Improvement of Heat Transfer Performance of Loop Heat Pipe for Electronic Devices

In this paper is presented the results on performance of the cooling model using Loop Heat Pipe (LHP) system. In recent years, ever-ending demand of high performance CPU led to a rapid increase in the amount of heat dissipation. Consequently, thermal designing of electronic devices need to consider some suitable approach to achieve high cooling performance in limited space. Heat Pipe concept is expected to serve as an effective cooling system for laptop PC, however, it suffered from some problems as follows. The heat transport capability of conventional Heat Pipe decreases with the reduction in its diameter or increase in its length. Therefore, in order to use it as cooling system for future electronic devices, the above-mentioned limitations need to be removed. Because of the operating principle, the LHP system is capable of transferring larger amount of heat than conventional heat pipes. However, most of the LHP systems suffered from some problems like the necessity of installing check valves and reservoirs to avoid occurrence of counter flow. Therefore, we developed a simple LHP system to install it on electronic devices. Under the present experimental condition (the working fluid was water), by keeping the inside diameter of liquid and vapor line equal to 2mm, and the distance between evaporator and condenser equal to 200mm, it was possible to transport more than 85W of thermal energy. The thickness of evaporator was about 5mm although it included a structure to serve the purpose of controlling vapor flow direction inside it. Successful operation of this system at inclined position and its restart capability are confirmed experimentally. In order to make the internal water location visible, the present LHP system is reconstructed using transparent material. In addition, to estimate the limit of heat transport capability of the present LHP system using this thin evaporator, the air cooling system is replaced by liquid cooling one for condensing device. Then this transparent LHP system could transport more than 100W of thermal energy. However, the growth of bubbles in the reserve area with the increase in heat load observed experimentally led to an understanding that in order to achieve stable operation of the LHP system under high heat load condition, it is very much essential to keep enough water in the reserve area and avoid blocking the inlet with bubbles formation.