The Study of Wick Structure and Pump-Assisted for Hybrid Flat Plate Loop Heat Pipe

Loop heat pipes have the advantages of phase changing, high heat flux capability, longer distance heat transport and small thermal resistance. The aim of this study is to develop the flat plate loop heat pipe (FP-LHP) cooling system for high heat flux equipment, such as nuclear power plants, supercomputers, and high speed networks. The hybrid FP-LHP combined active mechanical pumping force with passive capillary structure. The operating characteristics of the hybrid FP-LHP was investigated in the range of flow rates from 0.3 L/min to 1.5 L/min. To evaluate the effect of the heat leakage, nickel and PTFE are used as materials of capillary structure for the hybrid FP-LHP. In testing the nickel-water hybrid FP-LHPs at flow rate 0.6L/min, the maximum heat load achieved is 1200W (heat flux 153.1 W/cm2) and the minimum thermal resistance is at a level of 0.09 ℃/W. The effect of the flow rate shows that there is a minimum value of the total thermal resistance at the optimal flow rate. For a nickel and a PTFE wick, the corresponding flow rate are 0.9 L/min and 1.2 L/min, respectively. To reduce the level of the hybrid FL-LHP operating temperature, acetone is used as working fluid. At the optimal flow rate and a maximum allowable of the evaporator’s wall temperature about 100℃, with the use of a nickel wick the maximum heat load is 700W (heat flux 89.3 W/cm2) and the minimum of the total thermal resistance is 0.07 ℃/W; for the PTFE wick the maximum heat load achieved is 900W (heat flux 114.8 W/cm2) and the minimum of the total thermal resistance is 0.04 ℃/W。A PTFE wick has the maximum heat load transfer 30% greater than a nickel wick. With the same testing conditions and for electronic cooling the evaporator’s wall temperature does not exceed 85℃. In this case, a nickel wick’s heat load is about 500W (heat flux 63.8 W/cm2) and for a PTFE wick this value is up to 800W (heat flux 102.9 W/cm2), which is 60% larger than a nickel wick. The minimum of the total thermal resistance for a nickel wick is 0.07 ℃/W and for a PTFE wick it is 0.04 ℃/W, which is lower than a nickel wick about 43%。Moreover, the results of testing indicate that the heat leakage has the limited influence for the hybrid FP-LHP system. From the Mercury Porosimeter, the PTFE wick has biporous distribution. With the big pore radius, it can not only decrease the hydraulic drag but also release the vapor smoothly. For the hybrid FP-LHP system, the most efficient wick structure is PTFE.