| Ultra-thin flat-plate heat pipe(UFHP)is a heat transfer device composed of working medium after the non-condensable gas in the sealed cavity is removed.It has strong heat transfer ability and good temperature uniformity,and is suitable for the equipment with heat dissipation and isothermal requirement.At present,ultra-thin flat-plate heat pipe is developing in the direction of more ultra-thin,and its heat transfer performance decreases sharply when it becomes thinner.In order to maintain its good heat transfer performance,the study of heat and mass transfer is indispensable.In this dissertation,visual experiments were carried out on ultrathin flat-plate heat pipe with 120-200 powder sintered wick and three-layer mesh wick with a cavity space thickness of 1mm and working in a horizontal position.The heat transfer performance,start-up characteristics and phase change and flow characteristics of the ultra-thin flat-plate heat pipe under different heat load were analyzed.The thermal resistance model is established to analyze the thermal resistance units of each part of the ultra-thin flat-plate heat pipe.The calculation results show that the total heat transfer resistance of the ultra-thin flat-plate heat pipe is mainly provided by the heat conduction resistance of the shell,the heat conduction resistance of the wick and the thermal resistance of the vapor chamber.The thermal resistance of the vapor chamber is greatly related to the thickness of the vapor chamber.When the thickness of the vapor chamber decreases from 1mm to 0.1mm,the thermal resistance value increases about 400 times.The structure of wick has an effect on the heat and mass transfer process of ultra-thin flatplate heat pipe.Therefore,visualization experiments of ultra-thin flat-plate heat pipe with two kinds of wick structure are carried out with a liquid filling ratio of 40%.Within the scope of the heat load of 3W～27W,the working medium of ultra-thin flat-plate pipe with sintered wick shows four characteristics: liquid-vapor interface evaporation,bubble growth,vapor area connecting and vapor-liquid entrainment.The total heat transfer resistance decreases by about41.1%,and the minimum thermal resistance reaches 0.939K/W when vapor area connecting.There is no vapor area connecting happened in the ultra-thin flat-plate heat pipe with mesh wick,and its total heat transfer resistance decreases by about 29.2%.The decrease of total thermal resistance is due to the decrease of the length of liquid working medium separating the vapor region in the vapor chamber,which indicates that the heat transfer performance of the ultra-thin flat-plate heat pipe is mainly affected by the heat and mass transfer process of the vapor chamber.The mesh wick structure has higher thermal conductivity resistance and inhibits the bubble escape,which leads to poorer starting characteristics than the sintered wick.Combined with the heat transfer performance and starting characteristics,the sintered wick can be used to obtain better performance of ultra-thin flat-plate heat pipe.The visualization results show that the liquid working medium in the vapor chamber has a great influence on the heat transfer ability of the ultra-thin flat-plate heat pipe.Visualization experiments of ultra-thin flat-plate heat pipe with liquid filling ratios of 20%,40% and 60%were carried out.When the heat load is 15 W,the liquid in the vapor chamber will be evaporated at 20% filling ratio,and the total thermal resistance will reach the minimum.When the thermal load is 25 W,the minimum total thermal resistance can be obtained with 40% liquid filling ratio.And when the liquid filling ratio is 60%,the heat transfer performance and start-up characteristics are poor.The smaller the liquid filling ratio is,the better the starting characteristic is.According to the results of different liquid filling ratio visualization experiments,the optimal liquid filling rate of ultra-thin flat-plate heat pipe is related to its heat load. |
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