Katsuo Komori

Numerical modeling on turbulent transport with combined forced and natural convection between two vertical parallel plates

The present study was conducted to numerically investigate the transport mechanism of turbulent combined convection between two vertical parallel plates that were uniformly heated. The aiding and opposing flows were simulated by two-equation turbulence models with combined velocity and temperature fields under various Reynolds and Grashof numbers for Pr = 0.71. It was revealed from a series of simulations that there is good correlation between the predicted and the empirical heat transfer ratios. The characteristic behaviors of heat transfer were also analyzed through a simulation of the transport mechanism. The authors confirmed that a two-equation model is adequate to tentatively predict heat transfer in engineering applications.

Heat transfer and fluid flow of natural convection along a vertical flat plate in the transition region : experimental analysis of the wall temperature field

Abstract Heat transfer and fluid flow of natural convection along a vertical flat plate were experimentally investigated in the transition region. Local heat transfer coefficients along a vertical flat plate were measured to distinguish the first transition region. The wall temperature and fluid flow were then visualized using a liquid crystal sheet and water-soluble fluorescent paint. Also discussed are the characteristic statistical quantities with the aid of visualizations for the velocity and temperature fields. It was revealed from a series of experiments that horseshoe-shaped low-temperature patterns appear on the wall and that they play a significant role in heat transfer. When the data was ensemble-averaged it was found that the characteristic time-and-space scales of the patterns are statistically independent of not only heat flux but also the position in which they occur. Moreover, W-shaped flow patterns, which possess three-dimensional and unstable structures, appear in the near-wall region. They play a significant role in the laminar to turbulent transition.

Characteristics of Heat Transfer and Fluid Flow in the Rectangular Duct with a Rotating Cylinder.

An experimental investigation was performed to study the effect of a rotating cylinder near the wall of a rectangular duct on heat transfer coefficient and fluid flow of forced convection. Local heat transfer coefficients were first measured by varing the rotational speed and the distance between cylinder and wall. The results showed that the positions of maximum and minimum in the profiles of the heat transfer at a fixed distance from the wall were almost invariable even when the cylinder was rotating. The wall temperature and fluid flow were also visualized using a liquid crystal sheet and fluorescent paint. It was noted that low-temperature streaks appeared on the wall, and that they strongly affected heat transfer characteristics. It was further determined that the position of the reattachment shifted upstream as rotational speed increased.

Wall temperature pattern and heat transfer of transient natural convection along a vertical flat plate

Transient natural convection along a vertical flat plate was investigated experimentally. To find out the fundamental characteristic of the transient region, wall temperature patterns on the heated wall were visualized using liquid crystal sheet and were analized by image processing. It is revealed from the visualization that the horseshoe-shaped low temperature pattern generated by the streamwise vortex appears on the wall and that it plays a significant role in heat transfer. The spanwise length and the life time of the horseshoe-shaped pattern can be estimated using buoyant parameters, such as (κν2/g β qw) 0.25 and (κ /g β qw) 0.5, which are the characteristic length scale and the characteristic time scale in transient natural convection.

One proposal on wall shear stress of the turbulent natural convection along a vertical flat plate.

Wall shear stress of the turbulent natural convection along a vertical flat plate was investigated semi-numerically. In the natural convection, measurements of the turbulent quantities near wall are usually difficult as a result of its strong temperature fluctuation. On the other hand, a number of attempts to describe turbulent heat transfer were made and reliable data were presented. Therefore, in this study, Surface Renewal model was introduced and wall shear stress of the turbulent naturalconvection was considered in relation to its heat transfer. It was revealed from a simple attempt that the dependence of the wall shear stress on Grashof number varied with various heat transfer correlations.

Heat transfer of natural convection around two vertically arranged horizontal cylinders.

In this study, heat transfer of natural convection around two vertically arranged horizontal cylinders was investigated experimentally. Heat transfer coefficients were measured in a wide variety of values of ratio H/D. The results showed that the heat transfer rates around two vertically arranged horizontal cylinders for 1<H/D<4 became larger than those for a single cylinder. However, a reduction of heat transfer for H/D<1 was observed.

Experimental studies of enhancement of forced-convection heat transfer by adding injection flow.

対流熱伝達の促進を図ることを目的として, 水平平板上に形成された層流境界層内にスリットから吹き出し流を付加し, 熱伝達率および壁面摩擦係数に及ぼす吹出しの効果を実験的に検討した.実験はレイノルズ数Re=u∞・l/ν=3.0×104～6.4×105, 吹出し比F=ρ0υ0/ρ∞u∞=0.005～0.1の範囲で測定した.この結果, 熱伝達率および摩擦係数はスリット後の全領域にわたって吹出し効果によって上昇し, 乱流への遷移を早める.また, 吹出し比の変化によって, これらの特性値も変化するため, 特性の制御が可能となり, 一定の熱伝達促進しかもたらさないタービュレンス・プロモータにくらべて有利である.ポンプ動力一定の条件下で伝熱面性能を評価するとき, スタントン数の最大増加比St/St0はF=0.1で1.39倍, F=0.05で1.29倍である.

Characteristics of Turbulent Flow and Mass Transfer in Divergent Channels of Rectangular Section

The characteristics of turbulent flow and mass transfer in divergent channels of rectangular cross-section were experinlentally investigated and the typical effects of main flow and the secondary flow on mass transfer were discussed. Sherwood number Shx, could be suitably expressed by Reynolds number Rex, and acceleration parameter K. Sherwood numbers Shx, in the entrance region of mass concentration were proportional to Rex, 0.83 for every diver-gent angle in the present experiment. The velocity profiles in the upper region were recognized as a composite structure consisting of those for the inner and outer layers. but it was difficult to predict the velocity profiles in the lower region with the plane jet profile.