The Dynamic Characteristics Study Of Double Droplets Successively Impact On Super-hydrophobic Tubes

Droplet impact phenomena are widely found in industry,agriculture and daily life,such as fuel injection in gas turbines,spray cooling,electrostatic spraying,and pesticide spraying.The research on the heat and mass transfer process after the droplets impact the surface will provide a reliable theoretical basis for practical application,and help to improve the production of related products and the equipment operationefficiency.In this paper,a three-dimensional geometrical model of doubledroplets impact on the super-hydrophobic tube surface is established.The gas-liquid phase interface is captured by the Coupled Level Set and Volume of Fluidmethod,and the continuum surface force model is used.The influence of different parameters on the dynamic characteristics is studied.The dynamic characteristics of the double droplets impacting on the super-hydrophobic tube surfaceare studied when the curvature ratio is 0.25,0.33 and 1,and the double droplets impact velocity is in the range of 0.75 to 2 m/s.The influence of the curvature ratio on the double droplets impact process is analyzed for the impact velocity of 1.5m/s.In addition,focused on0.33 curvature ratio case,droplet spacing and eccentricity on the process of double droplets impacting the super-hydrophobictube surface are investigated.In this paper,the dynamic characteristics of droplet impact are described by combining the liquid film motion process diagram and characteristic parameters.The velocity vector field and pressure nephogram are used to analyze the mechanism of liquid film breakup and bubble entrainment.The impact of the trailing droplet promotes the liquid film extension and does not significantly promote the spread with curvature ratio of 1.There is a phenomenon that the edge of the leading liquid film exceeds the three-phase contact line,and the extension liquid film usually has two sorts of breakup: the liquid film edge breakup and the liquid film three-phase contact point breakup.As the impact velocity increases,the liquid film fluctuates significantly.When the curvature ratios are 0.33 and 0.25 respectively,the spread extent increases with impact velocity.The finger liquid film appears when the leading droplet spread.When the local capillary wave wavelength of the liquid finger film is greater than the critical capillary wave wavelength,the liquid finger film breakup happens.Under the three curvature ratios,the liquid crown film exits after the trailing droplet impact with the impact velocity varied from 1.25 to 2m/s.The bubble entrainment occurs atthree curvature ratio cases when the impact velocity ranges from 0.75 to 1.25m/s.While this phenomenon is not observed when the impact velocity exceeds 1.5 m/s.The bubble that is entrained has two consequences: escaping from the liquid film or causing the liquid film central breakup.The routine of bubble escaping from the liquid film is mainly determined by the flow direction of the fluid.The bubble causing the liquid filmbreakup generally exists in the center of the liquid film,and theRayleigh Taylorinstability causes the liquid film central breakup.The effects of droplet spacing(2.2,4 and 6 mm)on the process of double droplets impact the super-hydrophobic tube surface are investigated for impact velocities of 0.75 and1.5 m/s.The change in droplet spacing affects the impact mode and the size of the liquid crown film.The effects of droplet eccentricity(0.5,1 and 2 mm)on the process of double droplets impact the super-hydrophobic tube surface are studied for impact velocity 0.75 m/s and 1 m/s.The change in eccentricity does not affect the impact model.In theeccentric impactprocess,there is a phenomenon that the plane on the right side of the liquid film is approximately perpendicular to the plane on the left side of the liquid film.Bubble entrainment occurs at both impact velocities,occurring at the trailing droplet impact point or under trailing droplet.As the eccentricity is increasing,therebound point of the liquid film moves to right side.Further,the shape of the liquid film was more concentrated when it rebounds,so it is more difficult to break.