Behavior Of A Droplet Impacting A Thin Water Film

Droplet impact process is a common physical phenomenon in daily life.It has been widely applied in spray cooling,desalination,internal combustion engine combustion,ink-jet printing and other industrial fields because of its strong heat and mass transfer performance.Deepening the understanding of the flow phenomenon and mechanism of droplet impacting on the film is conducive to providing the theoretical basis for various practical engineering applications.In this paper,the dynamic behavior of droplet impinging on thin water film is analyzed in depth by experimental observation with high-speed camera technology.At the same time,CLSVOF method is used to simulate the impact of a water droplet on thin water film.The experiment of a glycerol aqueous droplet impinging on thin water film was carried out by laser induced fluorescence method.The process of droplet impacting on thin water film under different impact conditions were discussed.The main research contents and conclusions are as follows:Firstly,the experimental platform for water droplet impinging on thin water film was constructed to analyze the dynamic process and influence mechanism of droplet.Based on the experimental results,the effects of liquid film thickness,impact velocity and surface characteristics on the dynamic behavior of liquid droplets were analyzed.The delayed splash is depressed by increasing film thickness.For thinner films,higher impact velocities increase the splash intensity,while for thicker films,higher impact velocities increase the crown height and diameter.Surface wettability has no effect on the morphology and diameter of the crown,but the base crown diameter expands faster on the hydrophobic surface.The central jet breakup occurs at the crown with passive delayed splash.Secondly,CLSVOF method was used to establish a two-dimensional numerical model to simulate the process of droplet impacting on thin water film,the experiment is used to verify the correction of the simulation model.Based on the pressure distribution,velocity distribution and vorticity cloud map inside the fluid after impact,the influences of various water film thickness and impact velocity on flow characteristics were analyzed,and the bubble entrainment and prompt splash during the evolution of liquid droplets were discussed.For the thinner liquid film,the high-pressure region is larger along the radial direction at the moment of impact,and the pressure drops faster at the same jet evolution time.At the same time and fluid depth,with the increase of water film thickness,the axial velocity increases,the radial velocity decreases,and the pressure decreases slightly.As the droplet impact velocity increases,the splash intensity increases,and the vorticity in the jet region changes significantly.Finally,Visualization of glycerol aqueous droplet impinging on thin water film was studied by laser induced fluorescence method.The effects of glycerol solution concentration,water film thickness and droplet velocity on the crown morphology after droplet impact were investigated.The phenomenon of central jet breakup was analyzed,and the mechanism of the crown detachment was revealed.Comparative experiments were conducted to investigate the components of crown and secondary droplets by adding fluorescent agents separately to the droplets and water films.The maximum crown height decreases with the increase of film thickness after the impact of low concentration glycerol aqueous droplet,while the maximum crown height increases with the increase of film thickness after the impact of high concentration glycerol aqueous droplet.The maximum central jet height generated by low concentration glycerol aqueous droplet increases with the increase of water film thickness,while the maximum central jet height generated by high concentration glycerol aqueous droplet decreases with the increase of water film thickness.The components of the secondary droplets produced by prompt splash come from the thin water film,while the components of the secondary droplets produced by the delayed splash come from both the glycerol aqueous droplet and the thin water film.In summary,experimental observation and numerical simulation are used in this paper to study the flow characteristics and influencing factors of droplet impact on thin liquid film.The research results provide a reference for the improvement of heat and mass transfer process technology in industrial production.