Control of Natural Convection by Local Cooling or Heating with a Row of Heat Pipes

本研究涉及一种利用由一排热管组成的热屏控制自然对流的方法，热管具有非常高的有效导热率。通过热屏对沿垂直加热壁的自然对流边界层和线热源上方的浮力羽流发展的影响进行了实验和分析研究。首先，将热屏与空气环境中的垂直加热墙平行放置。利用热视频系统和热电偶目测得到的实验温度分布表明，当屏幕温度接近环境温度时，热边界层比无屏幕系统的热边界层变薄。另一方面，当筛网温度接近垂直壁面温度时，热边界层厚度增大。因此，从垂直壁面传热量随着筛温对环境温度的降低而增强，随着筛温对壁面温度的升高而减小。实验还表明，筛网靠近垂直加热壁的位置对控制自然对流换热是有效的。通过二维数值计算阐明了控制对流型的机理，发现热筛通过影响进出流流体的温度来改变浮力。其次，对线热源产生的浮力羽流进行了实验研究。热屏位于热源上方，以控制烟羽的发展。利用马赫-曾德尔干涉仪对羽流的目视观测和实验温度分布显示了在屏幕位置羽流的膨胀或收缩。渗透率小的热屏可以改变浮力羽流的特性，吸热或放热的效果明显，但随着渗透率的增加，热屏的作用减弱。实验结果表明，热屏可以通过改变烟羽的温度水平来抑制或促进烟羽的振荡。数值研究证实，传热速率取决于筛网的渗透率，由于筛网的温度分布具有较强的均匀性，因此吸热或放热可以改变浮力。结果表明，热屏可以控制垂直壁面自然对流和浮力羽流的特性，这取决于热屏的位置、渗透率和温度水平。少

This study is concerned with a method for controlling natural convection by utilizing a thermal screen consisting of a row of heat pipes with very high effective thermal conductivity. Experimental and analytical studies have been made on the effects of the heat extraction or heat release through the thermal screen on the developments of the natural convecion boundary layr along a vertical heating wall and of the buoyant plume above a line heat source.First, the thermal screen is placed in parallel with a vertical heating wall in an air environment. The experimental temperature distributions obtained by the visual observations using a thermal video system and thermocouples show that the thermal boundary layr becomes thinner than that of the system without screen when the temperature of the screen is closer to the ambient temperature. On the other hand, in the case that the temperature of the screen is close to that of the vertical wall, the thickness of thermal boundary layr increases. … More Therefore, heat transfer from the vertical wall is enhanced as the screen temperature decreases to the environment temperature and reduced as it increase to the wall temperature. The experiments also show that the closer location of the screen to the vertical heating wall is effective for the control of the natural convection heat transfer. Two dimensional-numerical calculation has been made to clarify the mechanism of controlling theconvective pattern and it is found that the thermal screen can change the buoyancy force by affecting the temperatures of in-flowing and out-flowing fluids.Secondly, experiments have been carried out for the buoyant plume arising from a line heat source. The thermal screen is located above the heat source to control the development of the plume. Visual observaions of the plume using a Mach-Zehnder interferometer and the experimental temperature distributions show the expansion or contraction of the plume at the location of the screen. The thermal screen with a small permeability can change the characteristics of the buoyant plume and the effect of the heat extraction or the heat release is marked, although the effect of the screen is reduced with the increase in permeability. Also, the experimental results show that the thermal screen can suppress or promote an oscillation of the plume by changing its temperature level. It is confirmed by the numerical study that the heat transfer rate depends on the permeability of the screen and the heat extraction or heat release can modify the buoyancy force because the screen introduce a strong uniformity in the temperature distributions.Then, it is concluded that the thermal sscreen can control the characteristics of both natural convection along the vertical wall and buoyant plume, depending on the location, permeability and the temperature level of the screen. Less