Experimental Study On Functional Gradient Wicks And LHP

Loop Heat Pipe(LHP),as an efficient two-phase heat exchanger,has gradually become a front technology for heat dissipation of electronic components with high heat flux because of its unique structure and performance advantages.With the development of miniaturization of loop heat pipe,the problem of heat leak of loop heat pipe has been paid more and more attention by researchers.The wick is a key component that affects the heat transfer performance of the loop heat pipe,providing the evaporation heat transfer surface and the driving force for the working fluid cycle,and connects the evaporator and the compensation chamber,whose effective thermal conductivity has an important influence on the heat leak problem of the loop heat pipe.In order to reduce the heat leak through the wick to the compensation chamber,a series of functional gradient wicks with progressive change of effective thermal conductivity along the direction of working fluid flow were developed in this study.The effects of progressive change of effective thermal conductivity of the wick on the performance of the wick and loop heat pipe were studied experimentally.The main work is as follows:1.A series of integrated Functional gradient wicks with progressive change of effective thermal conductivity along the direction of working fluid flow were designed and fabricated,while the thickness ratio of wicks with different effective thermal conductivity was different between wicks.The wick sintering process using nickel(Ni)and polytetrafluoroethylene(PTFE)as materials was explored.2.The basic parameters and performance parameters of the Functional gradient wicks were characterized experimentally.The results of immersion method showed that the porosities of the wicks were all about 48.64%.The pore structure and morphology of the wicks were observed by scanning electron microscope,and the pore radius distribution of nickel wick and PTFE wick were measured by mercury intrusion method.The results showed that nickel wick has a relatively uniform pore radius,while PTFE wick showed typical pore radius distribution of biporous wicks.The effective thermal conductivities of wicks were measured by steady state method,whose results showed that the effective thermal conductivity of nickel wick is larger than that of PTFE wick.The results of mass transfer experiment showed that nickel wick has better mass transfer performance,but the mass transfer performance of Functional gradient wicks has no linear relationship with the thickness ratio of wicks.In addition,the experimental results showed that the evaporative heat transfer performance of the Functional gradient wicks was better than that of the single effective thermal conductivity wicks.3.A loop heat pipe evaporator with detachable and replaceable wick was designed and the corresponding loop heat pipe was fabricated.The effects of different Functional gradient wicks on the starting performance of the loop heat pipe and the operational performance of the variable working conditions were studied experimentally.The experimental results showed that some loop heat pipes with Functional gradient wick can effectively improve the heat leak problem,reduce the startup time and the operating temperature of the system,and suppress the temperature fluctuation of the loop heat pipe.