STUDY ON TIME-SPATIAL CHARACTERISTICS OF HEAT TRANSFER BY VISUALIZATION OF INFRARED IMAGES AND DYE FLOW

平成10年度～平成12年度科学研究費補助金(基盤研究C)(2)研究成果報告書研究概要：大きなスケールの渦の流動を染料で可視化,それに対応する温度場を赤外線映像装置で瞬時に測定し,時間・空間的に変化する伝熱現象を調べた.まず縦渦は円管内に三角形翼を傾けて設置することで発生させた.圧力分布,等速度分布,摩擦係数,熱伝達率の周方向変化より,主縦渦が旋回しながら流動する現象が明らかになった.渦の吹き下ろし部で熱伝達が促進され,逆に吹き上がり部において減少する.このような場で熱流束と摩擦係数との非相似性が発生している.流路内に鈍頭物体を隙間を変えて置くことでカルマン渦,再付着流れ,および隙間流れが干渉する場を得た.カルマン渦が放出される場合は巻き上げられた側渦はカルマン渦と逆向きに下壁面にDown washしたのちに,ちぎれて下流に流れる.温度は側渦が立ち上がろうとする位置で最大となり,渦がDown washするところで急激に低下する.とくにカルマン渦に巻き込まれながら渦芯が大きくなるが,それは中央部のフレッシュな流体を渦芯内に巻き込んでおり,それが下面にDown washする時に壁面は一段と冷却される.隙間が小さくなると,カルマン渦が放出される場合とそうでない双方が存在し,伝熱の特徴はそのどちらかを示す双安定的な性状となる.さらに隙間が小さくなると,壁面をスイープする流れが熱伝達率を高めており,その温度変化は不規則で卓越周波数は存在せず,側渦による伝熱の促進とは明らかに異なる伝熱機構である.物体を壁面においた場合は再循環領域が形成される.大きなスケールの渦が衝突する周期は一定ではなく,再付着点近傍では大きなスケールの渦が下面に再付着すると表面は冷やされ,その後渦が下流にちぎれて流れるのに追従して低温度域は下流に移動し,つぎの流体塊が再付着するまで,少し高めの温度分布となる.その状態を繰り返しており,付着,離脱を繰り返すことによって伝熱は促進される.The effect on time-spatial heat transfer characteristics of large-scale vortices was investigated by means of observing both dye flow and infrared images. Longitudinal vortices were artificially generated by a single winglet vortex generator in a pipe. The patterns of iso-velocity, static pressure, friction factor, and heat transfer coefficients show good correspondences to phenomena of the main vortex spirally flowing downstream. The quantitative analogy between the heat transfer and the shear stress is confirmed except for some regions, where the effects of the down-wash or blowaway of the secondary flows is caused by the main vortex rotating on its own axis on the surface at the wall. When the blunt body placed at the center of the duct height, the side vortex is observed to occur at the wall in an interlocking motion with a Karman vortex. These periodical phenomena of growing side vortices play a role in the increase of heat transfer. At narrow clearances, Karman vortex was not shedding, the maximum heat transfer is near the separation point of flow after it has passed the clearance space. When the body was attached to the duct surface, the vortices in the shear layer flow in good order to downstream direction, then combine with each other in downstream region, and after that large vortex attaches the surface. Near the reattachment point, the temperature shows high and low values alternatively. Attaching vortices to the surface caused to decrease the temperature on there, and the low temperature region moves downstream according to the vortices. The reattachment point was kept a high temperature until next vortex attaches to there successively. The heat transfer was augmented by the reattachment and detachment of large-scale vortices.未公開：P.8以降（別刷論文のため

Heat Transfer in a Rectangular Duct with Thin Plates Inserted in Staggered Arrangement

This paper investigates how the local heat transfer coefficients and the friction facter are affected by the pitch and the clearance between thin plate and duct wall when thin plates (thickness: 5 mm, length: 20 mm) are set in a staggered arrangement in a rectangular duct in order to augment heat transfer from the duct wall. Average heat transfer coefficient in fully developed region increases with a decreasing pitch and attains a maximum at P=4 (P: pitch), and its value is two times of that in smooth duct; the friction facter $\lambda$, on the other hand, increases five times at P=4, two times at P=12 in comparison with value of smooth duct. The degree of augmentation is also analysed taking account of pressure loss. In the low-Reynolds number turbulent regime, $\eta$, which is the ratio of the average heat transfer for the present system to that for a smooth duct under the condition of equivalent pumping power, is larger than unity

An Experimental Investigation of Temperature Effectiveness of an Adiabatic Flat Plate Covered by a Three-Dimensional Wall Jet Flow. 2nd Report : Wall Jet Flow Issuing from Binary Square Nozzle

In the previous report, Experimental results of the jet development and the temperature effectiveness on the adiabatic flat surface in the three-dimensional wall jet flow are discussed. This paper deals with a continuous experimental study in which the flow and the temperature fields on the adiabatic flat surface in the threedimensional, incompressible, turbulent wall jet flow issuing from two square nozzles are discussed. Maximum velocity decays and growthes of half-velocity widthes on both central lines of the nozzle and the flow, growth of equi-velocity lines, and distribution of the temperature effectiveness are reported with varying the central distance of the nozzles, the injection velocity and the temperature of the injection jet. The flow fields of such jets are found to be characterized by two distinct regions ; One shows two peaks flow and the other shows one peak flow which is observed at far downstream. The flow patterns as well as the temperature distribution differ at both regions. Transition points between these two situations are much depending on the central distance of the nozzles, but less on the initial velocity. Flow visualization, with using oil-film technique, has been tried to observe and grasp the induced flow situation (or entrainment of surrounding fluid) along the wall, and reasonable explanation for the velocity defects in the neighbourhood of the potential core regions could be obtained by its photographs

The Effects of Two Dimensional Thin Wire Placed within Laminar Boundary Layer on Heat Transfer in a Flat Plate.

This investigation was carried out to clarify the effects of two dimensional thin wire, placed within laminar boundary layer on the local heat transfer rate using two dimensional wind tunnel. The temperature differece between main flow and the plate was made by heated plate under the condition of a constant heat flux, and from its resalts the local heat transfer coefficient in a flat plate was calculated. especialy the thin wire was attached a flat plate, we measured the intencity of turbulence by the hot wire anemometer in the boundary layer and main flow. Comparing the results heat transfer and turbulance, we found that the intentensity of turbulence and its region were larger, the local heat transfer rate in its region was augmented, and laminar flow was transformed into the turbulent flow just to behind the wire. and placed in the same condition of gradient of velocity, the wire diameter was larger, the local heat transfer rate was more increased in the region behind the thin wire. However the wire diameter was in certain size, the velocity gradient was not influence on heat transfer