Effects of inclination angles on heat transfer performance of a loop thermosyphon for CPU cooling

Abstract

A loop thermosyphon is known that it has higher heat transfer performance without a pump. Thus, we have applied new loop thermosyphon to cool CPUs of a rack mount server. Thermosyphon uses gravity to return refrigerant from the condensation part to the evaporation part. Therefore, when it is working, the water level in the condenser side should be higher than the evaporator side. In addition, the height of the new loop thermosyphon is designed low with 67.4mm to put it on a rack mount server. Hence, when the surrounding system and the loop thermosyphon incline, it is thought that water surface exists inside of the condenser. It causes decrease of heat transfer performance of loop thermosyphon. Hence, it is important to make clear how the performance changes by inclination angle of the system. To this purpose, the heat transfer performance of the loop thermosyphon has been measured as a function of inclination angles. As the result, it is found that thermal resistance of the condensing part and sub-cool degree at out-let of the radiator increase with increase of inclination angle. And the sub-cool degree has a correlation with vapor temperature of the loop thermosyphon. In addition, we confirmed the tendency of this correlation can be predicted by thermal circuit model of the radiator. These results show the thermal resistance changes of the loop thermosyphon by inclination angle are caused by water surface level changes in the condensing part.