Research On The Flow Characteristics Of The Small Droplet Impingement Cooling With Thin Liquid Film

Through numerical simulation and experimental observation, the influence of the droplet impingement on the evolution of the film at the solid surface is analysed. The stability of the film on the hot solid surface impacted by the small cooling droplets is also studied. The influencing factors in the course of coalition of impinging droplet and film are specified, and the effect of each factor on the stability of the liquid film impinged by the small droplets is also elucidated, which may provide the theoretical foundation for the application of small droplets impingement cooling in engineering.The main results of this dissertation are described as follows:(1)With the high-speed camera, the flow resulted from the impact of a water droplet and an ethanol droplet on the film of the same liquid with different thicknesses, is investigated. Based on the experimental results, the influence of the impact speed, the diameter of the droplet, the viscosity and surface tension of the liquid, and the thickness of the film on the evolution of the film are discussed. The fowllowing conclusions are reached. The viscosity of the fluid affects the thickness and stability of the crown formed after the impact of the droplet on the film. The surface tension appears to affect the height of the crown and retard the splash. With increasing thickness of the film, the crown height will be reduced and the thickness of the crown will be increased. The number of the splashing droplets is related with the diameter of the impacting droplet.(2)Through analyzing numerically the flow in the liquid film with the VOF method, the hydrodunamic mechanism resulting in the crown caused by a droplet impingement on the liquid fim is elucidated. Induced by the droplet impingement, continuous high-speed radial flow in the liquid film is formed in the original ambient static fluid. At the striking position of the high-speed fluid to the static fluid, a velocity discontinuity appears, which propogates outwards in the film. The crown appears as a result of the collision of the hige-speed and static fluid on both side of the discontinuity. The height of the crown becomes greater with liquid flowing upward into the crown.(3)The influence of the thickness of the liquid film, the Weber number and the Reynolds number of an impinging droplet on the evolution of the film is numerically analysed. The numerical results herein are in qualitative agreement with the experimental results. The main results of droplet impact on the film are as follows. When the droplet impacts on the film, if the Weber number and the Reynolds number are large, the liquid will be unstable, whereas, if the Weber number and the Reynolds number are small, the liquid will be stable. However, reducing the diameter and the velocity of the droplet and increasing the surface tension and viscosity of the liquid of the droplet, will be favorable to restrain the formation of the crown and splash and to keep the film stable.(4) The collisions of two and three droplets with the liquid film are numerically analysed. The following conclusions are drawn. When the droplets impact the liquid film one by one continuously, if the vertical spacing between two droplets is small, the crown will not appear easily and the liquid film will be stable. If the vertical spacing between two droplets is large, the crown will appear easily and the liquid film will be unstable. When the droplets parallelly impact the liquid at the same time, the evolution of the liquid film is farely alike with different spacings between two droplets, but the rising amplitude is different. However, the flow pattern will be more complicated if more droplets impact on the liquid film.(5)The influences of the small refrigerant droplet impingement, evaporation, surface tension etc on the stability of the liquid film are numerically analysed. For small droplet impingement cooling, the liquid film on the solid surface will be stable if the refrigerant with larger latent heat and thermal conductivity is used. Choosing the refrigerant with larger surface tension will also be favorable for the stability of the liquid film. It is possible to keep stable liquid film on the solid surface with appropriate mass flow of the refrigerant. If the mass flow of the refrigerant is small, the liquid film will rupture, whereas, if the mass flow of the refrigerant is large, the liquid film will become too thick to have good capability of cooling. The dynamic pressure of small droplet impingement is one of the factors causing the liquid film to be unstable. Therefore, the momentun of the droplet should be controlled before it impacts on the liquid film.