| With the development of microelectronics industry,aerospace industry,laser medical and other fields,the large amount of heat generated by highly integrated electronic components and high-power equipment seriously affects the performance and safety of the equipment.Therefore,an efficient and safe method of high heat flux heat dissipation becomes a key issue.Spray cooling gradually replaces traditional heat dissipation methods due to its small heat transfer temperature difference requirements,low working fluid requirements,and high cooling efficiency.It has attracted great attention from scholars at home and abroad in the field of high heat flux density heat dissipation.In this thesis,through the liquid nitrogen spray cooling experimental platform,the transient evaporative heat transfer characteristics of liquid nitrogen spray under high superheat were analyzed,and the effect of changing the structure of the heat transfer surfaces on the spray cooling performance was studied.In this thesis,the surface of square micro-pillar structure with different micro-pillar sizes was first made by using fiber laser etching mechanism.Through the method of wire mesh sintering,copper meshes with different meshes and layers were further sintered on the basis of the surface of the original square micro-pillar structure to prepare a composite structure surface and characterize its microstructure surface morphology.In addition,a liquid nitrogen spray evaporation cooling experimental platform was built to conduct experimental research on the liquid nitrogen evaporation heat transfer characteristics on the surface of the composite structure,and the effects of superheat,micropillar size,mesh size and number of layers on the heat transfer characteristics of liquid nitrogen on the surface of the composite structure were analyzed.In the study,the lumped heat capacity method was used to calculate the total heat transfer heat resistance,heat flux density,and liquid nitrogen evaporation heat transfer coefficient,and analyzed its variation law.The experimental results show that the surface of the composite structure has a faster average cooling rate and a higher liquid nitrogen evaporation heat transfer coefficient than the surface of the square micro-pillar structure and the surface of the wire mesh sintered structure.In addition,as the size of the square micropillars decreases,the average cooling rate of the surfaces of the square micropillar structure increases,and the evaporative heat transfer coefficient increases.The addition of copper mesh enhances the wettability of the surfaces,promotes the formation of a thin liquid film region of liquid nitrogen,and improves the evaporative heat transfer coefficient of liquid nitrogen.As the number of copper meshes continues to increase,its strengthening effect continues to increase.Sintered multi-layer copper mesh can further reduce the pore structure and enhance the capillary force on liquid nitrogen,increase the evaporative heat transfer coefficient of liquid nitrogen,and strengthen the spray cooling performance.With the decrease of superheat,the heat flux density and the heat transfer coefficient of liquid nitrogen on the surface of the composite structure showed a trend of increasing first and then decreasing. |
