| Loop heat pipe is a type of high efficient and passive heat transfer device.It shows great application potential in the field of cooling with high heat flux for its excellent heat transfer performance.As the power source and phase change place of the whole loop heat pipe system,the porous wick plays the decisive role on its performance.In this paper,a composite porous wick with spherical-dendritic powders is proposed.Dendritic copper,nickel and spherical copper,nickel,stainless steel powders are chosen as the material to sinter composite porous wicks at 750°C by orthogonal design experiment.The pore size distribution of porous wicks is counted by SEM image,and the porosity and effective thermal conductivity of porous wicks is measured experimentally.The experimental apparatus is set up to research its evaporation heat transfer characteristics.The effect of material,combination of powder,ratio and size of pore former on the performance of porous wicks are compared based on the analysis result.The main conclusions are as follows:(1)Basic parameters of porous wicksThe material of metal powder has the greatest influence on the microstructures and effective thermal conductivity of porous wicks.The wicks with spherical stainless steel combined with dendritic copper powders have the minimum porosity of 53.8%,the minimum ratio of small pore of 11.8%,and the maximum effective thermal conductivity of 10.3 W/m·K.The wicks with spherical copper combined with dendritic nickel powders have the maximum porosity of 73.3%,and the maximum ratio of small pore of 88.7%,and the minimum effective thermal conductivity of 2.5W/m·K.The effect of ratio of pore former follows.With the increase of porosity and ratio of big pore,the ratio of pore former in porous wicks increase,and the effective thermal conductivity decreases.The combination of powder and size of pore former have less influence on the microstructure and effective thermal conductivity of porous wicks.(2)Evaporation heat transfer characteristics of porous wicksThe evaporation heat transfer process of porous wicks has two operating states including stable and fluctuant,and three evaporation modes:Bubble growth-break inside vapor grooves;Liquid-vapor interface evaporation inside vapor grooves;Meniscus evaporation inside porous wicks.In the three evaporation modes,the wicks with spherical stainless steel combined with dendritic copper powders have the lowest evaporator wall temperature.In the first and second evaporation mode,increasing the effective thermal conductivity of the porous wicks can effectively reduce the evaporator wall temperature.The material of metal powder has the greatest influence on the evaporator wall temperature,and the other factors have little influence.In the third evaporation mode,increasing the ratio of big pore of the porous wicks can effectively reduce the evaporator wall temperature by increase evaporation area of the porous wicks,and the ratio and size of pore former has the greatest influence on the evaporator wall temperature.Compared with the single experiment,the continuous operating conditions can effectively increase the evaporation rate of the porous wicks and reduce the evaporator wall temperature.And the evaporator wall temperature shows lower value in the operation of decreasing heat load.There is no dry out inside the porous wicks exceeding the critical heat load,but the evaporation heat transfer cannot meet the requirement of heat dissipation.The combination of powders has the greatest influence on the critical heat load of porous wicks,and the critical heat load of porous wicks increases with the increase of the proportion of dendritic powder.The effect of material of metal powder follows and the effect of pore former is minimum.At the volume ratio of 3:7,40%of 30-45um pore former,the composite porous wick with spherical nickel and dendritic copper powders has the largest critical heat load of the 530W,and the critical heat flux of48.2W/cm~2. |
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