Research On Heat And Mass Transfer Characteristics Of Ultra-thin Flat Heat Pipe Based On Wire Mesh Wick

As a heat dissipation device with high heat flux density,flat heat pipes are widely used in various electronic devices.With the highly integrated development of electronic components,the thickness of the flat heat pipe is continuously reduced.The ultra-thinness of the flat heat pipe affects the phase change flow and heat transfer performance of the working fluid.Therefore,it is an urgent task to explore the phase change,flow law and optimal working condition of the working fluid inside the ultra-thin flat heat pipe.The following researches take the wire mesh core ultra-thin flat heat pipe as the research object,and conducts a systematic and in-depth study on it through theoretical numerical analysis and visual experimental exploration.Based on the heat and mass transfer theory of fluid phase change,the heat and mass transfer model of each part of the ultra-thin flat heat pipe is established.The results show that the thicker the vapor cavity,the smaller the flow pressure drop of the vapor,and the thicker the wick,the smaller the flow pressure drop of the liquid working fluid.The heat transfer limit of the ultra-thin flat heat pipe using water as the working fluid is higher than that of the ultra-thin flat heat pipe using alcohol as the working fluid.When alcohol is used as the working fluid,the thermal resistance is mainly distributed in the wick part.Combined with the analysis of the numerical simulation results,the greater the thermal load,the greater the overall pressure drop of the heat pipe.the greater the heat flux density,the greater the flow velocity of the vapor in the evaporation section.The performance of the heat pipe was tested on a water-cooled test bench.The results showed that the ultra-thin flat heat pipe with single-layer wire mesh wick had the lowest thermal resistance,and the heat pipe with three-layer wire mesh had the highest thermal resistance.As the working inclination angle increases,the heat transfer performance continues to increase,but the larger the working inclination angle,the smaller the influence of the inclination angle change on the heat transfer performance.Under the test conditions,the best liquid filling rate for the thermal resistance of the ultra-thin flat heat pipe with single-layer wire mesh wick,double-layer wire mesh core and three-layer wire mesh core is 25%-35%,25%,and 25%,respectively.The thermal resistance is reduced by about 20.67% compared with the maximum thermal resistance.As the thermal load increases,the thermal resistance first decreases and then increases.The best working heat load of the three kinds of ultra-thin flat heat pipes with wire mesh wick is about 20 W.Through visual air-cooling experimental research,it is found that as the working heat load increases,the ultra-thin flat heat pipe phase change modes include evaporative heat transfer,subcooled boiling,nucleate boiling,and drying zones that exceed the boiling heat transfer limit.The multi-layer wire mesh can observe the process of nuclear bubbles overflowing from the lower wire mesh on a microscopic level.The limitation of the overflow of nuclear bubbles causes its thermal resistance to be mainly distributed in the evaporation section.Combining working fluid flow analysis and temperature fluctuation analysis,the analysis results show that the oscillation of the vapor-liquid working fluid will affect the temperature of the heat pipe bottom plate,and the bottom plate temperature fluctuation lags behind the working fluid flow cycle,and the cycle size is approximately equal to the flow cycle Integer multiples.