EFFECT OF WICK THICKNESS ON THE HEAT TRANSFER PERFORMANCE OF LOW-TEMPERATURE COPPER-WATER HEAT PIPE

Copper-water heat pipes are commonly used for cooling electronic components and devices. For some applications of heat pipe, such as satellite cooling for telecommunication or low-temperature refrigeration for cold chains, the operating temperature can be as low as -40°C. Water freezes at temperatures below 0°C, which prevents water from flowing in the wick and affects the heat-transfer performance of the heat pipe. Even though several studies have been conducted, it is still not clear if copper-water heat pipes will operate well in frozen conditions. This study provides an experimental measurement of the heat-transfer performance of copper-water heat pipes at saturation temperatures of 20°C and -40°C. Two sintered wick heat pipes with thicknesses of 0.5 mm and 0.9 mm were tested and compared. The test results show that at the condensing temperature of 20°C, the filling liquid and wick porous layer result in a significant temperature gradient between the working fluid and the surface of the pipe, which increases the thermal resistance of the heat pipe. This leads to thermal resistances of the 0.9 mm heat pipe, which are higher than those of the 0.5 mm heat pipe. For the heat pipe with a wick thickness of 0.5 mm at the condensing temperature of -40°C, the adiabatic and condensing sections of the wick layer remain within the frozen zone. No ice can be melted by the core vapor flow and, therefore, the heat pipe cannot work properly. For the heat pipe with a 0.9 mm wick thickness, even though the outside surface of the condensing and adiabatic sections are still kept at -40°C, the frozen ice at the inner surface of the wick is heated by vapor, which allows the ice to melt into the liquid phase. Condensed liquid is able to flow back to the evaporating section through the inner layer of the wick, allowing the heat pipe to start operating.