Analysis On Heat And Mass Transfer Characteristic Of Ultra-thin Vapor Chamber Based On Different Heat Loads

As an efficient heat transfer element,the vapor chamber(VC)has been widely used in heat dissipation modules of high flux devices.With the development of highly integrated electronic devices,decrease of the VC thickness result in an increase in the flow resistance of the working fluid in the VC,and a sharp increase in the pressure drop of the vapor core.The sharp increase of pressure drop leads to a continuous deterioration of the heat transfer performance and maximum heat transfer limit.Study of the influences of ultra-thinning on the VC heat and mass transfer characteristics is a topic that not be ignored at present.In order to solve the deterioration of heat transfer performance of the ultra-thin vapor chamber(UTVC),the following researches based on theoretical analysis,visualization study and numerical simulations were carried out to investigate the thermal mass transfer characteristics and influencing factors of the UTVC under different heat loads.A theoretic model of the thermal resistance network of UTVC based on the theory of heat and mass transfer of fluid phase change was established.Influences of vapor core thickness on fluid flow and heat transfer characteristics in vapor core and wick structure were considered under different heat loads.Besides,influences of the vapor core thickness on the total thermal resistances of the VC,thermal conduction resistance and the phase change thermal resistance at the liquid-vapor interface have been considered and analyzed in the paper.The results show that the total thermal resistance of UTVC rises sharply as the vapor core thickness reduced to 0.3mm.When the vapor core thickness reduced to 0.1mm,the percentage of vapor core thermal resistance reaches over 80%.The vapor core thickness has become one of the main factors affecting the heat transfer enhancement of UTVC.The heat transfer process of the UTVC mainly depends on the phase change and circulating flow of the working fluid.The gravity affects the flow characteristics of the working fluid.Therefore,by constructing a changeable inclination angle experimental device platform of UTVC,thermal performance of the UTVC under different inclination angles and heat fluxes was investigated.Temperature of the evaporation section and condensation section,and thermal resistance of the VC were applied to evaluate the heat transfer capacity.Relationships between heat flux of evaporation section and the corresponding optimal tilt position were analyzed.Results show that higher-level match between the heat flux of evaporation section and inclination angle could lead to a lower thermal resistance of the VC.A comparison analysis of the boiling process and heat transfer characteristics in the space between anhydrous ethanol and deionized water in three groups of wire mesh and powder sintered structured wicks was made by establishing a visual experimental platform for boiling heat transfer in a limited unconfined space that consists of wick structures of VC.Results of the visual experiment reveal that the working fluid boiling in the limited unconfined space undergoes three processes:isolated bubble,bubble column and nucleate boiling.The boiling heat transfer intensity of the working fluid is related to the wick structures.Relationship between the boiling heat transfer coefficient and the porosity of wick structure was obtained according to the results.The phase change process of the working fluid in VC is complex and difficult to observe.In order to explore the influences of ultra-thin limited confined space on flow and boiling heat transfer characteristics of working fluid in the VC,the quartz glass was applied as the visualization window to establish visualization experiment device with the limited sealing space.The visualization experiment studied the influences of inclination angle and wick structures on the flow and boiling heat transfer of working fluid,and revealed the differences between boiling phenomenons of working fluid in limited confined space and limited confined space.The bubbles formed by the working fluid boiling changed to flat bubbles by the limited space thickness.The phase change process in the wick structure of evaporation section changed to liquid film evaporation,and the vaporized working fluid continuously supplies to the flat bubbles.By investigating the flow characteristic of working fluid under the action of gravity and flat gas bubble,it was found that the working fluid flow back to the evaporation section with the help of gravity and impacted the working fluid at the bottom.The impaction enhanced the convective heat transfer intensity between the working fluid and the heated surface.The experiment results show that the boiling heat transfer coefficience of the 120-200 powder sintered wick structure is the largest among the six wick structures.Combined with the results of visual experiment and solving the surface energy of the liquid film in the pore of wick structure of UTVC using gradient descent method,the meniscus-shaped liquid film was obtained.Based this liquid film morphology,the numerical model was established to analyze the effects of factors such as the variation of surface tension of working fluid,contact angle,particle diameter and the superheat of working fluid on the evaporation characteristics of the liquid film.The results show that annular convection flow caused by the surface tension variation of working fluid enhances the evaporation heat transfer intensity of the liquid film near the solid wall,while the decrease of solid-liquid contact angle increases the surface area of the film and enhances the heat transfer intensity.The column structures prevent the VC from deformation due to the large pressure difference,but the column may hinder the flow of working fluid in the vapor core,increasing the flow pressure drop.Therefore,by establishing a numerical model of the VC,the effects of factors including column size,spacing,number and structure types were considered.The relationship between the four factors and thermal performance of VC was analyzed.It is found that the copper column wrapped with sintered wick layer has the greatest enhancement on the heat transfer capacity of the VC.The sintered wick layer provides an extra flow path for the working fluid to flow from the condensation section to the evaporation section,reducing the flow pressure loss.Since the existing numerical model for VC simplifies the vapor core and neglects the influence of ultra-thinning,the calculation error of the numerical model for UTVC increases.Therefore,this paper established an optimized numerical model for the UTVC,and compared with the experiment data of a T shaped UTVC to verify the accuracy of this model.Compared with the original numerical model,the accuracy of the optimized model has increased by approximately 51.7%.