Investigate On The Properties Of LHP With Foam Metal Wicks And The Optimization Of The Wick

Loop Heat Pipe(LHP)is an efficient two-phase heat exchange device.Because of its structural and performance advantages,it has been a leading device for heat dissipation of electronic components.As the integration of electronic devices becomes higher,and develops in the direction of high performance and miniaturization,the miniaturization of LHPs becomes important.In this paper,a small loop heat pipe(SLHP)with an evaporator thickness of 10 mm was fabricated,and its performance was studied.It was found that during the preparation of the wick of the SLHP,a series of problems will occur in the preparation of the sintered wicks,so the nickel foam wick is prepared,and it is successfully applied to the SLHP.By comparing the performance of SLHP with nickel foam wick,LHP with high thermal conductivity sintered nickel wick and low thermal conductivity sintered PTFE wick.It was found that the thermal conductivity of the wick will affect the operating performance of the LHP.The high thermal conductivity wick will reduce the thermal resistance of the LHP,so the stable operating temperature is lower,and the low thermal conductivity wick will reduce the heat leak and the startup time becomes shorter.And the miniaturization of the LHP has a great impact on its performance.Due to its low internal working liquid's mass,its thermal resistance is higher under the same heat flux and the working temperature is higher.And because of the high thermal conductivity and small thickness of the nickel foam wick,the heat leak is significantly higher than that of the other two wicks,so the startup time of the SLHP is longer.Therefore,it is very important to optimize the nickel foam wick in the SLHP.Before optimizing the nickel foam wicks,its basic parameters and properties were fully characterized and compared with the sintered nickel wicks.It was found that for the nickel foam wicks,the pore size and effective thermal conductivity can be controlled by controlling porosity.The large porosity wick has a larger pore size and a lower effective thermal conductivity,and the nickel foam wick has a standard double pore size distribution.Comparing the performance of the two wicks with the same porosity,it is found that the nickel foam wicks has better mass and heat transfer performance.At the same time,comparing the nickel foam wicks with different porosities,it is found that the mass transfer performance of nickel foam wicks with porosity below 75%will increase with the increasing of porosity,but due to its effective thermal conductivity decrease,the nickel foam wicks with a porosity of about 60%has the best heat transfer performance.For the optimization of nickel foam wicks,two main methods are adopted.One is to prepare the nickel foam wick into two layers with different porosities,so that the two layers have different pore size and effective thermal conductivity.The other is basing on the nickel foam wick,a layer of PTFE wick is compounded to increase the difference in thermal conductivity between the two layers.By experiments,it was found that for the nickel foam composite wicks,when the small porosity layer is close to the heating plate and the large porosity is close to the liquid storage tank,its mass transfer performance and evaporation heat transfer performance are improved,and under the premise that the two layers' porosity meets the faster liquid working fluid transmission rate,the bigger difference between the porosity of the two layers of nickel foam,the greater improvement in heat transfer performance.For the PTFE-nickel foam wicks,due to the poor mass transfer performance of the PTFE layer,the mass transfer rate of the composite wick is reduced,but the heat transfer coefficient during the heat transfer experiment is still improved.Due to the effective thermal conductivity of the PTFE layer is very low,so the heat dissipated through the wick is reduced,the wick has a greater heat transfer coefficient at low power.Therefore,a reference scheme is proposed for the optimization of a wick,the side of the wick near the heating surface of the evaporator has a high thermal conductivity and a small pore size,and the side near the compensation chamber has a low thermal conductivity and a large pore size.