were produced occasionally during the collision process. The production of secondary drops was observed more frequently when the Surface of falling liquid film was wavy. The process of the production of secondary drops from the falling water film was investigated using the video images to understand the difference between the drop impacts onto the quiescent film and onto the falling film. It was shown that the secondary drops Could be produced when the primary drop collided against the bottom or the top of the wave. In the former case, it was considered that the impingement angle was overestimated in the present work since the effect of surface waviness was not taken into account. This would imply that the occurrence of splashing was predicted more precisely if the local inclination of film surface could be included in evaluating the impingement angle. In the latter case, it was inferred that the liquid flow formed below the film surface by the drop impact penetrated the film surface to produce secondary drops.
International conference proceedings, English

Production of secondary drops during the single water drop impact onto a plane water surface
Tomio Okawa; Takuya Shiraishi; Toshiaki Mori
Experiments in Fluids, 41, 6, 965-974, Dec. 2006, Peer-reviwed, The normal impact of single water drops onto a plane water surface was studied experimentally to reveal the amount of secondary drops produced from the rim of crown-like interfacial structure. Within the experimental ranges tested, the ratio of the total mass of secondary drops to the mass of primary drop was approximately within 0-1 and correlated well as the function of dimensionless parameter K that consisted of the impact Weber number We and the Ohnesorge number Oh (K = We Oh -0.4). The dependences of the number and the mean diameter of secondary drops on K and dimensionless film thickness were also investigated. © Springer-Verlag 2006.
Scientific journal, English

Numerical simulation of two-dimensional bubbles initially flattened along a flat plate
T Okawa; Kataoka, I; M Mori
NUMERICAL HEAT TRANSFER PART A-APPLICATIONS, TAYLOR & FRANCIS INC, 49, 4, 393-409, Sep. 2006, Peer-reviwed, Several researchers have reported the detachment of vapor bubbles from a vertical heated wall in upward flow boiling of water. In these experiments, the bubble shape was frequently flattened along the wall at inception but became more rounded before the onset of detachment. In this study, the motion of two-dimensional bubbles is investigated numerically to elucidate the mechanism of bubble detachment. The numerical results show that the change in bubble shape due to surface tension force induces the liquid flow from the periphery to the base of bubble and consequently the liquid inertia lifts the bubble from the wall.
Scientific journal, English

An experimental study on the mass transfer rate of droplets in annular two-phase flow
T Okawa; N Shimada; A Kotani; Kataoka, I
JSME INTERNATIONAL JOURNAL SERIES B-FLUIDS AND THERMAL ENGINEERING, JAPAN SOC MECHANICAL ENGINEERS, 49, 2, 271-278, May 2006, Peer-reviwed, Deposition rate of droplets in annular flow was measured to elucidate the effect of the shape of flow obstacles that were placed in a flow channel. In the present experiments. the test section was a vertical round tube of 5 mm in inside diameter, air and water were used as test fluids, and double film extraction technique was adopted for the measurement. Seven flow obstacles were tested; three obstacles were tubular in shape while other four obstacles were also cylindrical but the cross-sectional areas were varied smoothly in the axial direction. It was revealed that the deposition rate of droplets markedly increased if the present small flow obstacle was placed in the test section tube. The increasing rate was dependent primarily upon the cross-sectional shape of flow obstacle at the maximum cross-sectional area.
Scientific journal, English

A STUDY ON SECONDARY DROPS PRODUCED BY DROPLET COLLISION WITH A LIQUID FILM
Tomio Okawa; Toshiaki Mori; Takuya Shiraishi; Isao Kataoka
PROCEEDINGS OF THE ASME FLUIDS ENGINEERING DIVISION SUMMER CONFERENCE, VOL 2, AMER SOC MECHANICAL ENGINEERS, 73-77, 2006, Peer-reviwed, The collision of single water drops with a plane water surface was investigated experimentally to elucidate the effect of impact angle on the amount of secondary drops. When the impact angle was not too small, the measured critical impact Weber number for the formation of secondary drops agreed with the experimental data reported in literature, provided that the absolute velocity of primary droplet was used in calculating the Weber number. The ratio of total mass of secondary drops to the mass of primary droplet increased markedly with the decrease of impact angle. Whereas, when the impact angle was too small, no secondary drops were produced within the experimental ranges tested.
International conference proceedings, English

Correlations for the mass transfer rate of droplets in vertical upward annular flow
T Okawa; Kataoka, I
INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER, PERGAMON-ELSEVIER SCIENCE LTD, 48, 23-24, 4766-4778, Nov. 2005, Peer-reviwed, New correlations for the deposition rate and entrainment rate of droplets in vertical upward annular flow were developed from simple models and available experimental data. In the correlation for the deposition rate, the superficial gas velocity was used as the parameter of primary importance at low droplet concentration while the droplet concentration itself at high droplet concentration. In correlating the rate of droplet entrainment, the ratio of interfacial shear force to the surface tension force acting on the surface of liquid film was the appropriate scaling parameter to correlate the experimental data measured in varied conditions. The experimental data for air-water annular flow were used in the development of the present correlations since extensive databases were available. It was however confirmed that the present model provides satisfactory agreements with the experimental data for high-pressure steam-water annular flow. (c) 2005 Elsevier Ltd. All rights reserved.
Scientific journal, English

On the rise paths of single vapor bubbles after the departure from nucleation sites in subcooled upflow boiling
T Okawa; T Ishida; Kataoka, I; M Mori
INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER, PERGAMON-ELSEVIER SCIENCE LTD, 48, 21-22, 4446-4459, Oct. 2005, Peer-reviwed, A photographic study was carried out for the subcooled flow boiling of water to elucidate the rise characteristics of single vapor bubbles after the departure from nucleation sites. The test section was a transparent glass tube of 20 mm in inside diameter and the flow direction was vertical upward; liquid subcooling was parametrically changed within 016 K keeping system pressure and liquid velocity at 120 kPa and 1 m/s, respectively. The bubble rise paths were analyzed from the video images that were obtained at the heat flux slightly higher than the minimum heat flux for the onset of nucleate boiling. In the present experiments, all the bubbles departed from their nucleation sites immediately after the inception. In low subcooling experiments, bubbles slid upward and consequently were not detached from the vertical heated wall; the bubble size was increased monotonously with time in this case. In moderate and high subcooling experiments, bubbles were detached from the wall after sliding for several millimeters and migrated towards the subcooled bulk liquid. The bubbles then reversed the direction of lateral migration and were reattached to the wall at moderate subcooling while they collapsed due to the condensation at high subcooling. It was hence considered that the mechanisms of the heat transfer from heated wall and the axial growth of vapor volume were influenced by the difference in bubble rise path. It was observed after the inception that bubbles were varied from flattened to more rounded shape. This observation suggested that the bubble detachment is mainly caused by the change in bubble shape due to the surface tension force. (c) 2005 Elsevier Ltd. All rights reserved.
Scientific journal, English

Bubble rise characteristics after the departure from a nucleation site in vertical upflow boiling of subcooled water
T Okawa; T Ishida; Kataoka, I; M Mori
NUCLEAR ENGINEERING AND DESIGN, ELSEVIER SCIENCE SA, 235, 10-12, 1149-1161, May 2005, Peer-reviwed, Rise characteristics of vapor bubbles after the departure from a nucleation site in forced convective subcooled flow boiling were studied visually using two synchronized high speed video cameras. The test section was a transparent glass tube of 20 mm in inside diameter, filtrated and deionized tap water was used as a working fluid, and the flow direction adopted was vertical upward. The outer surface of test section tube was electrically heated to generate vapor bubbles inside of the tube. In the present experiments, the mass flux and liquid subcooling were varied within 94-1435 kg/m(2) s and 2.2-10 K, respectively. Since the observations were performed at low heat fluxes to avoid the significant increase in the number of active nucleation sites, the obtained bubble images were clear enough to carry out the detailed image analysis for the rise characteristics of individual bubbles. The following three different bubble rise paths were observed after the departure from nucleation sites: some bubbles slid upward the vertical wall for long distance, while other bubbles were detached from the wall after sliding for several millimeters and then migrated toward the bulk liquid; after the migration, some of the detached bubbles were collapsed in subcooled liquid but others remained close to the wall and were reattached to the wall. The results of detailed image analyses suggested that the variation in bubble shape from flattened to more rounded was of primary importance for the occurrence of bubble detachment from the wall. (c) 2005 Elsevier B.V. All rights reserved.
Scientific journal, English

Flow regimes of upward gas - Liquid two-phase flow in a vertical pipe with an axisymmetic sudden expansion
Koichi Kondo; Kenji Yoshida; Tadayoshi Matsumoto; Tomio Okawa; Isao Kataoka
Nihon Kikai Gakkai Ronbunshu, B Hen/Transactions of the Japan Society of Mechanical Engineers, Part B, Japan Society of Mechanical Engineers, 71, 711, 2679-2685, 2005, Peer-reviwed, Experimental study was made on the multi-dimensional behavior of upward air-water two-phase flow through a vertical pipe with an axisymmetric sudden expansion, which is one of the typical multi-dimensional channel geometry. The aims of this study are to clarify the multidimensional behavior of bubbly flow or slug flow affected by sudden expansion channel geometry, and to accumulate the experimental data for two-phase flow analysis, which is applicable to predict the multi-dimensional behavior with appropriate accuracy. The observation using a high-speed video camera was performed to clarify the multi-dimensional dynamic flow behavior with the sudden expansion. It revealed the characteristic phenomena such as gas-slug breakup and bubble deformation and bubble size below and above the sudden expansion point. Furthermore the effect of the sudden expansion channel on the flow regime map can be also described by using these experimental result.
Scientific journal, Japanese

Void fraction distribution of upward bubbly flow in a vertical pipe with sudden expansion
Koichi Kondo; Kenji Yoshida; Tadayoshi Matsumoto; Tomio Okawa; Isao Kataoka
Nihon Kikai Gakkai Ronbunshu, B Hen/Transactions of the Japan Society of Mechanical Engineers, Part B, Japan Society of Mechanical Engineers, 71, 703, 796-803, 2005, Peer-reviwed, Experimental study was made on the multi-dimensional behavior of upward gas-liquid two-phase flow through a vertical pipe with an axisymmetric sudden expansion. In this study, the measurement on the void fraction distribution was carried out for the sudden expansion channel. The void fraction distributions below and above the sudden expansion point were measured at the different axial and radial positions using a point-electrode resistivity probe for various gas and liquid flow conditions. The results of measured void fraction distributions showed that how the two-phase flow develops along the direction of the downstream of the sudden expansion. Furthermore, the development of the void fraction distribution also revealed quite complicated behaviors depending upon flow rates of gas and liquid phases and bubble size. Based on the measured void fraction distribution in the sudden expansion, cross-sectional averaged void fraction and phase distribution parameter were evaluated at the different tube cross sections in the sudden expansion. As a result, it revealed that the void fraction along the flow direction in the sudden expansion might be predicted by using the appropriate distribution parameter representing the void fraction distributions in the sudden expansion.
Scientific journal, Japanese

Prediction of critical heat flux in annular flow using a film flow model
T Okawa; A Kotani; Kataoka, I; M Naito
JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY, ATOMIC ENERGY SOC JAPAN, 40, 6, 388-396, Jun. 2003, Peer-reviwed, A new set of correlations for the film flow analysis was developed to predict the critical heat flux in annular flow regime accurately. All the correlations adopted here were based on experimental data or considerations of the processes in annular flow; the resulting model required no parameters that should be adjusted from the measured data of critical heat flux. The 4,375 data of critical heat flux in forced flow of water in vertical uniformly heated round tubes were used to test the basic performance of the model. The comparisons between the calculated and measured critical heat fluxes showed that the predicted results by the present model agree with the experimental data fairly well if the flow pattern at the onset of critical heat flux condition is considered annular flow. The predictive capability was not deteriorated even in the cases of small diameter tube, short length tube as well as low vapor quality. Good agreements were also achieved in the preliminary tests against the critical heat flux data for Freon 12 and Freon 21. It is expected from these results that the present model satisfactorily expresses the important phenomena to predict the critical heat flux in annular flow regime.
Scientific journal, English

Experimental observation of single bubble rise characteristics in various situations
Tomio Okawa; Tomoe Tanaka; Kazuhiro Torimoto; Masanori Nishiura; Isao Kataoka
American Society of Mechanical Engineers, Fluids Engineering Division (Publication) FED, 257, 1 B, 861-866, 2002, Peer-reviwed, The effects of liquid temperature and injection method on single bubble rise characteristics in clean still water were experimentally investigated. It was confirmed that the shape and rise velocity of a bubble strongly depend on the method of bubble formation. For the rise velocity in high temperature water, the correlation for fully contaminated liquid might be appropriate even in the clean water. Because of the importance in the numerical simulation of bubbly two-phase flow, the experimental information on rise path oscillation such as amplitude and frequency was also reported.
International conference proceedings, English

Mechanistic prediction of dryout heat flux in annular two-phase flow
Tomio Okawa; Saeyun Kim; Isao Kataoka; Masanori Naitoh
International Conference on Nuclear Engineering, Proceedings, ICONE, 3, 429-435, 2002, Peer-reviwed, Incorporating the recently developed correlations for the rates of droplet deposition and entrainment with the film flow model, critical heat flux due to liquid film dryout in steam-water annular flow was predicted. In the present calculations, the predicted critical heat flux is affected not only by the deposition and entrainment rates but also by the entrainment fraction at the transition to annular flow (Transition quality was estimated by Wallis's correlation). For this reason, the entrainment fraction at the transition was correlated in terms of dimensionless transition length to annular flow and density ratio. A simple correlation for the occurrence of dryout was also proposed. It was demonstrated by numerical simulations that the present method is to predict the available 1,340 data of dryout heat flux with the mean square relative error of 0.068.
International conference proceedings, English

技術ロードマップから見る2030年の社会 第6回 動力エネルギーシステム分野/エンジンシステム分野
中田俊彦; 氣駕尚志; 大川富雄; 浅野等; 今村宰; 塚本達郎; 内田登; 北村高明; 北村高明
2023, 日本機械学会誌, 126, 1255, 0021-4728, 202302289752426057

Dr. Romney B. Duffey on His 80th Birthday
Igor Pioro; Richard R. Schultz; Hideki Kamide; Sama Bilbao y Leon; Nils Diaz; Alexander V. Bychkov; Leon Cizelj; Francesco D’Auria; Yanping Huang; Yasuo Koizumi; Tomio Okawa; Robert Walker; Robert Youngblood; Enrico Zio
01 Jul. 2022, Journal of Nuclear Engineering and Radiation Science, 8, 030201, 1-4, Invited, Others, 2332-8983, 2332-8975, 85133889436

Synchronized images during falling liquid film cooling
Yutaro Umehara; Tomio Okawa
Last, Mar. 2022, 混相流, 36, 1, Japanese, Invited, Others, False

原子炉における機構論的限界熱流束評価技術の 確立に向けて Part 1：軽水炉燃料における現在の限界熱流束予測手法 と課題
大川富雄、淀忠勝、成島勇気、高野賢治
Lead, 日本原子力学会, Dec. 2021, 日本原子力学会誌, 63, 12, 1-5, Japanese, Invited, Introduction scientific journal, False

原子炉における機構論的限界熱流束評価技術の確立に向けて Part 2：機構論的限界熱流束予測評価手法確立に向けた研究とその課題 と課題
大川富雄, 森昌司, 劉維, 小瀬裕男, 吉田啓之, 小野綾子
Lead, 日本原子力学会, Dec. 2021, 日本原子力学会誌, 63, 12, 1-5, Japanese, Invited, Introduction scientific journal, False

Nanoparticle coating in heat pipe design for miniaturised cooling
Tomio Okawa, Menglei Wang
Lead, Research Outreach, 02 Jul. 2021, Research Outreach, 123, 22-25, English, Invited, Introduction commerce magazine, True, 2517-7028

Fundamentals of Thermal and Nuclear Power Generation
Yasuo Koizumi; Tomio Okawa; Shoji Mori
Fundamentals of Thermal and Nuclear Power Generation is the first volume in the JSME Series in Thermal and Nuclear Power Generation. The first part of this volume provides a thorough and complete reference on the history of thermal and nuclear power generation, which has informed and sculpted todays industry. It prepares readers for subsequent publications in the series that address more advanced topics and will particularly benefit early career researchers and those approaching the industry from an alternative discipline. Modern thermal and nuclear power generation systems and technologies are then explored, including clear analysis on the fundamentals of thermodynamics, hydrodynamics, thermal engineering, combustion engineering, and nuclear physics. The impact of these technologies on society is considered throughout, as well as supply issues, accident risk analysis, and important emission and sustainability considerations. This book is an invaluable resource for researchers and professional engineers in nuclear and thermal energy engineering, and postgraduate and undergraduate students in power generation, especially nuclear and thermal., 01 Jan. 2021, Fundamentals of Thermal and Nuclear Power Generation, 1-301, Others, 85126373605

Heat transfer improvement in heat exchanger for thermoacoustic engine by sintered porous pipes
OTOMO Yusuke; ENOKI Koji; KOBAYASHI Takuto; UEDA Yuki; AKISAWA Atsushi; KAWASAKI Jumpei; OKAWA Tomio
Recently thermoacoustic engine is being used to recover latent and sensible heat. However, the heat exchanger which is loaded by thermoacoustic engine has low performance because of large thermal resistance of the gas side. The purpose of this study is improving heat exchanger equipped with sintered fiber metal porous materials into the pipes in order to enhance the heat flux. Porous materials have the characteristic of mixing the fluid in contact and expanding heat transfer area. In addition, these heat pipes have a less contact thermal resistance because of the sintered porous materials into tube wall. Conducted experiments to measure heat exchange for each pipes show that porous pipes have between 1.5 times and 2 times better heat exchange performance than conventional ones under the condition Reynolds number Re = 4500. Furthermore, heat exchanger using sintered fiber metal porous materials increase until 10 times the heat transfer rate in comparison with the conventional heat exchangers., The Japan Society of Mechanical Engineers, 2019, The Proceedings of the National Symposium on Power and Energy Systems, 2019.24, E111, Japanese, 2424-2950, 130007773973

20314 A study on void development in subcooled flow boiling
KAJIHARA Tomoyuki; MIYANO Naoki; OKAWA Tomio; KUDO Yoshiro
Subcooled flow boiling plays an important role in boiling water reactors because it influences the heat transfer performance from fuel rods, two-phase flow stability, and neutron moderation characteristics. In this work, flow visualization of water subcooled flow boiling in a vertical heated channel was carried out to investigate the mechanisms of void fraction development. It was observed that with an increase in the wall heat flux, new nucleation sites were activated and larger bubbles were produced at low frequency. It was considered that these large bubbles significantly contributed to the void fraction development., The Japan Society of Mechanical Engineers, 14 Mar. 2014, 日本機械学会関東支部総会講演会講演論文集, 2014, 20, "20314-1"-"20314-2", Japanese, 110009949384, AA11902161

S083031 Visualization Study to Investigate the NVG Mechanisms in Subcooled Flow Boiling
AHMADI Rouhollah; UENO Tatsuya; OKAWA Tomio
Measurement of void fraction and observation of bubble dynamics were carried out to investigate the mechanisms causing the net vapor generation (NVG) in subcooled upward flow boiling. The working fluid was distilled water and the heated surface of rather low contact angle was used in the present experiment Since no bubbles could stay at their nucleation sites even at the onset of nucleate boiling (ONB), departure of bubbles from the nucleation sites was not the cause of NVG in the present experimental setup. Following the departure from the nucleation sites, bubbles were lifted off the heated surface to be propelled into the subcooled bulk liquid at low pressures whilst they slid along the vertical heated surface at elevated pressures. As a result, the bubble life time at ONB was considerably shorter in the experiments conducted under low pressures. The distinct difference of bubble dynamics suggested that the mechanisms of NVG are dependent on the system pressure. At low pressures, some bubbles could be reattached to the surface after the lift-off at sufficiently low liquid subcooling Since the bubble life time was markedly elongated, the reattachment of bubbles to the vertical heated surface was considered to trigger the NVG. It was also found that the bubble reattachment usually took place following the coalescence of bubbles produced successively at the same nucleation site. The stochastic bubble motion induced by coalescence was hence considered to be a key phenomenon causing NVG. The NVG mechanism at high pressures is investigated in future studies., The Japan Society of Mechanical Engineers, 09 Sep. 2012, Mechanical Engineering Congress, Japan, 2012, "S083031-1"-"S083031-5", Japanese, 110009993565, AA12588255

ICONE19-43556 Bubble behavior in subcooled flow boiling in a vertical rectangular channel
Ahmadi Rouhollah; Ueno Tatsuya; Okawa Tomio
In this study bubble behavior in subcooled flow boiling in an upward rectangular channel was discussed. Filtrated and deionized tap water was used as a working fluid. Heated surface is produced by cupper with high wetability or low contact angle (18°). Subcooled flow boiling is beginning with onset of nucleate boiling (ONB) which surface temperature is higher than saturation temperature while liquid temperature is less than saturation temperature. At various pressure, mass fluxes and inlet temperatures ONB heat flux is detected within observation incipient nucleate bubbles through high speed video camera. Comparing with available ONB correlations, it is revealed that surface wettability is a significant parameter. Among ΔT_ correlation only Basu et al. (2002)'s correlation has a good consistency with experimental data, and other correlations are most underpredicted. Nucleation rate of bubbles after ONB remain in very small amount until a specific condition nominated onset of significant void (OSV) or incipient point of net vapor generation (IPNVG) that void fraction drastically increased. Levy (1967) was the first to postulate bubble departure mechanism for OSV and many researchers believed this assumption and analytically and empirically proposed OSV correlation. However, in all the experimental conditions even at incipient bubble nucleation, bubbles are observed depart or lift-off from activated nucleation site on the heated surface. Bubble nucleation behavior as whether bubble Lift-off from hot surface or sliding along it are investigated. It is performed for low void fraction condition. It is concluded that lifting-off condition occurred when bubble size and subcooling temperature is high and sliding phenomena in the opposite condition happened. Bubble size dramatically drop down after atmospheric pressure. This categorization is useful for developing new OSV mechanism regarded to actual situation., The Japan Society of Mechanical Engineers, 01 Aug. 2011, Proceedings of the ... International Conference on Nuclear Engineering. Book of abstracts : ICONE, 2011, 19, "ICONE1943556-1"-"ICONE1943556-7", English, 110009956960, AA1190311X

B110 Analytical Study on Boiling Transition under Flow-Power Oscillating Condition for Hyper ABWR
Nozaki Kenichiro; Okawa Tomio; Hotta Akitoshi; Ikeda Hideaki
In the envisioned high-power density core plant, the degradation of stability is concerned that is attributed to the shortening of rod heat conduction time constant upon the dense fuel grid and the increment of pressure drop caused by the wall friction. To confirm the influence of such degradation on the fuel soundness, the bundle geometry critical power test was held under flow-power oscillation condition which envisages the unstable phenomenon occurring under in the flow transient event. This paper describes the analysis results of the cited test with sub-channel code NASCA, and the discussion of the mechanism of boiling transition under flow-power oscillating condition., The Japan Society of Mechanical Engineers, 18 Jun. 2008, National Symposium on Power and Energy Systems, 2008, 13, 85-88, Japanese, 110007703804, AA11901431

909 Experiments on the Deposition Rate of Droplets in a Flow Channel Containing an Obstacle
Fujita Takayuki; Minamitani Jun; Okawa Tomio; Kataoka Isao
The Japan Society of Mechanical Engineers, 17 Mar. 2006, 関西支部講演会講演論文集, 2006, 81, "9-8", Japanese, 110006669298, AN00360659

OS8-06 An experimental study on the mass transfer rate of droplets in annular two-phase flow
OKAWA Tomio; KOTANI Akio; SHIMADA Naoya; KATAOKA Isao
The deposition rate of droplets in annular two-phase flow in a small vertical tube was measured using the double film extraction method. Based on the present experimental data and available ones, new phenomenological models for the deposition rate and entrainment rate were developed. It was also shown experimentally that the deposition rate is markedly increased if a small flow obstacle is placed in the test section tube. An empirical correlation to account the influence of flow obstacle on deposition rate was proposed., The Japan Society of Mechanical Engineers, 28 Jun. 2005, National Symposium on Power and Energy Systems, 2005, 10, 135-138, Japanese, 110006187878, AA11901431

An experimental study on bubble rise path after the departure from a nucleation site in vertical upflow boiling
T Okawa; T Ishida; Kataoka, I; M Mori
Visual study was conducted to elucidate the rise characteristics of vapor bubbles after the departure from a nucleation site in forced convective boiling. The flow direction was vertical upward and nearly saturated water was used as a working fluid. The outer surface of a transparent glass tube of 20 mm in inner diameter was electrically heated to generate vapor bubbles inside of the tube. The number density of bubbles in the test section was kept low to observe the behavior of individual bubbles using high speed cameras. The cross-sectional area-averaged velocity of bulk flow was set at 0.5 and 1.0 m/s. The approximate maximum diameter of each bubble was 1.6-2.9 mm. The following observations were made on the behavior of typical bubbles generated in the present experimental conditions: (i) bubbles grew at a nucleation site for a short period of time less than 1 ms with the shape flattened along the vertical heating surface; (ii) bubbles then slid upward the vertical wall for a few millimeters with the gradual increase in their size; (iii) after that, bubbles were detached from the heating surface and migrate towards the bulk flow but they remained close to the wall; (iv) at the instant of detachment, bubble rise velocity was already comparable with the local velocity of surrounding liquid and the rapid growth seen immediately after the nucleation was already completed; (v) after the detachment, bubble size was decreased in slightly subcooled bulk fluid and some of them were collapsed; (vi) bubbles that did not experience the collapse in subcooled liquid reversed the direction of radial migration and was eventually reattached to the wall (vii) after the reattachment, bubbles generally slid upward the vertical surface for a long distance with the gradual growth due to the heat from the heating wall, though the bubbles that experienced larger gap with the wall after the first detachment showed rebounding motion with the reduced amplitude. The detachment from the wall and the reattachment to the wall of bubbles were clearly observed as the typical bubble behavior after the departure from a nucleation site in the present experiments. It was expected that the inertia force and shear induced lift force have the important roles in these phenomena. (C) 2004 Elsevier Inc. All rights reserved., ELSEVIER SCIENCE INC, Mar. 2005, EXPERIMENTAL THERMAL AND FLUID SCIENCE, 29, 3, 287-294, English, 0894-1777, 1879-2286, WOS:000226571600005

An experimental study on bubble rise path after the departure from a nucleation site in vertical upflow boiling
T Okawa; T Ishida; Kataoka, I; M Mori
Visual study was conducted to elucidate the rise characteristics of vapor bubbles after the departure from a nucleation site in forced convective boiling. The flow direction was vertical upward and nearly saturated water was used as a working fluid. The outer surface of a transparent glass tube of 20 mm in inner diameter was electrically heated to generate vapor bubbles inside of the tube. The number density of bubbles in the test section was kept low to observe the behavior of individual bubbles using high speed cameras. The cross-sectional area-averaged velocity of bulk flow was set at 0.5 and 1.0 m/s. The approximate maximum diameter of each bubble was 1.6-2.9 mm. The following observations were made on the behavior of typical bubbles generated in the present experimental conditions: (i) bubbles grew at a nucleation site for a short period of time less than 1 ms with the shape flattened along the vertical heating surface; (ii) bubbles then slid upward the vertical wall for a few millimeters with the gradual increase in their size; (iii) after that, bubbles were detached from the heating surface and migrate towards the bulk flow but they remained close to the wall; (iv) at the instant of detachment, bubble rise velocity was already comparable with the local velocity of surrounding liquid and the rapid growth seen immediately after the nucleation was already completed; (v) after the detachment, bubble size was decreased in slightly subcooled bulk fluid and some of them were collapsed; (vi) bubbles that did not experience the collapse in subcooled liquid reversed the direction of radial migration and was eventually reattached to the wall (vii) after the reattachment, bubbles generally slid upward the vertical surface for a long distance with the gradual growth due to the heat from the heating wall, though the bubbles that experienced larger gap with the wall after the first detachment showed rebounding motion with the reduced amplitude. The detachment from the wall and the reattachment to the wall of bubbles were clearly observed as the typical bubble behavior after the departure from a nucleation site in the present experiments. It was expected that the inertia force and shear induced lift force have the important roles in these phenomena. (C) 2004 Elsevier Inc. All rights reserved., ELSEVIER SCIENCE INC, Mar. 2005, EXPERIMENTAL THERMAL AND FLUID SCIENCE, 29, 3, 287-294, English, 0894-1777, 1879-2286, WOS:000226571600005

Experiments for liquid phase mass transfer rate in annular regime for a small vertical tube
T Okawa; A Kotani; Kataoka, I
The double film extraction technique was used to measure the deposition rate and the entrainment rate of droplets for vertical upward annular two-phase flow in a small diameter tube. The test section was around tube of 5 mm in inside diameter, air and water were used as test fluids and the system pressure was varied within 0.14-0.76 MPa. It was shown in the present experimental conditions that the deposition rate was primarily influenced by the droplet concentration in the gas core and that the entrainment rate was correlated well with the dimensionless number denoting the ratio of interfacial shear force to surface tension force acting on the surface of liquid film. These results were consistent with available empirical correlations that were developed using the experimental data for larger diameter tubes. (C) 2004 Elsevier Ltd. All rights reserved., PERGAMON-ELSEVIER SCIENCE LTD, Jan. 2005, INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER, 48, 3-4, 585-598, English, 0017-9310, WOS:000226625300011

Experiments for liquid phase mass transfer rate in annular regime for a small vertical tube
T Okawa; A Kotani; Kataoka, I
The double film extraction technique was used to measure the deposition rate and the entrainment rate of droplets for vertical upward annular two-phase flow in a small diameter tube. The test section was around tube of 5 mm in inside diameter, air and water were used as test fluids and the system pressure was varied within 0.14-0.76 MPa. It was shown in the present experimental conditions that the deposition rate was primarily influenced by the droplet concentration in the gas core and that the entrainment rate was correlated well with the dimensionless number denoting the ratio of interfacial shear force to surface tension force acting on the surface of liquid film. These results were consistent with available empirical correlations that were developed using the experimental data for larger diameter tubes. (C) 2004 Elsevier Ltd. All rights reserved., PERGAMON-ELSEVIER SCIENCE LTD, Jan. 2005, INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER, 48, 3-4, 585-598, English, 0017-9310, WOS:000226625300011

Applicability of One-Dimensional Two-Fluid Model to Air-Water Bubbly Flow in a Vertical Pipe with Sudden Expansion
KONDO Koichi; YOSHIDA Kenji; OKAWA Tomio; KATAOKA Isao
日本原子力学会, 25 Dec. 2004, Transactions of the Atomic Energy Society of Japan, 3, 4, 323-330, Japanese, 1347-2879, 10013705001, AA11643165

Effects of a flow obstacle on the deposition rate of droplets in annular two-phase flow
T Okawa; A Kotani; N Shimada; Kataoka, I
Double film extraction technique was used to elucidate the effects of a flow obstacle on the deposition rate of droplets in annular two-phase flow. In the present experiments, the test section was a round tube of 5 rum in inside diameter, air and water were used as test fluids and the flow direction was vertical upward; a short tube that was 2 mm in inside diameter, 3 mm in outside diameter and 20 mm in length was used as a flow obstacle. It was shown that the deposition rate of droplets was markedly increased if the present flow obstacle was concentrically placed in the flow channel. In all the experimental conditions tested, the deposition rates when the flow obstacle was placed were approximately 1.5 times larger than the deposition rates when no obstacle was placed. An empirical correlation was developed to elucidate the influence of flow parameters on the enhancement of deposition rate by the present flow obstacle of simple geometry., ATOMIC ENERGY SOC JAPAN, Sep. 2004, JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY, 41, 9, 871-879, English, 0022-3131, WOS:000225245100001

Effects of a flow obstacle on the deposition rate of droplets in annular two-phase flow
T Okawa; A Kotani; N Shimada; Kataoka, I
Double film extraction technique was used to elucidate the effects of a flow obstacle on the deposition rate of droplets in annular two-phase flow. In the present experiments, the test section was a round tube of 5 rum in inside diameter, air and water were used as test fluids and the flow direction was vertical upward; a short tube that was 2 mm in inside diameter, 3 mm in outside diameter and 20 mm in length was used as a flow obstacle. It was shown that the deposition rate of droplets was markedly increased if the present flow obstacle was concentrically placed in the flow channel. In all the experimental conditions tested, the deposition rates when the flow obstacle was placed were approximately 1.5 times larger than the deposition rates when no obstacle was placed. An empirical correlation was developed to elucidate the influence of flow parameters on the enhancement of deposition rate by the present flow obstacle of simple geometry., ATOMIC ENERGY SOC JAPAN, Sep. 2004, JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY, 41, 9, 871-879, English, 0022-3131, WOS:000225245100001

Prediction of the critical heat flux in annular regime in various vertical channels
T Okawa; A Kotani; Kataoka, I; M Naitoh
The axial variation of film flow rate in annular regime was analyzed to predict the critical heat flux in water saturated flow boiling in various vertical channels. In the analyses, several quantities including the deposition and entrainment rates of droplets were evaluated with the correlations that were based on the separate experimental data of film flow rate and droplet flow rate in annular flow; model closure was achieved without introducing arbitrary constants. The predicted results were compared with extensive data of critical heat flux in flow boiling. Fairly good agreements were observed for the critical heat fluxes in round tubes with uniform and axially non-uniform heating and in non-circular shaped channels without sharp corners. It is hence expected that the important processes for the onset of critical heat flux condition in annular regime were satisfactorily described in the model. However, the critical heat fluxes were generally overestimated in thin rectangular channels with sharp corners. The non-uniform distribution of liquid film in channel cross-section would be the primary reason of the disagreement.(C) 2004 Elsevier B.V. All rights reserved., ELSEVIER SCIENCE SA, Apr. 2004, NUCLEAR ENGINEERING AND DESIGN, 229, 2-3, 223-236, English, 0029-5493, WOS:000221080400010

Prediction of the critical heat flux in annular regime in various vertical channels
T Okawa; A Kotani; Kataoka, I; M Naitoh
The axial variation of film flow rate in annular regime was analyzed to predict the critical heat flux in water saturated flow boiling in various vertical channels. In the analyses, several quantities including the deposition and entrainment rates of droplets were evaluated with the correlations that were based on the separate experimental data of film flow rate and droplet flow rate in annular flow; model closure was achieved without introducing arbitrary constants. The predicted results were compared with extensive data of critical heat flux in flow boiling. Fairly good agreements were observed for the critical heat fluxes in round tubes with uniform and axially non-uniform heating and in non-circular shaped channels without sharp corners. It is hence expected that the important processes for the onset of critical heat flux condition in annular regime were satisfactorily described in the model. However, the critical heat fluxes were generally overestimated in thin rectangular channels with sharp corners. The non-uniform distribution of liquid film in channel cross-section would be the primary reason of the disagreement.(C) 2004 Elsevier B.V. All rights reserved., ELSEVIER SCIENCE SA, Apr. 2004, NUCLEAR ENGINEERING AND DESIGN, 229, 2-3, 223-236, English, 0029-5493, WOS:000221080400010

610 Liquid Velocity and Turbulent Intensity Distributions of Bubbly Flow in a Vertical Pipe with Sudden Expansion
KONDO Koichi; YOSHIDA Kenji; OKAWA Tomio; KATAOKA Isao
Experimental study was made on the multi-dimensional behavior of upward gas-liquid two-phase flow through a vertical pipe with an axisymmetric sudden expansion, which is one of the typical multi-dimensional channel geometries. In this study, the liquid velocity and turbulent intensity along the flow direction at the below and above of the sudden expansion were measured by using a hot-film anemometer for various flow conditions. The development of the liquid velocity and the turbulent intensity along the flow direction of the downstream of the sudden expansion showed quite complicated behaviors depending upon flow rates of gas and liquid phases and bubble size. These results would become significant data for the development of multi-dimensional analysis in two-phase flow., The Japan Society of Mechanical Engineers, 17 Mar. 2004, 関西支部講演会講演論文集, 2004, 79, "6-19"-"6-20", Japanese, 110004301348, AN00360659

Experiments for equilibrium entrainment fraction in a small vertical tube
2004, Proceedings of 5th International Conference on Multiphase Flow, Yokohama, Japan

Bubble rise characteristics after the departure from a nucleation site in water saturated and subcooled flow boiling
2004, Proceedings of Third International Symposium on Two-Phase Flow Modelling and Experimentation, Pisa, Italy

Prediction of void fraction distribution for turbulent bubble flow in a vertical pipe with sudden expansion
2004, Proceedings of Third International Symposium on Two-Phase Flow Modelling and Experimentation, Pisa, Italy

Correlation for the deposition rate of droplets in vertical upward annular two-phase flow
2004, Proceedings of 6th International Topical Meeting on Nuclear Reactor Thermal Hydraulics, Operation and Safety, Nara, Japan

Experiments for deposition rate of droplets in a small diameter tube
2004, Proceedings of Sixth International Topical Meeting on Nuclear Reactor Thermal Hydraulics, Operation and Safety, Nara, Japan

混相流ハンドブック
2004, 朝倉書店

Experiments for equilibrium entrainment fraction in a small vertical tube
2004, Proceedings of 5th International Conference on Multiphase Flow, Yokohama, Japan

Prediction of void fraction distribution for turbulent bubble flow in a vertical pipe with sudden expansion
2004, Proceedings of Third International Symposium on Two-Phase Flow Modelling and Experimentation, Pisa, Italy

Bubble rise characteristics after the departure from a nucleation site in water saturated and subcooled flow boiling
2004, Proceedings of Third International Symposium on Two-Phase Flow Modelling and Experimentation, Pisa, Italy

Correlation for the deposition rate of droplets in vertical upward annular two-phase flow
2004, Proceedings of 6th International Topical Meeting on Nuclear Reactor Thermal Hydraulics, Operation and Safety, Nara, Japan

Experiments for deposition rate of droplets in a small diameter tube
2004, Proceedings of Sixth International Topical Meeting on Nuclear Reactor Thermal Hydraulics, Operation and Safety, Nara, Japan

Handbook of Multiphase Flow
2004, Asakura Pub.

Numerical Simulation of Liquid Film Falling down an Inclined Flat Plate
Okawa Tomio; Kataoka Isao
The Japan Society of Mechanical Engineers, 18 Sep. 2003, Fluids engineering conference ..., 2003, 158-158, Japanese, 1348-2882, 110002509974, AA11284390

Temperature effect on single bubble rise characteristics in stagnant distilled water
T Okawa; T Tanaka; Kataoka, I; M Mori
Rise characteristics of spherical and ellipsoidal bubbles in normal- and high-temperature distilled water were visually observed. In the high-temperature experiments, the measured results of the rise velocity of a single bubble and the existence of rise path oscillation roughly agreed with the correlations for the bubbles in contaminated liquid. Also, applicability of an available correlation for the frequency of rise path oscillation was confirmed and a new correlation was developed to evaluate the amplitude of oscillation. It is expected that these results are to contribute to the further improvement of the prediction methods of multidimensional void distribution. (C) 2002 Elsevier Science Ltd. All rights reserved., PERGAMON-ELSEVIER SCIENCE LTD, Feb. 2003, INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER, 46, 5, 903-913, English, 0017-9310, WOS:000180522900013

Studies on the modification characteristics of gas-phase turbulence in annular flow
TANAKA Hidenobu; YOSHIDA Kenji; MATSUMOTO Tadayoshi; OKAWA Tomio; KATAOKA Isao; MATSUURA Keizo
Experimental and numerical studies were made to investigate the effects of liquid film to gas-phase turbulence modification on annular flow. By using the hot-wire anemometer, time averaged axial velocity profile, turbulence fluctuation profile, energy spectrum, and auto-correlation coefficient of gas-phase axial velocity component are measured. By using the point-electrode prove, the time-averaged liquid film thickness is also measured. Considering the effect of wavy interface of liquid film, numerical simulations for gas-phase turbulence structures in annular flow are carried out. Liquid film is assumed and modeled to be the surface roughness of the wall moving with the velocity of liquid film., The Japan Society of Mechanical Engineers, 2003, 関西支部講演会講演論文集, 2003, 78, "14-33"-"14-34", Japanese, 110002493995, AN00360659

Bubble characteristics of steam-water two-phase flow in a large-diameter pipe
K Yoneda; A Yasuo; T Okawa
Bubble characteristics of steam-water two-phase upward flow were observed in a vertical pipe 155 mm in inner diameter. Experiments were conducted under volumetric flux conditions of J(G) < 0.25 m/s and J(L) < 0.6 m/s, and three different inlet boundary conditions to investigate the developing state of the flow. The radial distributions of flow structure were obtained by horizontally traversing optical dual void probes through the pipe. The spectra of bubble chord, length and gas velocity were also obtained to study the characteristics of bubbles in detail. The flow reached a quasi-developed state within relatively short height to diameter aspect ratio of about HID = 4 compared to a small-diameter pipe flow. The PDF profiles of bubble chord length and gas velocity were able to approximate fairly well by model function using Gamma distribution and log-normal distribution, respectively. (C) 2002 Elsevier Science Inc. All rights reserved., ELSEVIER SCIENCE INC, Aug. 2002, EXPERIMENTAL THERMAL AND FLUID SCIENCE, 26, 6-7, 669-676, English, 0894-1777, WOS:000178284700010

Bubble characteristics of steam-water two-phase flow in a large-diameter pipe
K Yoneda; A Yasuo; T Okawa
Bubble characteristics of steam-water two-phase upward flow were observed in a vertical pipe 155 mm in inner diameter. Experiments were conducted under volumetric flux conditions of J(G) < 0.25 m/s and J(L) < 0.6 m/s, and three different inlet boundary conditions to investigate the developing state of the flow. The radial distributions of flow structure were obtained by horizontally traversing optical dual void probes through the pipe. The spectra of bubble chord, length and gas velocity were also obtained to study the characteristics of bubbles in detail. The flow reached a quasi-developed state within relatively short height to diameter aspect ratio of about HID = 4 compared to a small-diameter pipe flow. The PDF profiles of bubble chord length and gas velocity were able to approximate fairly well by model function using Gamma distribution and log-normal distribution, respectively. (C) 2002 Elsevier Science Inc. All rights reserved., ELSEVIER SCIENCE INC, Aug. 2002, EXPERIMENTAL THERMAL AND FLUID SCIENCE, 26, 6-7, 669-676, English, 0894-1777, WOS:000178284700010

Numerical simulation of lateral phase distribution in turbulent upward bubbly two-phase flows
T Okawa; Kataoka, I; M Mori
New constitutive models for the interfacial forces acting on bubbles were developed for accurately predicting the lateral phase distribution in turbulent bubbly two-phase flow in vertical channels. Several experimental measurements have revealed that the lateral void profile in bubbly two-phase flow varies from the void peaking near the wall to the almost flat distributions as the liquid velocity increases. However, within the authors' knowledge, the effect of liquid velocity on the void profile has not been successfully predicted by the existing models; this would indicate the strong limitation of the existing multidimensional two-phase flow models. In view of these, the validity of the present constitutive models was tested in varied conditions of the liquid velocity as well as the bubble size. Since several assumptions were required in the models mainly due to the insufficient knowledge of the bubble motion, further improvements should still be needed. Nevertheless, the predicted lateral phase distributions were found to be in reasonably good agreement with available experimental data. It is hence expected that the present constitutive models can effectively be used in the practical applications and also be the base of the more sophisticated ones. (C) 2002 Elsevier Science B.V. All rights reserved., ELSEVIER SCIENCE SA, Apr. 2002, NUCLEAR ENGINEERING AND DESIGN, 213, 2-3, 183-197, English, 0029-5493, WOS:000175213500008

Flow Regime of Vertical Upward Two-Phase Flow through Round Tube with Sudden Expansion
KONDO Koichi; HASHIMOTO Tomoyuki; YOSHIDA Kenji; MATSUMOTO Tadayoshi; OKAWA Tomio; KATAOKA Isao
Experimental studies were made on the multi-dimensional behavior of upward gas-liquid two-phase flow through the vertical round tube with an axisymmetric sudden expansion, which is one of the typical multi-dimensional channel geometry. The aims of this study are to clarify the multi-dimensional behavior of bubbly or slug flow affected by sudden expansion channel geometry, and to accumulate the experimental data for two-phase flow analysis, which is applicable to predict with appropriate accuracy the multi-dimensional its behavior. As the first step in this study, direct observation using high-speed video camera was performed and revealed the multi-dimensional dynamic flow behavior affected by the sudden expansion part. Characteristic phenomena were observed such as bubble break-up, deformation due to the strong share of liquid flow, or liquid micro jet penetration through the gas-slug, and so on. The void fraction profiles in sudden expansion were also measured using a pointed-electrode resistivity probe. From these results, the flow regime map at the below or above of the sudden expansion part were classified., The Japan Society of Mechanical Engineers, 2002, 関西支部講演会講演論文集, 2002, 77, "11-17"-"11-18", Japanese, 110002493065, AN00360659

Prediction of dryout heat flux using film flow model
2002, Thermal Science and Engineering, Vol. 10, No. 4, pp. 33-34

Critical power analysis with mechanistic models for nuclear fuel bundles, (I) Models and verifications for boiling water reactor application
M Naitoh; T Ikeda; K Nishida; T Okawa; Kataoka, I
The critical power analysis code for BWR fuel bundles. "CAPE-BWR". was developed. The objective of the development is to predict dryout phenomena of liquid film on fuel rod surfaces without tuning any parameters even for fuel bundle design improvements. The major features of the code are modular structure with mechanistic models and parallel computation. The calculation methods were divided into three steps: subchannel, liquid film flow and spacer effect analyses. The code was validated by the rod bundle test analyses. The overall comparison of calculated critical power with 166 measured data points showed -0.3% average difference with the standard deviation of 6.3%. The spatial domain decomposition method was applied for parallel computation of the spacer effect analysis. The parallelization efficiency was about 80%. The calculated dryout location agreed well with the measured one at the full-scale 8 x 8 bundle test. The code could trace the tendencies of the critical power depending on power distribution, spacer geometry and fluid conditions within a practical range of difference. From the calculation. difference of the critical power due to the spacer geometry was clarified to be caused by the difference of droplet deposition characteristics onto the liquid film., ATOMIC ENERGY SOC JAPAN, Jan. 2002, JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY, 39, 1, 40-52, English, 0022-3131, WOS:000174437800006

New entrainment rate correlation in annular two-phase flow applicable to wide range of flow condition
T Okawa; T Kitahara; K Yoshida; T Matsumoto; Kataoka, I
Assuming the rate of droplet entrainment is characterized by the ratio of the interfacial shear force to the surface tension force acting on the phase interface, new correlation representing the rate of droplet entrainment in annular-dispersed two-phase flow was developed. Although the correlation is based on the simple assumption, the quasiequilibrium droplet flow rates measured in many experiments were predicted reasonably well (root mean square error of entrainment fraction was roughly halved comparing with several existing correlations). Its applicability to the nonequilibrium situation was also demonstrated by the numerical calculations using a one-dimensional three-fluid model. (C) 2001 Elsevier Science Ltd. All rights reserved., PERGAMON-ELSEVIER SCIENCE LTD, Jan. 2002, INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER, 45, 1, 87-98, English, 0017-9310, WOS:000172076200009

Photosynthetic productivity of conical helical tubular photobioreactors incorporating Chlorella sp under various culture medium flow conditions
M Morita; Y Watanabe; T Okawa; H Saiki
The characteristics of the flow of culture medium significantly affects the photosynthetic productivity of bioreactors incorporating microalgae. Therefore, in order to optimize the performance of a conical helical tubular photobioreactor (CHTP) designed to be useful in practical applications, we characterized the flow pattern of the culture medium through the reactor. The effects of medium flow conditions on the photosynthetic productivity of Chlorella sp. were investigated using several different CHTP units with 0.50-m(2) installation areas which were designed to vary the direction and rate of flow driven by airlift. In addition, the performance of two- and four-unit systems constructed by combining individual CHTP units was evaluated. We found that when medium flowed from the bottom to the top of the photostage, it exhibited smoother flow of culture medium than when flowing from top to bottom, which led to higher photosynthetic productivity by the former. Consistent with theoretical calculations, varying the lengths of vertical flow passages caused flow rates to vary, and higher flow rates meant smoother circulation of medium and better photosynthetic performance. Flow of medium through a four-unit CHTP system was similar to that in single units, enabling a photosynthetic productivity of 31.0 g-dry biomass per m(2)-installation area per day to be achieved, which corresponded to a photosynthetic efficiency of 7.50% (photosynthetically active radiation (PAR; 400-700 nm)). This high photosynthetic performance was possible because smoother medium flow attained in single units was also attained in the four-unit system. (C) 2001 John Wiley & Sons, Inc., JOHN WILEY & SONS INC, Jul. 2001, BIOTECHNOLOGY AND BIOENGINEERING, 74, 2, 136-144, English, 0006-3592, WOS:000169296600006

614 A study on forced convective transient gas-liquid two-phase flow
TSUJITA Yoshihiro; NAKAZUMI Minoru; OKAWA Tomio; MATSUMOTO Tadayoshi; KATAOKA Isao
This paper shows the experimental and analytical study of forced convective transient airwater two-phase flow. In experiment, the flow rates of water and air were independently at the entrance, the transient bubbly two-phase flow was realized in vertical channel. Then, measurement the transient variation of pressure, differential pressure and void fraction at measuring position. Concerning the measure of void fraction , use the character of air which is not conductive. Moreover , supply the data for the analysis., The Japan Society of Mechanical Engineers, 16 Mar. 2001, 関西支部講演会講演論文集, 2001, 76, "6-25"-"6-26", Japanese, 110002492115, AN00360659

615 Study On Transient Gas and Liquid Two-Phase Natural Circulation
NISHIMORI Yutaka; AOYAMA Shigeki; OKAWA Tomio; MATSUMOTO Tadayoshi; KATAOKA Isao
This paper is an investigation of the transient two-phase natural circulation. Laboratory equipment was composed of single-phase flow section and two-phase flow section. Single-phase flow section was 4.5m in length. Two-phase flow section was vertical and 2m in length. Flow is vertically upward. The inner diameter of the pipe was 20mm. The measurement was carried out on transient variation of the pressure, the void fraction and the flow rate of water induced by the supplied air flow. Comparisons are made between the data and the calculations based on basic equation of two-phase flow., The Japan Society of Mechanical Engineers, 16 Mar. 2001, 関西支部講演会講演論文集, 2001, 76, "6-27"-"6-28", Japanese, 110002492117, AN00360659

Numerical Simulation of Single Bubbles Located in Turbulent Flow in Vertical Pipes Using Bubble Tracking Method
OKAWA Tomio; ARAKI Koji; YOSHIDA Kenji; MATSUMOTO Tadayoshi; KATAOKA Isao; MORI Michitsugu
日本混相流学会, 2001, Japanese journal of multiphase flow, Vol. 15, No. 2, pp. 165-174, 2, 165-174, English, 0914-2843, 10010263636, AN10088286

Numerical implementation of interfacial drag force for one-dimensional, two-way bubble tracking method
T Okawa; Y Suzuki; Kataoka, I; M Aritomi; M Mori
A two-way bubble tracking method, in which each bubble is individually tracked on the Lagrangian coordinates, is one of the most promising two-phase flow models to partly take the place of a fully-averaged two-fluid model. In this method, conservation equations on different coordinate systems must be solved simultaneously, so that construction of the phase coupling models becomes particularly important. In the present study, taking into account the downward liquid flow formed around a bubble rising in a vertical pipe and using the bubble shape function proposed by Trapp et ad. at the same time, we developed simple phase coupling models for one-dimensional, two-way bubble tracking method. The calculated results of a single bubble rising in a circular pipe filled with stagnant liquid showed that the proposed model not only successfully mitigates the numerical fluctuation of the bubble rising velocity but also naturally predicts the effect of containing wall on the terminal bubble velocity; the present model would therefore be reasonable from both numerical and physical points of view. It was also revealed that the mean void fraction in bubbly two-phase flow is reasonably to be predicted if the effect of other bubbles on the downward liquid flow is appropriately evaluated., TAYLOR & FRANCIS LTD, Apr. 2000, JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY, 37, 4, 387-396, English, 0022-3131, 1881-1248, WOS:000087390500008

Numerical implementation of interfacial drag force for one-dimensional, two-way bubble tracking method
T Okawa; Y Suzuki; Kataoka, I; M Aritomi; M Mori
A two-way bubble tracking method, in which each bubble is individually tracked on the Lagrangian coordinates, is one of the most promising two-phase flow models to partly take the place of a fully-averaged two-fluid model. In this method, conservation equations on different coordinate systems must be solved simultaneously, so that construction of the phase coupling models becomes particularly important. In the present study, taking into account the downward liquid flow formed around a bubble rising in a vertical pipe and using the bubble shape function proposed by Trapp et ad. at the same time, we developed simple phase coupling models for one-dimensional, two-way bubble tracking method. The calculated results of a single bubble rising in a circular pipe filled with stagnant liquid showed that the proposed model not only successfully mitigates the numerical fluctuation of the bubble rising velocity but also naturally predicts the effect of containing wall on the terminal bubble velocity; the present model would therefore be reasonable from both numerical and physical points of view. It was also revealed that the mean void fraction in bubbly two-phase flow is reasonably to be predicted if the effect of other bubbles on the downward liquid flow is appropriately evaluated., TAYLOR & FRANCIS LTD, Apr. 2000, JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY, 37, 4, 387-396, English, 0022-3131, 1881-1248, 10004752848, WOS:000087390500008

901 Analysis of Flow Characteristics in Annular Dispersed Flow by a Multi-Fluid Model
KITAHARA Tsuyoshi; YOSHIDA Kenji; OKAWA Tomio; MATSUMOTO Tadayoshi; KATAOKA Isao
Prediction of dryout in annular-dispersed flows in the boiling heat transfer is particularly important. A three-fluid model is often used for analyses of the annular-dispersed flows ; its applicability to the prediction of the flows in round tubes has been well validated. However its applicability to the annular-dispersed flows in more complicated channels is uncertain. In the present study, the computer program based on a multi-fluid model is developed for predicting the flow properties of annular-dispersed flow accurately in complicated channels and applied it to the annular-dispersed flow in annuli., The Japan Society of Mechanical Engineers, 16 Mar. 2000, 関西支部講演会講演論文集, 2000, 75, "9-1"-"9-2", Japanese, 110002491711, AN00360659

902 Analysis of Turbulent Structure and Void Fraction Distribution in Boiling Two-Phase Flow with Various Cross Section Tube
MUTA Masayoshi; KIM Saeyun; YOSHIDA Kenji; OKAWA Tomio; MATSUMOTO Tadayoshi; KATAOKA Isao
Analyses were carried out on turbulence structure and void fraction distribution in boiling two-phase flow for various channel geometries. Basic conservation equations of mass, mometum and energy and turbulence in boiling two-phase flow were solved couppled with basic equation of void fraction distribution. Turbulence source terms of two phase flow were considered and fully two-way coupling model between gas and liquid chases used. The results show that bubble number density and void fraction distributions in boiling two-phase flow in rectangular channel has sharp wall peak in the central region of the channel whereas it has flattened distribution near wall region which reasonably agree with experimental observation. It is indicated that multi-dimensional turbulence structure is quite important in predicting void fraction distribution in boiling two-phase flow., The Japan Society of Mechanical Engineers, 16 Mar. 2000, 関西支部講演会講演論文集, 2000, 75, "9-3"-"9-4", Japanese, 110002491713, AN00360659

626 Development of Numerical Simulation Method of Dispersed Two-Phase Flow Using a Euler-Lagrange Model
ARAKI Koji; HASHIMOTO Katsuteru; OKAWA Tomio; YOSHIDA Kenji; MATSUMOTO Tadayoshi; KATAOKA Isao
A bubble tracking method is one of the most promising candidates for partly replacing the two-fluid model, which has been widely used in two-phase flow engineering analysis. In the present study, we confirmed the usefulness of this method by calculating the motion of the bubbles rising in the turbulent flow in the vertical rectangular duct, taking into account the lateral lift force, wall force acting on the bubbles, and reasonable agreement is found with the available experimental data., The Japan Society of Mechanical Engineers, 16 Mar. 2000, 関西支部講演会講演論文集, 2000, 75, "6-51"-"6-52", Japanese, 110002491591, AN00360659

Mathematical well-posedness of a two-fluid equations for bubbly two-phase flows
Tomio Okawa; Isao Kataoka
It is widely known that two-fluid equations used in most engineering applications do not satisfy the necessary condition for being mathematical well-posed as initial-value problems. In the case of stratified two-phase flows, several researchers have revealed that differential models satisfying the necessary condition are to be derived if the pressure difference between the phases is related to the spatial gradient of the void fraction through the effects of gravity or surface tension. While, in the case of dispersed two-phase flows, no physically reasonable method to derive mathematically wellposed two-fluid model has been proposed. In the present study, particularly focusing on the effect of interfacial pressure terms, we derived the mathematically closed form of the volume-averaged twofluid model for bubbly two-phase flows. As a result of characteristic analyses, it was shown that the proposed two-fluid equations satisfy the necessary condition of mathematical well-posedness if the void fraction is sufficiently small., Japan Society of Mechanical Engineers, 2000, Nihon Kikai Gakkai Ronbunshu, B Hen/Transactions of the Japan Society of Mechanical Engineers, Part B, 66, 646, 1281-1287, Japanese, 0387-5016, 110002393130, 71449125032

Mathematical well-posedness of a two-fluid equations for bubbly two-phase flows
Tomio Okawa; Isao Kataoka
It is widely known that two-fluid equations used in most engineering applications do not satisfy the necessary condition for being mathematical well-posed as initial-value problems. In the case of stratified two-phase flows, several researchers have revealed that differential models satisfying the necessary condition are to be derived if the pressure difference between the phases is related to the spatial gradient of the void fraction through the effects of gravity or surface tension. While, in the case of dispersed two-phase flows, no physically reasonable method to derive mathematically wellposed two-fluid model has been proposed. In the present study, particularly focusing on the effect of interfacial pressure terms, we derived the mathematically closed form of the volume-averaged twofluid model for bubbly two-phase flows. As a result of characteristic analyses, it was shown that the proposed two-fluid equations satisfy the necessary condition of mathematical well-posedness if the void fraction is sufficiently small., Japan Society of Mechanical Engineers, 2000, Nihon Kikai Gakkai Ronbunshu, B Hen/Transactions of the Japan Society of Mechanical Engineers, Part B, 66, 646, 1281-1287, Japanese, 0387-5016, 71449125032

New interfacial drag force model including effect of bubble wake, (II) - Model validation using experimental data of steam-water bubbly flow in large-diameter pipes
T Okawa; K Yoneda; Zhou, SR; H Tabata
In the previous study, we proposed a new interfacial drag force model based on experimental data of steam water bubbly flow in a large-diameter pipe. This is because our experimental results had suggested that effect of bubble wake should he included in the interfacial drag force model, although it half not been taken into account in the existing models. A preliminary method for including the effect of bubble wake was hence developed and used in the new model. A new bubble size prediction method was also adopted in the model. In the present study, after improving the measuring equipment and signal processing procedure, another series of experiments was carried out. Using the new experimental data, the methods of predicting bubble size and effect of bubble wake were slightly modified. To test the validity of the new model, predicted results were compared with available experimental data sets of steam-water bubbly two-phase flow in large-diameter pipes. One-dimensional and two-dimensional two-fluid models were used for the calculation. Comparisons shelved that the new model is in good agreement with the experimental data whereas the model which does nut take into account the effect of bubble wake overestimated the void fractions., TAYLOR & FRANCIS LTD, Nov. 1999, JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY, 36, 11, 1030-1040, English, 0022-3131, 1881-1248, WOS:000084632400007

New interfacial drag force model including effect of bubble wake, (II) - Model validation using experimental data of steam-water bubbly flow in large-diameter pipes
T Okawa; K Yoneda; Zhou, SR; H Tabata
In the previous study, we proposed a new interfacial drag force model based on experimental data of steam water bubbly flow in a large-diameter pipe. This is because our experimental results had suggested that effect of bubble wake should he included in the interfacial drag force model, although it half not been taken into account in the existing models. A preliminary method for including the effect of bubble wake was hence developed and used in the new model. A new bubble size prediction method was also adopted in the model. In the present study, after improving the measuring equipment and signal processing procedure, another series of experiments was carried out. Using the new experimental data, the methods of predicting bubble size and effect of bubble wake were slightly modified. To test the validity of the new model, predicted results were compared with available experimental data sets of steam-water bubbly two-phase flow in large-diameter pipes. One-dimensional and two-dimensional two-fluid models were used for the calculation. Comparisons shelved that the new model is in good agreement with the experimental data whereas the model which does nut take into account the effect of bubble wake overestimated the void fractions., TAYLOR & FRANCIS LTD, Nov. 1999, JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY, 36, 11, 1030-1040, English, 0022-3131, 1881-1248, 10002841537, WOS:000084632400007

Numerical Simulation of Turbulence Generation due to Boiling Bubble
KATAOKA Isao; UCHIDA Kyosuke; MATSUMOTO Tadayoshi; OHKAWA Tomio
29 Jul. 1999, 年会一般講演, 18, 253-254, Japanese, 1342-8004, 10007478438, AA11169596

Effects of Source Terms and Virtual Mass Force on Mathematical Nature of a Two-Fluid Model(2nd Report, Examination of Mathematical Nature)
OKAWA Tomio; TOMIYAMA Akio
As is by now well known, partial differential equations based on a one dimensional, one-pressure two fluid model are mathematically ill-posed as an initial-value problem. Up to the present, however, eigenvalue analysis of the two fluid model has been conducted only for a simplified two-fluid model without any source terms, since a necessary condition of well-posedhess can be derived from differential terms only. Consequently, effects of source terms have not been systematically investigated. In the previous report, we proposed the method to examine mathematical nature of the complete two-fluid model including source terms ; the effects of gravity were evaluated using this method. As a result, it was confirmed the two fluid model can be mathematically ill-posed due to source terms. In the present study, the effects of interfacial drag force and wall friction force were examined using the same method. The effects of virtual mass force were also evaluated, since its influence has not been clarified in spite of the fact that it can affect the numerical stability., The Japan Society of Mechanical Engineers, 1999, Transactions of the Japan Society of Mechanical Engineers. Series B., 65, 629, 86-91, Japanese, 0387-5016, 110002395183, AN00187441

Effects of source terms and vertual mass force on mathematical nature of a two-fluid model(2nd report, Evaluation of mathematical nature)
Tomio OKAWA; Akio TOMIYAMA
1999, Transactions of the Japan Society of Mechanical Engineers(Ser. B), 65, 629, 86-91, 0387-5016

New interfacial drag force model including effect of bubble wake, (I) - Model development for steam-water bubbly flow in large-diameter pipes
T Okawa; K Yoneda; Y Yoshioka
Several experimental results show that bubbles can easily be captured in the wake formed by leading bubbles when multiple bubbles ate rising in a liquid. It is suggested from this experimental result that the effect of bubble wake should be included in the constitutive relationships representing the interfacial drag force. In the present study, steam-water bubbly now experiments were performed to develop a new interfacial drag force model including the effect of bubble wake. Since the validity of the existing constitutive equations have been tested mainly for two-phase flow in small-diameter pipes, our study focused on two-phase flow in a large-diameter pipe. Using a one-dimensional two-fluid model, the applicability of the new interfacial drag force model to our experimental conditions was investigated. As a result, it was shown that the present model markedly improves the accuracy of the predicted results. It was therefore demonstrated that the present bubble wake model is effective at least for the conditions which were used for model development. Its applicability to different conditions will be discussed in a subsequent study., TAYLOR & FRANCIS LTD, Dec. 1998, JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY, 35, 12, 895-904, English, 0022-3131, 1881-1248, 10002080550, WOS:000078458400007

New interfacial drag force model including effect of bubble wake, (I) model development for steam-water bubbly flow in large-diameter pipes
Tomio Okawa; Kimitoshi Yoneda; Yuzuru Yoshioka
Several experimental results show that bubbles can easily be captured in the wake formed by leading bubbles when multiple bubbles are rising in a liquid. It is suggested from this experimental result that the effect of bubble wake should be included in the constitutive relationships representing the interfacial drag force. In the present study, steam-water bubbly flow experiments were performed to develop a new interfacial drag force model including the effect of bubble wake. Since the validity of the existing constitutive equations have been tested mainly for two-phase flow in small-diameter pipes, our study focused on two-phase flow in a large-diameter pipe. Using a one-dimensional two-fluid model, the applicability of the new interfacial drag force model to our experimental conditions was investigated. As a result, it was shown that the present model markedly improves the accuracy of the predicted results. It was therefore demonstrated that the present bubble wake model is effective at least for the conditions which were used for model development. Its applicability to different conditions will be discussed in a subsequent study. © 1998 Taylor &
Francis Group, LLC., 1998, Journal of Nuclear Science and Technology, 35, 12, 895-904, English, 0022-3131, 0032255042

Effects of Source Terms and Virtual Mass Force on Mathematical Nature of a Two-Fluid Model (1st Report, Effects of Gravity)
OKAWA Tomio; TOMIYAMA Akio
According to mathematical theorems on initial-value problem, real-valued characteristic roots are necessary for a system of partial differential equations. However, a one-dimensional, one-pressure two-fluid model has complex-valued characteristic root, so that it is called mathematically ill-posed. Up to the present, eigenvalue analysis of a two-fluid model has been conducted only for a simplified two-fluid model without any source terms. Although these simplified analyses can yield a necessary condition for well-posedness, they cannot give a sufficient condition. A necessary and sufficient condition can be obtained if and only if the eigenvalues of a complete two-fluid model with source terms are evaluated. Furthermore, numerical stability of the two-fluid model is also affected by source terms. The examination of source terms is, hence, indispensable to clarify the mathematical and numerical nature of a two-fluid model. In this report, the method to examine the mathematical nature of a complete two-fluid model with source terms is presented. Based on this method, it is revealed that the degree of ill-posedness of the two-fluid model is greatly affected by gravity., The Japan Society of Mechanical Engineers, 1998, Transactions of the Japan Society of Mechanical Engineers. Series B., 64, 624, 2472-2477, Japanese, 0387-5016, 110002397175, AN00187441

Numerical Simulation of CAES Champagne Effect (2nd Report, Preliminary Calculation for Plant Design)
OKAWA Tomio; TANAKA Nobukazu; YAMAMOTO Ryosuke; MATSUI Goichi; MONJI Hideaki
Evaluation of champagne effect is indispensable for successful commercialization of a compressed air energy storage (CAES) system which is one of the most promising candidates of energy storage system. In the present study, we examined optimum design of the CAES system through numerical calculations of the champagne effect based on a two-fluid model. As a result, the following conclusions were obtained. (1) Charging rate of air into a lower reservoir should be gradually changed during startup and shutdown periods to avoid pressure oscillation in the lower reservoir. (2) If the water level in the lower reservoir is too low when the charging is finished, the inertial force makes the water level enter into the U-bend, which also induces the pressure oscillation in the lower reservoir. (3) If appropriate procedures are adopted during the startup and shutdown period of air charging system, the water level in the lower reservoir does not enter into the U-bend. The construction cost can hence be reduced by shortening the U-bend., The Japan Society of Mechanical Engineers, 1998, Transactions of the Japan Society of Mechanical Engineers. Series B., 64, 628, 3964-3969, Japanese, 0387-5016, 110002397383, AN00187441

Numerical Simulation of CAES Champagne Effect (Ist Report, Development of Computer program
OKAWA Tomio; TANAKA Nubukazu; YAMAMOTO Ryosuke; MATSUI Goichi; MONJI Hideaki
Fluctuations of electric power demands in Japan are so heavy that development of electric sources for peak load generation is an important problem. Compressed air energy storage (CAES) system is one of the most promising candidates of energy storage system, and considered to follow pumping-up electric power plants. The CAES system stores compressed air in an underground reservoir using low-cost power from base load plants during off-peak periods. The stored air is used for gas-turbine power generation during peak periods. For successful design of the CAES system, accurate prediction of CAES champagne effect is required to avoid a loss of the stored air caused by blowout. In the present study, a computer program based on a one-dimensional two-fluid model including a simple gas release model was developed for prediction of the CAES champagne effect. This program was applied to simulation of experimental results of a high-solubility carbon dioxide/water laboratory model of the CAES system. The computer program developed was found to show good agreement with the experimental results if the mass transfer coefficient, that determines the gas release rate, was set to 0.4 mm/s., The Japan Society of Mechanical Engineers, 1998, Transactions of the Japan Society of Mechanical Engineers. Series B., 64, 621, 1398-1404, Japanese, 0387-5016, 110002396944, AN00187441

Effects of source terms and virtual mass force on mathematical nature of a two-fluid model(1st report, Effect of gravity)
Tomio OKAWA; Akio TOMIYAMA
1998, Transactions of the Japan Society of Mecanical Engineers(Ser. B), 64, 624, 2472-2477, 0387-5016

Numerical simulation of CAES champagne effect(2nd report, Preliminary calculation for plant design)
Tomio OKAWA; Nobukazu TANAKA; Ryosuke YAMAMOTO; Goichi MATSUI; Hideaki MONJI
1998, Transactions of the Japan Society of Mechanecal Engineers(Ser. B), 64, 628, 3964-3969, 0387-5016

Numerical simulation of CAES champagne effect(1st report, Development of computer program)
Tomio OKAWA; Nubukazu TANAKA; Ryosuke YAMAMOTO; Goichi MATSUI; Hideaki MONJI
1998, Transactions of the Japan Society of Mechanical Engineers(Ser. B), 64, 621, 1398-1404, 0387-5016

Numerical Analysis of Performance of Air-lift pump Combined with the Ocean CAES system
YAMAMOTO Ryosuke; TANAKA Nobukazu; OHKAWA Tomio
Jul. 1996, 年会一般講演, 15, 447-448, Japanese, 1342-8004, 10002906103, AA11169596

二流体モデルに対する高次風上差分法の適用性(第3報:数値解の精度に関する検討)
1994, 混相流, 8, 4, 291-300

二流体モデルに対する高次風上差分法の適用性(第2報:二流体モデル方程式に基づく考察)
1994, 混相流, 8, 2, 126-134

Applicability of high-order upwind schemes to two-fluid(1st report, Examination on the accuracy of numerical solutions)
OHKAWA T.
1994, Japanese Journal of Multiphase Flow, 8, 4, 291-300, 10004114591, AN10088286

Applicability of high-order upwind schemes to two-fluid model(1st report, Examination based on the two-fluid model)
OHKAWA T.
1994, Japanese Journal of Multiphase Flow, 8, 2, 126-134, 10004114590, AN10088286

非圧縮性多次元二流体モデルの数値解法(構成方程式の評価に適する解法)
冨山明男; 片岡勲; 大川富男; 平野雅司
1993, 機械学会論文集B編, 59, 566, 3003-3008, 0387-5016, 110002402717

二流体モデルに対する高次風上差分法の適用性(第1報:線形モデル方程式に基づく考察)
1993, 混相流, 7, 3, 241-249

Numerical solution method for incompressible multidimensional two-fluid model(A method suitable for the evaluation of constitutive equations)
TOMIYAMA A.
1993, Transactions of the Japan Society of Mechanical Engineers(Ser. B), 59, 566, 3003-3008, 0387-5016, 10004114597

Applicability of high-order upwind schemes to two-fluid model(1st report, Examination Based on a linear model equation for the two-fluid model)
1993, Japanese Journal of Multiphase Flow, 7, 3, 241-249

二相流数値解析における質量保存式の選択が数値解に及ぼす影響
1992, 混相流, 6, 4, 395-405

Effect of mass ewuations for two-phase flow analyses on numerical solutions
1992, Japanese Journal of Multiphase Flow, 6, 4, 395-405

Numerical experiment on interaction of solitons describing recurrence of initial state
R. Takahashi; T. Ohkawa
A soliton is the wave which propagates in the form of stable bulk with concentrated energy. Zabusky and Kruskal discovered it in 1965 by solving numerically the K-dV equation, and consequently a variety of analytical methods were established for studying its physical property to introduce the new field of modern physics. This paper succeeds in demonstrating computationally interaction of solitons concerning recurrence of the initial state suggested by Zabusky and Kruskal where ambiguity about their numerical analysis has prevented us from obtaining stable solution representing recurrent phenomenon for a long time. Here numerical experiment enables us to examine instability occasioning in the leapfrog method with a function of the initial condition which served as a key parameter. It is clarified that: (i) this finite difference approximation is available to describe only weakly coupled interaction of solitons, (ii) on linear analysis point of view, instability results from increasing unstable numerical mode of the higher Fourier components which was attributed to dispersive property of this difference method and (iii) instability originates in a sharp solitary wave which was standing still at the point of its birth. © 1989 Springer-Verlag., Springer-Verlag, Jul. 1989, Computational Mechanics, 5, 4, 273-281, English, 0178-7675, 34249975921

Numerical experiment on interaction of solitons describing recurrence of initial state
R. Takahashi; T. Ohkawa
A soliton is the wave which propagates in the form of stable bulk with concentrated energy. Zabusky and Kruskal discovered it in 1965 by solving numerically the K-dV equation, and consequently a variety of analytical methods were established for studying its physical property to introduce the new field of modern physics. This paper succeeds in demonstrating computationally interaction of solitons concerning recurrence of the initial state suggested by Zabusky and Kruskal where ambiguity about their numerical analysis has prevented us from obtaining stable solution representing recurrent phenomenon for a long time. Here numerical experiment enables us to examine instability occasioning in the leapfrog method with a function of the initial condition which served as a key parameter. It is clarified that: (i) this finite difference approximation is available to describe only weakly coupled interaction of solitons, (ii) on linear analysis point of view, instability results from increasing unstable numerical mode of the higher Fourier components which was attributed to dispersive property of this difference method and (iii) instability originates in a sharp solitary wave which was standing still at the point of its birth. © 1989 Springer-Verlag., Springer-Verlag, Jul. 1989, Computational Mechanics, 5, 4, 273-281, English, 0178-7675, 80005115744, 34249975921

Thermal and Nuclear Power Generation
English, Others, Elsevier, 2020

新しい気液二相流数値解析
コロナ社, 2002

Applicability of high-order upwind difference methods to the two-fluid model
Advances in Multiphase Flow, Elsevier, 1995

Applicability of high-order upwind difference methods to the two-fluid model
Advances in Multiphase Flow(A. Serizawa, T. Fukano, J. Bataille, Eds. ), Elsever, 1995

液膜内核沸騰における液滴飛散率の評価
名手 海人; Raka Firman; 大川 富雄; 成島 勇気; 古市 肇; 上遠野 健一
Oral presentation, Japanese, 第28回動力・エネルギー技術シンポジウム
18 Jun. 2024
17 Jun. 2024- 18 Jun. 2024

4 複数のプレートを使用した場合の濡れ速度の増加に対する液体と壁の特性の影響
Akbari Akbari; 梅林宏行; 大川富雄
Oral presentation, English, 第28回動力・エネルギー技術シンポジウム
18 Jun. 2024
17 Jun. 2024- 18 Jun. 2024

飽和プール沸騰の限界熱流束に及ぼす加熱壁内熱伝導の影響
柳田大智; 大川富雄; 小泉安郎
Oral presentation, Japanese, 第28回動力・エネルギー技術シンポジウム
17 Jun. 2024
17 Jun. 2024- 18 Jun. 2024

強制対流サブクール沸騰中の気泡⽣成頻度に及ぼす発泡核間⼲渉の影響
大川富雄; 辻村玲摩
Oral presentation, Japanese, 第61回日本伝熱シンポジウム
29 May 2024
29 May 2024- 31 May 2024

Development an IVR System with Honeycomb Cooling Technology that combines CHF Improvement and Rapid Cooling of High-Temperature Bodies (3) Elucidation of Rapid Cooling Mechanism of High-Temperature Bodies by Nanofluids
Som Onn Ouch; Tomio Okawa
Oral presentation, English, 日本原子力学会2024年春の年会
27 Mar. 2024
26 Mar. 2024- 28 Mar. 2024

Modeling and experimental validation for the relation between large bubble formation and departure from nucleate boiling
Binh Thanh Nguyen; Ryoma Tsujimura; Tomio Okawa
Oral presentation, English, 日本原子力学会2024年春の年会
26 Mar. 2024
26 Mar. 2024- 28 Mar. 2024

二相流CFDに基づく機構論的DNB予測手法の開発 (4)二相流CFDによるボイド率分布解析
小瀬裕男; 大川富雄
Oral presentation, Japanese, 日本原子力学会2024年春の年会
26 Mar. 2024
26 Mar. 2024- 28 Mar. 2024

二相流CFDに基づく機構論的DNB予測手法の開発 (3)強制対流沸騰下における大気泡生成モデルの構築
小野綾子; 大川富雄; 吉田啓之
Oral presentation, Japanese, 日本原子力学会2024年春の年会
26 Mar. 2024
26 Mar. 2024- 28 Mar. 2024

二相流CFDに基づく機構論的DNB予測手法の開発 (1)全体概要と流動沸騰実験
大川富雄; Nguyen Thanh-Binh; 辻村玲摩
Oral presentation, Japanese, 日本原子力学会2024年春の年会
26 Mar. 2024
26 Mar. 2024- 28 Mar. 2024

急収縮ノズルから流出する液噴流の流動特性に関する研究
孫国富; 大川富雄; 青柳光裕; 内堀昭寛; 岡野 靖
Oral presentation, Japanese, 日本原子力学会2024年春の年会
26 Mar. 2024
26 Mar. 2024- 28 Mar. 2024

液膜内核沸騰における液滴飛散割合の評価
名手海人; Raka Firman; 大川富雄; 成島勇気; 古市肇; 上遠野健一
Oral presentation, Japanese, 日本機械学会関東学生会第63回学生員卒業研究発表講演会
13 Mar. 2024
13 Mar. 2024- 13 Mar. 2024

液滴・液膜衝突時に生じる二次液滴の性質
桒原悠人; 大川富雄; 白方嵩晃
Oral presentation, Japanese, 日本機械学会関東学生会第63回学生員卒業研究発表講演会
13 Mar. 2024
13 Mar. 2024- 13 Mar. 2024

鉛直矩形管における気相混入による二相流熱伝達促進
古屋歩; 木村優佑; 小泉安郎; 大川富雄; 西中一貴
Oral presentation, Japanese, 日本機械学会関東学生会第63回学生員卒業研究発表講演会
13 Mar. 2024
13 Mar. 2024- 13 Mar. 2024

二相流CFDに基づく機構論的DNB予測手法の開発
大川富雄
Poster presentation, Japanese, NEXIP交流会, Invited
26 Dec. 2023

強制対流サブクール沸騰中におけるOSV及びDNBメカニズムの新解釈
大川 富雄
Invited oral presentation, 第26.0回相変化界面研究会, Invited
21 Dec. 2023

New interpretations of the OSV and DNB phenomena in subcooled flow boiling
Tomio Okawa; Thanh Binh Nguyen; Ryoma Tsujimura
Invited oral presentation, English, Beiyang Power Forum, Tianjin University, Invited
27 Nov. 2023

ナノ粒子層をウィックとするヒートパイプの伝熱性能に及ぼす充填率と作動流体の影響
王萌蕾; 劉依凡; 大川富雄
Oral presentation, Japanese, 日本機械学会 熱工学コンファレンス2023
14 Oct. 2023

部門設立30周年記念出版 Vol.1 動力エネルギー工学の基礎
大川富雄
Oral presentation, Japanese, 第27回動力・エネルギー技術シンポジウム
21 Sep. 2023

A study of tube inclination and jacket plate effect on freeze plug performance in molten salt reactors
Ilham Muhammad; Tomio Okawa
Oral presentation, English, 日本原子力学会2023年秋の大会
07 Sep. 2023

サブクールフローボイリングにおけるOsvの発生に及ぼす気泡の合体の役割
グエン タイン ビン; 大川富雄; 辻村玲摩
Oral presentation, English, 日本原子力学会2023年秋の大会
07 Sep. 2023

Integration of AI Technology and Thermal Hydraulics for the Development of a Data-Driven Methodology for Plant Safety Assessment (3) Robust condensation bubble feature extraction in subcooled flow boiling using object detection and tracking technique
Wen Zhou; Shuichiro Miwa; Tomio Okawa
Oral presentation, English, 日本原子力学会2023年秋の大会
07 Sep. 2023

人工知能技術と熱流動の融合によるデータ駆動型プラント安全評価手法の開発 (2) 強制対流サブクール沸騰における発泡核間干渉とAI画像解析の応用
辻村玲摩; グエンビン; 大川富雄; 周文; 三輪修一郎
Oral presentation, Japanese, 日本原子力学会2023年秋の大会
07 Sep. 2023

急収縮ノズルから流出する液噴流の流動様式と分裂挙動に関する研究
孫国富; 大川富雄; 青柳光裕; 内堀昭寛; 岡野靖
Oral presentation, Japanese, 日本原子力学会2023年秋の大会
06 Sep. 2023

Study on the effect of air flowrate and subcooled degree on boiling entrainment rate from a liquid film flowing downwards on a vertically heated surface
Raka Firman; Baskara Permana; Tomio Okawa; Nade Kaito; Yuki Narushima; Kenichi Katono; Hajime Furuichi
Oral presentation, English, 日本原子力学会2023年秋の大会
06 Sep. 2023

Comparison of quench speeds of 1 and 2 cooling surfaces using metal plates
アクバリ; 大川富雄
Oral presentation, English, 日本機械学会2023年度年次大会
04 Sep. 2023

グローバル気泡合体機構モデルに基づく大型気 泡の形成と有意なボイドの発生との相関性
グエン タイン ビン; 大川富雄; 辻村玲摩
Oral presentation, English, 日本機械学会2023年度年次大会
04 Sep. 2023

ナノ粒子層によるヒートパイプの伝熱性能の改善
王萌蕾; 劉依凡; 大川富雄
Oral presentation, Japanese, 日本機械学会2023年度年次大会
04 Sep. 2023

液滴・液膜衝突時の二次液滴生成特性
大川富雄; 白方嵩晃; 桑原悠人
Oral presentation, Japanese, 混相流シンポジウム2023
26 Aug. 2023

G221 ナノ粒子層をウィックとするヒートパイプの伝熱性能に及ぼす充填率の影響
王萌蕾; 劉依凡; 大川富雄
Oral presentation, Japanese, 第60回日本伝熱シンポジウム
26 May 2023

Measurement of Boiling Entrainment Rate from a Liquid Film Flowing Downwards on a Vertical Heated Surface
Raka Firman; Baskara Permana; Tomio Okawa; Yuki Narushima; Hajime Furuichi; Kenichi Katono
Oral presentation, English, 日本原子力学会春の年会
14 Mar. 2023

プール沸騰の限界熱流束に及ぼす伝熱面内熱拡散の影響
柳田 大智; 大川 富雄; 小泉 安郎
Oral presentation, Japanese, 日本原子力学会春の年会
14 Mar. 2023

長円形ノズルから流出する液噴流の分裂と液滴スプラッシュに関する研究
孫 国富; 詹 翼; 大川 富雄; 青柳 光裕; 内堀 昭寛; 岡野 靖
Oral presentation, 日本原子力学会春の年会
14 Mar. 2023

Experiment on boiling entrainment from a falling liquid film
T. Okawa, J. Tabuchi, R. Firman, Y. Narushima, H. Furuichi, and K. Katono
Invited oral presentation, English, 1st European-American-Japanese Two-Phase Flow Group Meeting, Invited, International conference
25 Oct. 2022

ナノ流体によるヒートパイプの伝熱性能の改善
王萌蕾, 大川富雄
Oral presentation, Japanese, 熱工学コンファレンス
08 Oct. 2022

強制対流サブクール沸騰における発泡核間の干渉が気泡の発泡頻度に与える影響
辻村玲摩, 大川富雄, グエン・ビン
Oral presentation, Japanese, 熱工学コンファレンス
08 Oct. 2022

Measurement of Bubbly Layer Void Fraction at the Point of Onset of Significant Void in Subcooled Flow Boiling
Binh Thanh Nguyen, Tomio OKAWA, Ryoma TSUJIMURA
Oral presentation, English, 日本原子力学会「2022年秋の大会」
08 Sep. 2022

長円形ノズルから流出する液噴流の分裂挙動に関する研究
孫国富、詹翼、大川富雄、青柳光裕、内堀昭寛、岡野靖
Oral presentation, Japanese, 日本原子力学会「2022年秋の大会」
08 Sep. 2022

プール沸騰CHFに及ぼす伝熱面材質の影響
大川富雄, 中濃昂輝, 小泉安郎, 柳田大智, Feng Yan
Oral presentation, Japanese, 第26回動力・エネルギー技術シンポジウム
14 Jul. 2022

シリカゾルにおけるヒートパイプへの応用及び伝熱性能の影響
劉依凡, Wang Menglei, 梅原裕太郎, 大川富雄, 清水大輔
Oral presentation, Japanese, 第26回動力・エネルギー技術シンポジウム
14 Jul. 2022

ナノ流体によるナノ粒子層をウィックとするヒートパイプの伝熱 性能の改善
王萌蕾、大川富雄
Oral presentation, Japanese, 第59回日本伝熱シンポジウム
20 May 2022

A New Mechanistic Model for Prediction of the Departure from Nucleate Boiling Heat Flux
Thanh Binh NGUYEN, Tomio OKAWA
Oral presentation, English, 第59回日本伝熱シンポジウム
18 May 2022

POSSIBLE ROLE OF BUBBLE COALESCENCE IN CAUSING OSV AND DNB IN SUBCOOLED FLOW BOILING
Tomio Okawa and Nguyen Thanh Binh
Keynote oral presentation, English, Japan-U.S. Seminar on Two-Phase Flow Dynamics, Invited, International conference
11 May 2022

ON SPLASHING DURING DROP IMPACT ONTO A QUIESCENT LIQUID FILM
Tomio Okawa
Keynote oral presentation, English, 32nd International Symposium on Transport Phenomena, Invited, International conference
21 Mar. 2022

現象論的クエンチモデルの開発及び適用性の検討
梅原裕太郎, 大川富雄
Oral presentation, Japanese, 日本原子力学会「2022年春の年会」
17 Mar. 2022

Experimental Study on Onset Condition of Boiling Entrainment from Falling Liquid Film with Gas Sheared Flow
Raka Firman, Junpei Tabuchi, Tomio Okawa, Yuki Narushima, Hajime Furuichi, Kenichi Katono
Oral presentation, English, 日本原子力学会「2022年春の年会」
17 Mar. 2022

インフォパワード・エネルギー概論
2024 - Present

Physics Ⅱ
2021 - Present

熱工学基礎
2021 - Present

熱力学応用
2021 - Present

熱力学
2021 - Present

日本伝熱学会

日本原子力学会

日本機械学会

日本混相流学会

広範囲の燃料格子仕様に適用する汎用沸騰遷移解析手法に関する技術開発
2004

気液界面相互作用モデルに関する研究
2001

A study on analytical models for phase interface
2001

オイラー・ラグランジェモデルを用いた分散性気液二相流数値解析に関する研究
2000

A study on numerical simulation of dispersed two-phase flow using Euler-Lagrange model
2000

超高温体の急冷機能を付与したハニカム冷却技術による新型原子炉のIVR開発
文部科学省, 原子力システム研究開発事業, 九州大学; 電気通信大学, Coinvestigator
Sep. 2023 - Mar. 2026

二相流CFDに基づく機構論的DNB予測手法の開発
文部科学省, 原子力システム研究開発事業, 電気通信大学; 九州大学; 日本原子力研究開発機構; 大和システムエンジニア, Principal investigator
Sep. 2023 - Mar. 2026

ナノ粒子層と旋回流を併用する超高熱流束除熱技術の開発 補助事業
公益財団法人JKA, Principal investigator, 2023M-387
Apr. 2023 - Mar. 2025

高温壁の液膜冷却時におけるクエンチ速度予測モデルの高度化
中部電力株式会社, Principal investigator
Apr. 2023 - Mar. 2025

Development of mechanistic CHF model in subcooled flow boiling
大川 富雄
Japan Society for the Promotion of Science, Grants-in-Aid for Scientific Research, The University of Electro-Communications, Grant-in-Aid for Scientific Research (C), 強制対流サブクール沸騰における限界熱流束(CHF)の機構論的予測では、小気泡の合体により形成される大気泡と加熱壁の間に薄液膜が形成され、これが消失すると壁温の急上昇を開始すると仮定する。これをLiquid Sublayer Dryoutモデル（以下LSDモデルと称する）と呼び、広範囲の実験条件で得られたCHFデータをよく予測することが知られている。ここで、LSDモデルに基づくCHFの予測では、大気泡の長さLB、大気泡の上昇速度UB、薄液膜の厚さdの三つを適切に与える必要があるが、これらの物理量に関する実験情報は現状では皆無に近い。このため、既存のLSDモデルでは、まず開発者が適当と考える方法でLB,UB,dを評価した上でCHFを計算し、CHFの計算結果を実験データと比較することでモデルの妥当性を検証している。この場合、CHFの計算値が実験データと一致していても、LB,UB,dの各々を正確に評価できている保証はない。この意味で、既存のLSDモデルは経験則の範疇を出ていない。したがって、通常の経験的相関式と同様に、モデル開発に用いた実験データベースの条件範囲外では、大幅な予測精度の低下を覚悟する必要がある。そこで本研究では、透明伝熱面、IRカメラ、高速度カメラ、レーザー変位計を用いた強制対流サブクール沸騰実験装置を構築するとともに実験データの収集作業を実施し、CHF状態に移行するときの熱流動状態がLSDモデルの想定と整合することを確認した。また、基本的な実験条件において、LSDモデルの根幹となる三つの物理量、すなわちLB,UB,dが計測可能であることを実証した。, 22K03940
Apr. 2022 - Mar. 2025

液噴流・液膜衝突時の液滴生成に関する研究
日本原子力研究開発機構, 日本原子力研究開発機構, Principal investigator
Apr. 2023 - Mar. 2024

Clarificaion of the relation between wetting front propagation velocity and pool boiling CHF
小泉 安郎; 大川 富雄
Japan Society for the Promotion of Science, Grants-in-Aid for Scientific Research, The University of Electro-Communications, Grant-in-Aid for Scientific Research (C), Coinvestigator, 核沸騰による除熱は、きわめて高い熱伝達率を示す高効率の冷却手法であり、その応用が様々な分野で期待されている。ただし、限界熱流束（CHF）を超過すると、沸騰様式が膜沸騰に移行して、熱伝達率が大きく低下する。このため、より高いCHF値を示す高性能伝熱面の開発が、世界各所で精力的に行われている。本研究では、伝熱面の表面性状によってCHF値が大きく変化する主要因と考えられる因子として、各種伝熱面の濡れ進展速度を計測し、別途計測するプール沸騰CHFとの関係を調べる。得られた結果より、濡れ進展速度を主要変数とするCHF相関式を開発することを目的としている。本研究により、伝熱実験をせずとも、各種伝熱面のCHFを予測することが可能となるため、CHF向上化方策の方向性が明確となり、より高いCHFを示す高性能伝熱面の開発を、効率的に行うことが可能となると期待できる。本研究の初年度である令和3年度には、様々な伝熱面に対して、濡れ進展速度とCHFの計測を可能とするため、板状の伝熱面を設置可能な試験容器を製作するとともに、その下部に高強度のレーザー光を照射することで、沸騰開始から限界熱流束に至るまでの範囲で、沸騰曲線を得るための準備を整えた。また、通常の伝熱面を用いてプール沸騰実験を実施することにより、広く知られている核沸騰熱伝達相関式とよく一致する結果が得られることを確認した。これより、本実験装置により、十分に高精度の実験を実施可能であることを確認した。, 21K03895
Apr. 2021 - Mar. 2024

Development of data-driven plant safety evaluation method by fusion of artificial intelligence technology and thermal hydraulics
三輪修一郎
文部科学省（東京大学からの再委託）, 原子力システム研究開発事業, 東京大学、日本原子力研究開発機構、株式会社原子力エンジニアリング, Coinvestigator
2021 - 2023

ヒートパイプ製造方法
Patent right, 大川 富雄, 王 萌蕾, 特願2019-085579, Date applied: 26 Jan. 2019, 国立大学法人電気通信大学, 特許第7284988号, Date registered: 24 May 2023

シリンジポンプ装置
Patent right, 榎木光治, 宮田一司, 秋澤淳, 大川富雄, 特願2017-072214, Date applied: Mar. 2017, 特開2018-173038, Date announced: Nov. 2018, 特許第6899132号, Date registered: 16 Jun. 2021

フリーズバルブ、原子炉、および太陽熱発電装置
Patent right, 大川富雄, インダルタ・クンコロ・アジ, 木下幹康, 吉岡律夫, 特願2019-092316, Date applied: 2019

ボイラ
Patent right, 大川富雄, 榎木光治, 特願2016-242453, Date applied: Dec. 2012, 特開2018-96631, Date announced: Jun. 2018

冷却剤
Patent right, 大川 富雄, 梅原 裕太郎, 榎木 光治, 特願2018-044725, Date applied: Mar. 2018

冷却剤、冷却装置および冷却方法
Patent right, 大川 富雄, 梅原 裕太郎, 榎木 光治, 特願2017-046260, Date applied: Mar. 2017

気液二相流シミュレーションの計算方法、プログラム、および装置
Patent right, 大川富雄, 杉中隆史, 湊明彦, 特願2012-149590, Date applied: Jul. 2012, 特開2014-013424, Date announced: 23 Jan. 2014

逆熱対流混相流体
Patent right, 片岡勲、他5名, 特願2000-69510, Date applied: 2000, 特開2001-262133, Date announced: 2001