Fabrication Of Biporous Wick And Its Application In Loop Heat Pipe

Loop heat pipe (LHP) is a capillary driven heat control and heat transfer device, a new technology developed from recent 20 years. Due to its high heat transfer capacity, long transfer distance, flexible tube, anti-gravity ability, non-movement part, and high reliability, loop heat pipe is widely applied to thermal control in spacecraft and electronic devices.Loop heat pipe consists of evaporator, condenser, vapor line and liquid line. The evaporator is the key component in LHP which is directly connected to heat source, it dissipates the waste heat by phase change of working fluid. The evaporator design fundamentally affects the operation of LHP. And the porous wick is the key part of evaporator, which provides the capillary force for the whole loop, so it is the most important yet most difficult component to fabricate in the LHP.In this thesis, fabrication of the biporous wick and experiment of LHP with secondary wick were carried out.With respect to the fabrication of wick, in high heat flux condition, the vapor and liquid flow rate are equally large in the evaporator. Due to the fine pore, monoporous wicks will produce high hydraulic resistance, which is one of the most important reasons causing dry-out in evaporator. In order to resolve this problem, the biporous wick was fabricated in this thesis, with two different kinds of pores allowing the wick to achieve a better performance through separation of liquid and vapor. The experimental setup for manufacturing porous wick was built, pore former was proportionally added into the nickel powder, two methods, cold-pressing sintering and direct loose sintering, was applied to fabricate porous wicks. Porosity was measured by Archimedes method, pore radius was observed by Electronic Scanning Microscope, and permeability of wicks was calculated by empirical equation. Result shows that wicks are successfully fabricated, the optimal wicks are found to be sintered at 700°C, using cold pressing sintering method, with pore former content 30% by volume; these wicks could reach the porosity of 77.40%, the permeability of 3.15×10-13m2, and have sufficient mechanical strength to meet the machining requirements. With regard to the experiment of LHP, the evaporator with secondary wick assisting primary wick pumping working fluid was designed, and the sintered biporous wick mentioned above was selected as the primary wick. The start-up characteristics, performance in variable working condition, temperature oscillation, and thermal resistance were investigated in the experiment. Results showed that the LHP with innovative structure had a very good start-up characteristics, the system could start up from 20 to 100W. In variable working condition, LHP can operate reliably from 20 to 160W. The temperature oscillations occurred mainly in the heat load between 30 to 90W. Under heat load of 160W(with heat flux of 16.6W/cm2), the evaporator wall temperature was 84.0°C. The biporous wick can be applied in the high heat flux condition.