Investigation of the Salt-leaching Polymer Wick Structure Applied to Loop Heat Pipe

Loop heat pipe (LHP), which is a passive two-phase thermal transport device with high heat transfer capacity and long transport distance, has a great potential for applications of spacecrafts and electronic cooling. At present, the wick structures of the LHP are almost manufactured by sintering of metal powder. However, the use of sintered metal wick with high thermal conductivity may cause heat transmit into the evaporator core easily, which is called the problem of “heat leakage”. Besides, the wick parameters of sintered metal are difficult to control in the sintering process. Therefore, the main purpose of this study are using polystyrene with low thermal conductivity as the material and manufacturing the polymer wick structures by the method of salt-leaching in biotechnology area. By the method of salt leaching, the porosity can be controlled by adjusting the volume ratio between polymer material and sodium chloride, and the pore radius also can be controlled by adjusting the particle size of sodium chloride. Hence, polymer wick structures with different porosity and pore radius are manufactured, and the wick parameters that included pore size, pore radius and permeability are also measured. The results show that the pore size parameter lies in the desired range, and the porosity is able to be controlled within 1.65 % MAPE. Summarizing the permeability of the polymer wicks with different porosity and pore radius, the formulas of polystyrene wick structure between permeability, pore radius and porosity expressed specific as lnκ=-20.47+lnrc+4.31ε2.5 is established. The formulas would help the design of LHP and the prediction of the heat transfer performance of LHP.he polymer wick structures with various wick parameters are installed into a LHP, and the heat transfer performance of LHP system is conducted and compared with sintered metal wick structures. The results of the test reveal when the smaller the thickness and pore radius are, and the larger the porosity is, the performance of the heat transfer would be much better. Hence, a wick structure with the thickness of 1.5 mm, the pore radius of 5.9 micron, the porosity of 80 % is installed into a LHP system to carry out the performance. The capacity of heat transfer in LHP with polymer wick structure approaches 320 W, the thermal resistant is 0.234 ℃/W under the evaporator temperature of 85 ℃. The comparison of the compensation chamber temperature between the predicted results and experimental data shows a good agreement (within 5.65 %). Furthermore, in compare with metal wick structure at the similar porosity and pore radius, the performance of the LHP with metal wick structures is little better than the LHP with polymer wick structure. The thermal conductivity of wick material is major cause of different performance. Polymer wick structure with low thermal conductivity can reduce the compensation chamber temperature, but also decrease the evaporative heat transfer coefficient at the outside surface of wick structure.ccording to the literature, the heat leak problem is more serious in the flat LHP than the cylindrical LHP. This problem also makes the thermal resistance of flat LHP higher. Hence this study tries to solve the difficulties by the use of low thermal conductivity polymer wick structure. Then, a composite double layer wick structure, which is composed of the primary wick structure sintered with metal powder and the secondary wick structure made of the low thermal conductivity polymer is applied to the flat loop heat pipe. The results show that when primary wick thickness of composited double-decked wick structure is 2 mm and the temperature of the evaporators is less than 100 ℃. The maximum heat transfer capacity increases from 40 W to 180 W. In addition, the thermal resistance decreases from 0.69 ℃/W to 0.38 ℃/W. n short, polymer wick structure, comparing with metal wick structure, has some advantages in the characteristics in its production, such as low manufacturing cost, easily controllable, perfect processing, and so forth. Moreover, these merits would promote its application in the LHP.