Study On The Heat Transfer Characteristics Of Liquid Film Evaporation Between Micro-columns Under Non-uniform Heat Flu

The integration and miniaturization of electronic devices will bring surge heat,the thermal safety of power batteries limits its development.Rapid and efficient dissipation of the heat generated in these micro-scale,narrow spaces is a key to ensure the safety of device.Heat pipe and vapor chamber technology utilizes the latent heat of liquid-gas phase change with high heat transfer efficiency.Its important component is the evaporator with porous wick structure,which forms an extended curved moon surface for heat transfer by liquid film evaporation,and generates capillary pressure to spontaneously complete the circulation of liquid.With the advantages of high manufacturing accuracy and low thermal resistance,micropillar arrays as a wick structure for evaporators have a promising application.In this dissertation,firstly,a model is used to study the maximum heat transfer capacity of the vertical micropillar evaporator under uniform heat flow and the horizontal and vertical evaporators under hot spot scenarios as affected by geometry,and an optimized design is made to enhance the evaporator heat dissipation capacity.Secondly,numerical simulations are used to study the flow and heat transfer performance changes along the wicking direction of the horizontal evaporator under both uniform heat flow and segmented non-uniform heat flow,highlighting the nonuniformity of evaporation.For vertically placed micropillar evaporators,the maximum error between model predictions and experimental results does not exceed 14%.The dryout threshold of the evaporator is influenced by the geometry with an optimal pitch ratio of about 0.35,and the dryout threshold increases linearly with increasing height and decreases with increasing dryout length.The variation pattern of liquid rise height with time shows that micropillar arrays with closer to the optimal spacing ratio and smaller recending contact angle will have better capillary performance,and the dryout threshold will increase as a result.The genetic algorithm optimization results show that increasing the micropillar height is an effective way to improve the heat transfer performance of the evaporator when the dryout length increases.The dryout threshold can be increased by about 13% at the optimal spacing ratio for the hexagonal arrangement compared to the square arrangement due to the effective enhancement of capillary pressure.In the hot spot case,the results show that the pattern of influence of geometry on the maximum heat transfer capacity of the evaporator does not change regardless of whether the evaporator placed horizontally or vertically.The diffusion effect of heat within the substrate is highlighted in the hot spot case,which is described using the characteristic length and solved by a heat transfer model.The results show that an increase in the characteristic length will lead to an effective enhancement of the evaporator heat dissipation capacity due to the increase of the liquid film evaporation area,while an increase in the thermal conductivity of the material or an increase in the substrate thickness can increase the characteristic length.Based on the idea of discretization,numerical simulations are performed to study the performance changes of the evaporator along the wicking direction in the case of uniform heat flow as well as segmented non-uniform heat flow,highlighting the actual change in the shape of the curved moon surface and the inhomogeneity of evaporation.The results show that the increase of liquid film area in the wicking direction leads to the increase of evaporation efficiency.The variation of evaporator superheat provides a reference for structural design,such as the most efficient arrangement in the middle of high heat flow density under segmented non-uniform heat flow,where the peak of evaporator superheat is minimized,and a more compact micropillar array in the high heat flow region reduces the peak further.In this dissertation,the heat transfer characteristics of liquid film evaporation in a micropillar evaporator under different heat flow scenarios are investigated.Macroscopically,the maximum heat transfer performance of the evaporator is investigated with geometry using a model,which provides a reference for the design of the evaporator geometry.The variation of the bending moon surface is also considered from the microscopic point of view to highlight the inhomogeneity of evaporation.The work will provide support for the application of heat pipe and vapor chamber technology to the field of micro-scale heat dissipation.