The Study of PTFE Wick Structure Applied to Flat Plate Loop Heat Pipe

Loop heap pipe (LHP) is a type of passive two-phase heat transfer device;compared with a traditional heat pipe, LHP possesses the following advantages: long heat transfer distance, low thermal resistance, and high heat transfer capacity. For a heat source with a flat surface, a saddle must be added to a normal cylindrical LHP’s wick, but doing so not only increases the total thermal resistance of the system but also renders the evaporator’s surface temperature non-uniform, effecting the overall performance of the LHP. To eliminate such problems caused by the addition of a saddle, this study investigates the use of LHP with a flat-plate evaporator. In order to target cooling of electronic products that often require small and lightweight heat transfer devices, most LHPs use metal as the wick’s material; however, metal wicks oxidize easily and also require high sintering temperature in the manufacturing process. Thus, this study looks for a different wick material that can allow for more cost-effective and more easily manufactured wicks; from literatures,high polymer materials have been studied for this purpose,and among them
polytetrafluoroethene (PTFE) have shown greater potential. Previous studies on flat-plate LHPs have also found that, compared to cylindrical LHPs, flat-plate LHPs encounter more serious heat leakage problems, causing the total thermal resistance values to be higher than those of cylindrical LHPs; PTFE, with a very low thermal conductivity value, is a great choice for wick material to solve this problem. Therefore,this study chooses PTFE as wick material to effectively, with the use of self-rewetting fluid as working fluid, delay the occurrence of drying-out and increasing the critical heat load. This study then investigates the feasibility of replacing the conventional nickel with PTFE as choice for wick material. In addition, the effect of self-rewetting fluid on the heat transfer performances of two LHPs—one with a nickel wick and one with a PTFE wick—are compared and studied.Experimental results show that,compared to those of previous studies mentioned in literatures, the heat transfer performance testing system established in this study is reliable. Concerning the effect of using self-rewetting fluid instead of water as working fluid, for flat-plate LHP with nickel wick, the use of self-rewetting fluid effectively decreases the overall operating temperature of the system. Under 85°C, which is the typical target temperature for electronic devices, using self-rewetting fluid instead of water can increase the LHP’s highest heat load from 50W to 100W and decrease the lowest thermal resistance from 1.56K/W to 1.07K/W; since the occurrence of dry-out is also delayed as a result, the critical heat load is increased from 250W to 325W.Concerning the effect of using PTFE instead of nickel as wick material, under the
vii aforementioned conditions, the performance of LHP with PTFE wick is actually worse than LHP with nickel wick; the critical heat load decreases from 325W to 75W, while the thermal resistance has no significant improvement, increasing from 0.42K/W to 1.41K/W. Since PTFE is hydrophobic by nature, it is concluded that, with water as
working fluid, it is difficult for the working fluid to travel through the wick; however,the pore size distributions of manufactured PTFE wicks are very similar to those of
biporous nickel wicks. Thus it is highly probable that, with the proper choice of working fluid, the biporous nature of PTFE wick can come into effect, showing great potential for high and even enhanced heat transfer performance of flat-plate LHPs with PTFE wicks.