高垣 直尚

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．

<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>

<p>The suppression of wind waves by surfactants and other chemicals has been confirmed for a long time. Because of the complexity of the phenomena, the suppressing factors and the mechanism are not completely clear. In this experiment, using three surfactants and three other chemicals, we investigated the effects of surface tension and viscosity on the wave height fluctuation in a small wind-wave tank. As a result, it was suggested that the wave height fluctuation is also strongly affected by dynamic surface tension and dynamic viscosity (viscoelasticity) other than static surface tension and static viscosity.</p>