| In the nature and daily production,the phenomenon of droplets impinging is common,among which the hydrodynamics,heat transfer and transport mechanism have always been the focus of academic research.Many studies have shown that the periodic variation of stretching and compression occurs when the droplet falls freely,which makes the droplet ellipsoidal before impacting wall.The droplet also deforms under the influence of air flow,electric and magnetic fields.After the droplets impact the wall continually,a thin liquid film form and attach to the wall.In many conditions,the liquid film has a certain speed.The subsequent droplets hit the moving liquid film,and the phenomenon is obviously different.The existence of bubbles in the liquid film will affect the heat transfer characteristics,working life of medium and equipment vibration.In this paper,a numerical model of ellipsoidal droplet impact on the wall and moving liquid film is established.The influence of the fluidity of the film on the impact process is analyzed.The main work contents and conclusions are as follows:(1)The coupling level set and volume of fluid method(CLSVOF)is used to simulate the process of ellipsoidal droplet impinging on the wall.It is found that the simulation results are in good agreement with the experimental results in the literature,which verifies the feasibility of the numerical model and method.The spreading and rebound processes of the impact of the ellipsoid droplets are researched.The results show that the maximum spreading radius of ellipsoidal droplet increases with the improvement of AR.A liquid column form in the process of droplet rebound,and the maximum radius of column(Rr)decreases with the increase of AR.The secondary droplet breakage occurs under the action of instability.The higher AR will promote the generation of secondary droplets during the rebound,while the lower AR has a significant inhibitory effect.The secondary droplets not appear when AR is less than the critical value 0.6.(3)Numerical simulation of ellipsoidal droplet impact on stationary and moving liquid film is carried out.The morphological changes,jet formation and asymmetry of pressure distribution in the process are analyzed.It is found that the neck jet forms more easily when the droplet impinges on the moving liquid film.Meanwhile,the moving liquid film will promote the jet into a highly asymmetric coronal splash,and the splash height increase with the improvement of AR.The initial velocity on the upstream side of film is opposite to the direction of spreading,so the discontinuous effect of movement is obvious.The increase of the film velocity restrains the splash height on downstream side,and facilitates that on upstream side.After the droplet impact the moving liquid film,the pressure under the impact point and both sides of the wall increased significantly than static film.The pressure distribution was also asymmetrical,and the pressure distribution coefficient increase with the raise of AR.(4)The process of ellipsoidal droplet impinging on static and moving film containing bubbles is studied.The influences of AR,film velocity and impinging velocity on the bubbles and film are analyzed.It is found that the morphology of splash in the static films with a little effect of bubbles.The deformation of bubbles is obviously affected by AR.The bubble and gas ring rupture on the wall,and cause jet at the bottom of the crater.When the droplet hit the moving film containing bubbles,both the bubbles and the film will undergo different changes: bubbles will rupture,moving on the wall and flow in the liquid film.The splash is asymmetrical and the size increases obviously with the increase of flow velocity.After the bubble burst in the moving film,the resultant velocity of liquid in the shell is inclined downward and toward the wall with no bottom jet generated.The burst time decreases with the increase of film velocity and AR.In this paper,a numerical model for the impact of ellipsoidal droplets on impacting the wall and moving liquid film are established.The impact process and characteristics of the ellipsoidal droplet are investigated.The influences of film velocity and AR on spray height,pressure distribution and bubble behavior are further analyzed.The research work can provide a reference for further study of deformed droplet impact moving film,and also deepen the understanding of bubble dynamics under the impact of droplets. |
