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

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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