Study On Behavior Characteristics Of Millimeter And Micron Droplet Beam Impacting On Metal Hot Surface

In recent years,spray cooling has become an efficient cooling technology due to its advantages of low heat transfer and thermal resistance,less demand for working medium,good temperature uniformity and no boiling lag.It has been widely used in industry,agriculture,fire fighting,aerospace,internal combustion engine injection and other fields,attracting extensive attention of many researchers.Most studies on spray cooling have focused on single droplet impinging on hot surfaces,as this provides advantages such as simplified control of influential parameters and improved measurement capability during droplet-surface interactions.However,the study of single droplet impinging on the hot surface can not accurately describe the mechanism of droplet flow impinging on the hot surface.Compared with the single droplet impinging on the hot surface,droplet flow impingement on the hot surface is more approximate to the real spray cooling,but it is more difficult to control the key parameters such as surface roughness,droplet size and impact velocity.In order to have a deeper understanding of the mechanism of spray cooling,optimize its application in the industrial field,and achieve the best cooling effect,it is necessary to pay attention to the heat transfer at the solid-liquid interface.In this study,an experimental platform for droplet impact on metal hot surface was built based on the visualization method,and the morphology changes and Leidenfrost temperature changes of droplet dynamic on the millimeter and micron single droplet and droplet flow impacting on the hot surface were revealed by droplet parameters and surface parameters.Finally,a mathematical model of heat transfer is established to calculate the change of heat transfer when the droplet impacts the hot surface.It provides theoretical basis for subsequent research and has positive significance for further mastering spray cooling mechanism.The research shows that as the surface temperature increases,The hot surface temperature of micron-scale single droplet and droplet flow is higher than that of millimeter-scale droplet during nuclear boiling and lower than that of millimeter-scale droplet during transition boiling.Millimeter-scale single droplet and droplet flow can break up during film boiling,while micron-scale droplets will bounce completely from the hot surface.With the increase of surface roughness,millimeter-scale single droplet and droplet flow will explode,the maximum spread factorβ_maxand the residence time of droplet on hot surface will increase,and the dynamic Leidenfrost temperature of droplet will increase.The dynamic Leidenfrost temperature of millimeter and micron single droplet and droplet flow increases,about 270℃-300℃.As the droplet size increases,the degree of fragmentation of millimeter-scale single droplets and droplet flow increases,the maximum spreading factorβ_maxof the droplets and the residence time on the hot surface are larger,and the dynamic Leidenfrost temperature of millimeter and micron single droplet and droplet flow increases accordingly,about 260℃-290℃,the larger the heat dissipation and heat transfer coefficient.With the increase of droplet impact velocity,both millimeter and micron single droplet and droplet flow will be broken when they impact on the hot surface and the degree of fragmentation will increase,and The dynamic Leidenfrost temperature of the droplet decreases accordingly,at about 260℃-280℃.The residence time of the droplet on the hot surface becomes shorter,and the maximum spreading factorβ_maxof the droplet and its heat dissipation and heat transfer coefficient on the hot surface increase.In the process of spray cooling,increasing the droplet size and impingement velocity can increase the cooling rate.