Effect Of Wall Wettability On Droplet Impacting On A Restricted Liquid Surface

The phenomenon of droplet impacting the free liquid surface phenomenon has many applications in industry,the current research on the free surface has been more than complete.However,there is still less research on the collision dynamics between droplet and limited liquid surface.The impact of droplets on a restricted liquid surface has applications which include spray cooling,fuel atomization,and seawater desalination.The study of droplet impacting on a restricted liquid level is not only beneficial for a deep understanding of the mechanism of free surface flow,but also can be used to guide specific industrial applicationsIn this study,the basic calculation model is established,and the static contact angle and the dynamic contact angle model are selected to calculate.By comparing with the experimental results,it is found that the Kistler dynamic contact angle model can predict well the dynamic process of the drop impacting the confined liquid surface in the tube.Finally,the VOF method combined with the Kistler dynamics contact model was built.A series of computations were performed for a droplet impacting on a restricted liquid surface.The numerical simulation results agreed well with the experimental results.According to the numerical results,A regime map,including the coalescence regime,breakup regime,and coalescence regime,was plotted.It was found that the increase in contact angle ? enlarges the breakup regime,but narrows the other two.In order to compare different working conditions,six special moments were selected.At the beginning of the collision,droplet behaviors was dominated by inertial force.The contact line at the wall started to move due to the mechanical wave.While the crater developed from formation to the deepest,the wall wettability did not affect the longitudinal development,but it would limit its radial development.An energy conversion model was established to analyze the regime map.It was observed that with the increase of ?,when the spreading height arrived maximum,the dissipation coefficient decreased and more energy was used for jet formation and breakup.Based on the above energy model,the theory of available and unavailable energy was added to further understand the collision regime.It was shown that with the increase of ?,the proportion of available energy increased,which was consistent with the result of energy model.The analysis about wave at the time corresponding the capillary convergence showed that the wavelength generated by 10° and 30° tubes was much longer than the wave height and it belonged to gravity wave.The other corresponding wave were capillary waves.With the increase of contact angle,the wavelength decreases,the wave energy increases and the convergence time decreases.More specifically,there were two mechanisms in the breakup regime:the high-speed jet caused by wave convergence,which consists of capillary wave and gravity wave,and low-speed jet caused by crater collapse.There were also two mechanisms in the different jet breakup:the capillary instability dominated breakup mechanism caused by Rayleigh instability and high-pressure stagnation,and the surface tension dominated breakup mechanism caused by the effects of gravity and surface tension.Moreover,the high-speed jet was of the same type as the free-surface hydraulic jets in surface wave collapse,while the low-speed jet was not.Based on these two mechanisms,the breakup regime could be divided into two smaller regimes.The 60° and 90° tubes always corresponded to the surface tension dominated breakup regime with low-speed jets,and other tubes corresponded to the capillary instability dominated breakup regime with high-speed jets.