河南 治

Abstract This paper presents an experimental investigation of local heat transfer characteristics of single-phase flow in a plate heat exchanger (PHE). The local heat transfer coefficient is evaluated using a test section with PHE geometry for measuring wall temperature distribution. The test section of 1.5 mm thickness is employed to consider the heat conduction effect of the heat transfer plate. The results indicated that the local heat transfer coefficient is influenced by the development of the thermal boundary layer along the flow direction and the maldistribution of water flows along both the direction perpendicular to the flow and the stacking direction. The harmonic mean heat transfer coefficient calculated by the measured local heat transfer coefficient agrees with the average heat transfer coefficient evaluated by the modified Wilson plot method within ±25% and within ±16% for the hot side and the cold side, respectively.

小型熱交換器の伝熱性能を向上させる手法の一つとして，熱交換器内部に渦発生体を設置し，流体の混合による温度境界層の破壊を行う方法が有効である．とくに，流れ方向に回転軸を持つ縦渦構造は，流路内で渦が崩壊しにくいため，高い伝熱促進効果が期待されている．本研究では渦発生体配置が流れの挙動変化と熱輸送性に与える影響について知見を得ることを目的とし，流路内の三次元流れと熱伝達の数値計算結果について報告する．

熱伝達性能を向上させるうえで，圧力損失増大の問題は不可分の関係にある．本研究対象のディンプルは，窪みというシンプルな形状であるため，低圧力損失で高伝熱性能が得られることが特徴である．すでに実機にも使用されているが，ディンプルの伝熱効果に関しては未知な部分が多い．高熱伝達を得るためには，Ligraniら¹⁾が行ったようにディンプルの生成する流れ構造を把握することが重要であるが，この評価方法は定性的であるため，他条件との比較を行うのが困難である．本研究はディンプルの基礎研究として，平板上に単一ディンプルを設置し，生成される流れの攪拌作用についてステレオPIVを用いて定量的調査を行った．実験は，バルクRe数=13,000程度の乱流場で行い，3種類のディンプル形状が生成する流れ構造を比較した．その結果，とくに実験ではこれまで未解明であった，ディンプル内部から外部における3軸方向速度成分とレイノルズ応力分布および以下の知見が得られた．円型，楕円型ディンプルの結果は，Acharyaら²⁾と同様，スパン方向中央軸上のディンプル後縁部を中心にレイノルズ応力分布が最も高くなった．Khooら³⁾は，この流れの混合現象が後壁に生じる摩擦係数に比例することを示し，これを得るには相応の圧損が生じることを主張した．しかし，本実験において円型以外のディンプル形状の結果を比較したところ，生成される流れの差異がより顕著にみられたのは，中心軸上ではなくスパン方向寄りのディンプル端断面上であり，その発生要因は後壁との衝突に依らないという結果が得られた．下図は，スパン方向中央断面から0.28D（楕円型）,0.39D（円型，涙型）離れた断面上のレイノルズ応力分布と時間平均流速を断面上の成分はベクトル，断面法線方向成分はコンター図で示した．（D : dimple print Diameter）円型ディンプルに対し，楕円型，涙型ディンプルは後縁部上から後方へかけて広くレイノルズ応力の分布が確認された．また同じ位置では，平均流速結果からディンプルの後壁に沿って底面垂直方向の速度成分が楕円型に若干，涙型に強くみられ，その付近でスパン方向速度成分の変動もみられた．このベクトルおよびコンターの変化もディンプル後方まで生じたことから，持続性のある3方向成分の流れ構造が楕円型および涙型ディンプルで生成されたといえる．

In planned planetary explorations, cryogenic liquids such as liquid hydrogen (LH2), liquid oxygen (LOX), and liquefied natural gases (LNG) are used as fuel and oxidants in the propulsion systems of spacecraft. Such explorations require long-term storage of those cryogens, as well as heat insulation technology to protect the heat from the outside and pressure control technology to suppress rises in pressure due to gas evaporation in the propellant tank. However, current vent systems that discharge the evaporated gas to the outside of the spacecraft are suboptimal because the propellant's uncertain position in the tank when the spacecraft operates in microgravity environments causes a significant loss of propellant during venting. In response, we examined a method using a mixing jet in a thermodynamic vent system (TVS) that adjusts the tank pressure by cooling the inside of the tank and reducing boil-off gas.In this study, a verification experiment on the TVS performed by jet mixing using simulated liquid (LN2) was carried out. Subcooled mixing jets were supplied under vent-free conditions, and it was possible to reduce the temperature and pressure in the test tank. On the other hand, it was found that depending on the liquid level and supply flow rate of the mixing jet, the tip of the mixing jet could not reach the free surface, and reduction of tank pressure could not be realized.

© 2020 We developed a simultaneous measurement technique for temperature distribution on a heated surface and gas-liquid interface motion of two-phase flows in microgap channel using temperature-sensitive paint (TSP). Since the TSP layer is optically transparent, the proposed method enables us to simultaneously measure the temperature distribution and the motion of gas-liquid interface through the TSP layer using an in-house stereoscope-like device. Since the refractive index of gas-phase differs from that of liquid-phase, the luminescent intensity of TSP in gas-phase varies from that in liquid-phase even though the temperatures are same in both phases. Then, we proposed a correction method for temperature distribution in gas-phase. To validate the proposed method, the temperature distribution measurements were conducted in liquid single-phase flow. The Nusselt numbers calculated from the measured temperatures were in good agreement with the Sieder-Tate equation. Then, we measured the temperature distributions for the boiling flows under several heated temperature conditions. The temperature distributions at growing bubble and bubble coalescence were successfully measured. TSP is useful method for investigating the temperature distribution in two-phase flows.

© 2019 Elsevier Ltd Boiling heat transfer using immiscible liquid mixtures is an innovative cooling method for electronic devices operated at high heat flux density. Immiscible liquid mixtures discussed here are composed of more-volatile liquid with higher density and less-volatile liquid with lower density such as combination of FC-72 and water. In the case of pool boiling in a vessel using appropriate composed ratio of immiscible liquid mixtures, more-volatile liquid on the heating surface is boiled and as the heat flux increases, the more-volatile liquid reaches the critical heat flux. Subsequently, the less-volatile liquid is replaced with the more-volatile liquid and moves onto the heating surface, and this liquid is started boiling. This phenomenon is called a boiling refrigerant transition (BRT) and is an important feature of pool boiling by immiscible mixtures. To clear the phenomena of BRT in pool boiling by immiscible mixtures, the experiments under the conditions of various heights of more-volatile liquid layer and various mixture composed ratio by using several combinations of immiscible mixtures are carried out. And a new model derived by Kelvin-Helmholtz instability at the liquid-liquid interface is proposed for the occurrence of BRT. The unique heat transfer characteristics of the immiscible liquid mixtures were obtained from the experimental results; occurrence of BRT depends on the height of more-volatile liquid, and the characteristics after BRT is corresponding to the characteristics of pure less-volatile liquid. Experimental results including data from past literatures agreed well with newly proposed model.

© 2020, Springer Nature B.V. In planned planetary explorations, cryogenic liquids such as liquid hydrogen (LH2), liquid oxygen (LOX), and liquefied natural gases (LNG) are used as fuel and oxidants in the propulsion systems of spacecraft. Such explorations require long-term storage for those cryogens, as well as heat insulation technology to protect the heat from the outside and pressure control technology to suppress rises in pressure due to evaporation gas in the propellant tank. However, current vent systems that discharge the evaporated gas to the outside of the spacecraft are suboptimal, for the propellant’s uncertain position in the tank when spacecraft operate in microgravity environments causes the significant loss of propellant during venting. In response, we examined a method using jet mixing in a thermodynamic vent system (TVS) that adjusts the tank pressure by cooling the inside of the tank and reducing boil-off gas. To that end, thermal and fluid analyses were conducted to design a test tank system before the ground verification test of the TVS using liquid nitrogen (LN2) as the simulated cryogen. To evaluate our results, we compared experimental and numerical results regarding the formation of thermal stratification. The experimentally performed verification of the TVS function revealed that jet mixing can lower the liquid’s temperature in the tank after the experimental apparatus was modified to supply a stable subcooled mixing jet.

© 2019 Published under licence by IOP Publishing Ltd. In the future space exploration mission, transfer vehicle using cryogenic propellant and oxidizer will be likely operated. Thermal management systems, such as thermal insulation, pressure control, and reduction of boil off gas must be installed on transfer vehicle since cryogenic fluid has high volatile characteristics. Final target of this study is the development of thermodynamic vent system (TVS) utilizing jet mixing. Numerical simulation and ground based experiment for thermal and fluid behaviours in tank partially filled with liquid nitrogen were conducted. Thermal stratification in tank was formed by creating heat input from side wall, and change of temperature field and break of thermal stratification by jet mixing were investigated. Numerical results were evaluated by comparing with experimental results. © Published under licence by IOP Publishing Ltd.

In the future exploration for deep space, cryogenic fluid will be used as propellant and oxidizer for spacecraft. The increasing of tank pressure is caused by boil off gas (BOG) induced by heat leakage into the tank from the surrounding environment. Reducing of BOG is derived from destroy of thermal stratification of a bulk liquid in the t ank. Therefore, development of Thermodynamic Vent System (TVS) for the purpose of prevent of loss of cryogenics propellant in future space system is needed. TVS combining jet mixing, spray, and heat spot removal by forced cooling using Electro-hydro-dynamics is under consideration by authors. Destroying thermal stratification without venting by using subcooling mixing jet will be developed for a key technology of TVS. In this paper, the behavior of the mixing jet by visualization method and description about the motion of the jet with a simple one-dimensional model without heat transfer is reported. In our simple dynamic model, a single sphere droplet as the tip of the jet is assumed and an equation of the motion is applied for the droplet. The results of analysis model and experimental data taken by shadowgraph system are compared. In addition, microgravity experiment is performed, and flow behavior in microgravity is described.

<p>In recent years, EGR coolers, which are heat exchangers for automobiles, are required to improve heat transfer performance and reduce size and weight for the purpose of improving fuel efficiency and reducing nitrogen oxides. Previous studies have revealed that longitudinal vortices induced by vortex generators installed near the surface of the heat exchanger promote heat transfer from the main vortex to the boundary layer. In this study, the heat transfer performance by the shape of the vortex generator (VG) using CFD is compared for the flow in the rectangular pipe channel. Moreover, the effect of the continuous shape is clarified from the calculation result of the case where the VG is installed in the offset fin channel.</p>

<p>The micro-tube heat exchanger efficiently exchanges heat by flowing fluid into the flow path and small diameter tube. In recent years, thanks to 3D printers, it has become possible to easily mold even shapes that could not be realized in the past. Based on these points, we investigated the flow of the rhomboid tube group that could not be created before and the effect of self-excited vibration on the temperature field.</p>

<p>EGR coolers installed in automobiles are required to improve heat exchange performance and to be smaller and lighter. Conventionally, a method has been adopted in which heat transfer fins generate vortices to promote heat transfer. In recent years, however, soot accumulation in exhaust gas has been regarded as a problem. Therefore, the purpose of this study is to clarify the relationship between the heat flow and the flow field formed by the shape and flow path arrangement of the flow path with the protruding vortex generator, and compare the performance with the conventional product.</p>

<p>The micro-tube heat exchanger efficiently exchanges heat by flowing fluid into the flow path and small diameter tube. In recent years, thanks to 3D printers, it has become possible to mold easily even shapes that could not be realized in the past. Based on these points, we investigated the flow of the rhomboid tube group that could not be created before and the effect of self-excited vibration on the temperature field.</p>

<p>Micro-tube heat exchangers are often used for heat exchange between fluid and gas. This heat exchanger is equipped with fine circular tubes to perform heat exchange efficiently and the miniaturization of tube diameter and the increase of installation density are promoted with the recent development of processing technology. However, the miniaturization of the circular tube leads to an increase in the wall thickness, it is necessary to calculate the temperature considering not only the heat transfer in the radial direction of the pipe but also the heat transfer in the axial direction of the pipe. Therefore, in this analysis, numerical analysis of tubes with wall thicknesses was conducted, and the flow characteristics of the fluid and the heat transfer characteristics inside the tube were investigated.</p>

<p>When dimples are installed on the heat transfer surface of the heat exchanger, the heat transfer efficiency is expected to be improved because the flow is mixed while suppressing the pressure loss. However, the performance of single dimple, which is the most basic, has not been clarified yet. In this study, for the purpose of quantitative evaluation of turbulence property generated by circular dimple in a flow field, local flow velocity was measured using PIV. As a result of evaluating Reynolds stress in three-dimension around the dimple, the stirring effect generated in dimple was confirmed.</p>

<p>EGR cooler is a heat exchanger that reduces NOx emissions and improves fuel consumption of cars. Up to now, fin shape Vortex generator have been used to improve the heat transfer performance, but in recent years, raised dimple shape have attracted attention. Although this dimple is effective for reducing of soot deposits, it does not generate vortices that improve heat transfer performance. The purpose of this research is to compare the shapes of protrusions using CFD for cost reduction, and propose the shapes of protrusions that meet the requirements.</p>

<p>Heat exchangers are installed in the cycle of car air conditioners to improve the fuel efficiency of automobiles. This higher performance of the internal heat exchanger leads to further improvement of fuel consumption and reduction of the influence of global warming. As the way to do it, research and development of laying dimples inside equipment is attracting attention for the purpose of increasing the heat transfer area and disturbing the fluid with low pressure loss to improve the heat exchange efficiency. However, there are still many unclear matters such as optimum arrangement and optimum shape of dimples, and the Reynolds number of the actual machine is on the order of hundreds of thousands, so it is necessary to clarify the influence of dimples on the turbulent flow field. Therefore, in this research, numerical analysis was performed by using in-house code in the channel where multiple dimples were laid to clarify the effect of laying dimples and the relationship between turbulence statistics and heat transfer characteristics. As a result, it was found that the Nusselt number at the bottom and the Reynolds stress in the vicinity become larger near the dimple trailing edge. In addition, it was found that the magnitude of the local Nusselt number between dimples in the center of the flow field and the peak value of Reynolds stress near the bottom decrease as going downstream.</p>

<p>Boiling heat transfer using immiscible liquid mixtures is an innovative cooling technique for high-heat-density electronic devices. Immiscible liquid mixtures which are composed of more-volatile liquid with higher density and less-volatile liquid with lower density such as combination of FC-72 and Water are discussed here. In the case of pool boiling using immiscible liquid mixtures, more-volatile liquid on the heating surface is started boiling firstly, and then the more-volatile liquid reaches the critical heat flux as the heat flux increases. Subsequently, the less-volatile liquid is replaced with the more-volatile liquid and moves onto the heating surface, and this liquid is started boiling. This phenomenon is called a Boiling Refrigerant Transition, and the characteristics of heat transfer and flow behavior during Boiling Refrigerant Transition are not clear. Therefore, here the experiments under the conditions of various heights of more-volatile liquid layer and various mixture composed ratio by using two combinations of immiscible mixtures are carried out. The experimental results show that characteristics of heat transfer including Boiling Refrigerant Transition are only depending on the height of more-volatile liquid layer. And the relation between the observation of boiling behavior on the heating surface and characteristics of heat transfer is discussed.</p>

Copyright © 2018 by the International Astronautical Federation (IAF). All rights reserved. A series of systematic experiments was performed using a boiling and two-phase flow loop under the microgravity condition provided by the International Space Station. Based on the understanding of gas-liquid interfacial behaviour and highly reliable data on heat transfer characteristics from this experiment, we clarify the details of elementary processes concerning boiling two-phase flow under the microgravity condition. This experiment also demonstrates a boiling cooling system in space. Through this experiment, we acquired data useful for designing a high-performance and compact thermal control system, aiming to develop it into an innovative space platform. The microgravity experiment was performed from July 2017 to March 2018. The experimental facility worked well, and a lot of data were able to accumulate. Under the certain regime of experimental condition, it was completely different from the behaviour of boiling bubbles on the ground. As a result, heat transfer also showed unique characteristics.

© 2018 International Heat Transfer Conference. All rights reserved. Boiling and Two-phase flow in narrow channel has been recently proposed for cooling the heat sources directly in application of thermal management for electronic devices. Many researches were carried out to investigate the mechanism of heat transfer in the narrow channel. In this study, the boiling heat transfer and gas-liquid behavior on the heated surface in narrow channel is investigated experimentally by using Temperature-Sensitive Paint (TSP). Experiments are performed in a single rectangular narrow channel having 10 mm width, 35 mm heated length and 0.5 mm height and using FC-72 as a test fluid. TSP coated on the heated wall located at upper side of the channel is used to obtain the local temperature and subsequently local heat flux distribution. In addition, simultaneous flow visualizations are conducted to observe and explore the flow boiling in narrow channel. Flow rate of FC-72 is 24-30 ml/min and heated water at 45-50 ◦C is used as a heat source for boiling of the test fluid. Pressure at the inlet of the test section is about 50 kPa. As a result, the heat flux in the thin liquid film region that is located around bubbles is 4-5 times higher than that of other region.

© 2018 by Begell House. Although the gas–liquid interface in boiling flow is very important, it is difficult to clearly show the relationship between the gas–liquid interface in the channel and heat transfer characteristics. The interfacial area concentration (IAC) is one of them, however its measurement method is not easy. Here, a two-dimensional projected gas–liquid interfacial area (2D-PGIA) is proposed as a simple index. 2D-PGIA is an index obtained by image processing from a video image which is two-dimensional information, and other parameters such as velocity vector of bubbles is not considered. Flow boiling experiments using vertical 4-mm-diameter transparent heated tube were carried out to investigate the relationship between heat transfer characteristics and 2D-PGIA. From the result analyzed images in bubble, slug, and annular flows, it is clear that the 2D-PGIA time profile well follows the temporal change of the inner wall temperature in all flow regimes. Especially in bubble flow, heat transfer increases as 2D-PGIA increases, 2D-PGIA reaches the maximum value just before DNB, and then 2D-PGIA drastically decreases when DNB occurs. From our experimental results, 2D-PGIA will be a useful index for understanding boiling heat transfer in a tube.

The payload capacity of launch vehicles must be increased in order to extend space exploration and development beyond low-Earth orbit into the solar system. A propellant system using a cryogenic fluid such as liquid oxygen or liquid hydrogen must reduce the amount of unusable propellant due to evaporation and boiling. However, in space exploration and development, where the safety and reliability of missions are critical, predictions of boiling heat transfer using existing technology are not sufficiently reliable for thermal management design, given the lack of pertinent knowledge and relevant research. Therefore, the objective of this research is to understand and accurately predict boiling heat transfer by developing numerical simulation tools for two-phase flows that consider phase change. This paper presents a recent research activity toward the development of chill-down process simulation technology. The cryogenic chilldown experiment conducted in a vertical pipeline and in a complicated channel was verified via simulation to show the effectiveness of the simulation tool under development.

Experiments were performed to verify the performance of experimental apparatus for the acquisition of reference data for flow boiling heat transfer under the terrestrial condition which is to be compared with that obtained under the microgravity condition onboard International Space Station (ISS) by using another apparatus with the same specification. Test section is a circular tube made of copper with an inner diameter of 4 mm and a heated length of 368 mm and oriented vertically on ground. To improve the accuracy of local heat fluxes, the compensation of heat flux distribution along the tube axis is discussed on the basis of the experimental results on the local heat transfer coefficients for a single-phase liquid flow. Correlations for local heat transfer coefficient of flow boiling are proposed here as functions of boiling number and Martinelli parameter in the regions of nucleate boiling and two-phase forced convection, respectively. Because the discrepancy of local heat transfer coefficient obtained from the apparatus for the terrestrial and the space experiments is caused by the difference of surface roughness in nucleate boiling region, a compensation factor is introduced in the correlation. The local heat transfer coefficients predicted by the proposed correlation are agreed well with those obtained by both apparatus.

<p>The payload capacity of launch vehicles must be increased in order to allow the exploration and development of space to be extended from low-Earth orbit into the solar system. A propellant system using a cryogenic fluid must reduce the amount of unusable propellant due to evaporation and boiling. However, in space exploration and development, where safety and reliability of missions are critical, predictions of the boiling heat transfer of current technology are not sufficiently reliable for thermal management design due to a lack of knowledge and relevant research. Therefore, the objective of this research is to understand and accurately predict boiling heat transfer by developing numerical simulation tools for two-phase flows that consider phase change. In this paper, recent research activity toward the development of chill-down process simulation technology is presented.</p>