| With the development of microelectronic technology and the performance improvement of electronic equipment,the heat flux of electronic devices increases continually.How to control the temperature of electronic devices and guarantee the normal operation of electronic devices has become an urgent problem.At the same time,With the miniaturization and lightweight of electronic equipment,the space of heat dissipation modules will continue to decrease due to high integration of electronic devices in the future.How to transfer the heat generated by electronic devices to the external environment in time has become the key to further improve the performance of electronic equipment.As an efficient phase change heat transfer element,ultra-thin flat heat pipe(UTFHP)has many advantages,such as high thermal conductivity,excellent operation stability,self-adaptive ability and so on.Therefore,UTFHP has been widely used in the thermal management of electronic devices in many fields,which has become a reliable method to replace the traditional cooling method.With the continuous decrease of the thickness of the flat heat pipe,the vapor-liquid circulation resistance obviously increases in confined micro-space,and the evaporation at vapor-liquid interfaces becomes the main heat transfer mode in the ultra-thin flat heat pipe.These problems create challenges to the improvement of the flow and heat transfer capacity of the UTFHP.How to improve the evaporation/condensation heat transfer performance and vapor-liquid circulation rate of UTFHP through controlling vapor-liquid flow has become a critical issue.In view of the above problems,the packing structures of particles with complex shapes were built.According to the experimental and calculation results,the best correlation for predicting pressure drop of different particle wick structures was proposed.On this basis,the heat transfer limit of UTFHP with different wick structures was obtained,and the most suitable wick structure for UTFHP was explored.The super-hydrophilic treatment of the wick structure was finished by thermal oxidation method,and the effect of super-hydrophilic treatment on the wicks’ surface nanostructure was investigated.To analyze the variation liquid transport capacity,the changes of the micro-morphology and liquid transport capacity of the wick structure before and after super-hydrophilic treatment were characterized by scanning electron microscope and high-speed camera.The experimental results show that the superhydrophilic treatment of the screen wick can significantly improve the surface wettability and droplet absorption rate.After oxidation treatment,the apparent contact angle of the wire mesh structure was reduced from 103° to 0°,and the droplet absorption time was decreased by 28.3%.On the basis of the above research,the UTFHP with super-hydrophilic wick was proposed by combining the thermal oxidation treatment and processing technology.The effect of hydrophilic wick on the thermal performance of UTFHP was investigated.By using the diffusion bonding technology,an ultra-thin flat heat pipe with a total thickness of 0.68 mm was designed and manufactured,and the striped wick structure was proposed to control vapor-liquid flow in confined micro-space for reducing flow resistance.An experimental research platform was set up to investigate the variation of temperature and thermal resistance of ultra-thin flat heat pipe under several working conditions.By fabricating the striped wick structure with different vapor-liquid channel width ratio,the effect of vapor-liquid channel distribution on heat transfer performance is compared,and the best vapor-liquid channel width ratio of ultra-thin heat pipe was obtained.The experimental results show that the reasonable selection of mesh wick structure and the design of vapor-liquid channel are beneficial to improve the heat transfer performance of ultra-thin flat heat pipe.The ultra-thin flat heat pipe with wick of mesh number of 200 shows the best heat transfer performance,and the best channel area ratio of vapor to liquid is 1:1.13.In order to solve the performance deterioration of the ultra-thin flat heat pipe with striped wick under the condition of anti-gravity,three types of gradient porous wick were designed and manufactured by combining mesh wick with different pore structure.The influence of multi-parameters on the performance of the ultra-thin flat heat pipe was obtained with the comparison of the transient and steady-state performance of the heat pipe at different inclination angles.The variation process of vapor-liquid phase change phenomenon was analyzed in depth based on the variation of thermal resistance and temperature of UTFHP.Then,the best pore size ratio of the gradient wick structure was obtained through the characterization of the pore structure,which provides a basis for the further improvement of the UTFHPs wick.The results show that the gradient wick structure obviously improves the heat transfer performance of the ultra-thin flat heat pipe under anti-gravity condition.Comparing the heat transfer performance of the UTFHP with different gradient wick structure,the best pore size ratio of the gradient wick structure is between 2.37 and 3.49.According to the heating load variation of electronic chips in actual operation,the heat transfer performance of UTFHP under variable heat flux was studied.The influence of initial heat flux,peak heat flux,fluctuation cycle of heat flux and inclination angle on the operation of heat pipe is discussed,which provides guidance for the improvement of ultra-thin flat heat pipe structure.The results show that the effect of peak heat flux on the dynamic performance of ultra-thin flat heat pipe is related to the relative magnitude of peak heat flux and capillary limit.Under the assistance of gravity,the degree of thermal hysteresis before and after heat flux fluctuation obviously decreases. |
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