Study On Mechanism Of Dynamics And Heat Transfer During Droplet Impingement

Droplet impacting on the surface widely exists in nature and industries.Experiments,simulations and theoretical analysis were carried on to explore the mechanism of dynamic and heat transfer during droplet impingement.Better understanding of the process can not only enrich the theory of gas-liquid flow with free interface,but also improve the spray control technology.The main contents and results were summarised as follows:Diesel droplet impacting on the dry surface and the wetted surface were experimentally recorded using high speed camera.With the increasing of impact velocity,when a droplet impacts on the dry surface,it successively presents spread,fingering edge and splash;when impacts on the wetted surface,the droplet presents spread,crown jet and jet splash;the effect of surface wave is obviously when 0.2 m/s<U0<0.9 m/s.The effects of impact velocity,initial diameter,properties of droplet and surface were explored.The difference of motion and impact characteristics between a droplet impacts on the dry and the wetted surface was also studied,the results showed that the secondary atomized droplets are smaller but has larger kinetic energy when a droplet impacts on the dry surface;the effect of impact velocity on spreading factor is more obvious,while it is less on the maximum spreading velocity when a droplet impacts on the wetted surface.The theoretical model of maximum spreading factor during a droplet impacts on the dry surface was also developed based on energy equation.A numerical model was developed using coupled Level set and Volume of Fluid method including surface tension,heat transfer,contact resistance,liquid-solid wettability and contact angle hysteresis.This model was verified by experiments.The mechanism of motion and secondary atomization during a droplet impacts on the dry surface as well as the mechanism of spread,jet formation and splash during a droplet impacts on the wetted surface were obtained according to results analysis.The results showed that the pressure gradient inside the droplet is the main factor in determining the formation and break up of edge jet when impacts on the dry surface,and it is important to the formation,development and splash of crown jet when impacts on the wetted surface;Rayleigh-Plateau instability and capillary wave play an important role in shrink and breaking of jet;the change of surface tension caused by Marangoni effect also decides the splash when impacts with heat transfer.Also,the effects of impact velocity,surface wettability and thickness of liquid film on impact characteristics were explored.Air entrapment during a droplet impacts on the dry and the wetted surface was studied.The evolution of entrapped air was obtained,which showed that the entrapped air presents shrinkage,breaking,coalescence and stripping with time;surface wettability and Weber number are closely related to the evolution of entrapped air.The mechanism of the formation and motion of entrapped air was explored.It was found that the pressure difference between liquid and gas before a droplet impacting is the main factor decides the formation of air entrapment;the pressure distribution inside the droplet determines the evolution of entrapped air.Also,the effects of impact velocity,surface wettability and thickness of liquid film on the evolution and characteristics of entrapped air were obtained.The mechanism of heat transfer during a droplet impacts on the dry and wetted surface was studied.The entrapped air was important to surface heat flux distribution at the very initial stage during impact.The relation between features of dynamic and heat transfer was explored at different condition.It was found that when a droplet impacts on the dry surface,the features of dynamic and heat transfer tends to be synchronous as impact velocity increases;when a droplet impacts on the wetted surface,the dynamic feature tends to lag behind that of heat transfer as impact velocity increases and thickness of liquid film decreases.The effects of impact control parameters were also obtained,which showed that impact velocity,surface temperature,droplet temperature,thickness of liquid film and liquid film temperature impacts obviously on wall heat flux;temperature of droplet and liquid film are not related to dynamic features of the droplet;wall temperature is closely related to spreading feature,Marangoni stress boundary is the main factor decides this relation.Also,the evaporation of a droplet impacts on the dry surface was studied.The impact center and spreading edge have larger evaporation rate,which easily forms liquid vapor;pressure gradient at the phase interface is the main factor decides the dynamic motion of entrapped air;at the later stage of impact,vapor at the impact center impacts obviously on the wall heat flux.