Study On Dynamic Characteristics Of Droplet Impact On Tube Surfaces With Different Wettability

In this paper,the three-dimensional numerical simulation of the dynamic characteristics for droplets impact on different wettability surfaces is performed using CLSVOF method.The dynamic characteristics of droplets impact on hydrophilic,hydrophobic and superhydrophobic surfaces are analyzed when the curvature ratios are 0.65,0.43 and 0.32,respectively.And the influence of impact velocity,viscosity coefficient,surface tension coefficient and eccentricity on the dynamic characteristics of the droplet impacted on the superhydrophobic tube surfaces is discussed.Finally,the mechanism of the process change after droplet impact on the surfaces is also analyzed.Under the condition of the same curvature ratio and impact velocity,the smaller the contact angle is,the more favorable the liquid film spreads,the larger the maximum dimensionless spreading coefficient that the liquid film can reach,the smaller the largest dimensionless liquid film center height that it can reach.Within the scope of this paper,when the impact velocity and contact angle are constant,the smaller the curvature ratio is,the role of gravity is weakened,and the more disadvantageous to the spreading process of the liquid film,the smaller the maximum dimensionless spreading coefficient of the liquid film is.When the impact velocity is constant,the poorer the surface wettability and the smaller the curvature ratio,the larger the entrained air bubbles during the retraction.When the curvature ratio is 0.32,the effects of impact velocity on the dynamic change of droplets impact on the superhydrophobic tube surfaces are discussed.According to the difference in the dynamic characteristics of droplets impact on the tube surfaces,the rebound state is divided into overall rebound,fracture rebound,and splashing rebound.In the range of the impact velocity for the study,the critical conditions for the transition of the rebound motion after the droplets impact on the superhydrophobic tube surfaces are given.On the superhydrophobic tube surface,the maximum dimensionless spreading diameter decreases with the viscosity coefficient.The smaller the viscosity coefficient of droplet is,the faster the spread,retraction and rebound take place,and the greater the rebound height is.With the increase of the surface tension coefficient,the time required for the liquid film to reach the maximum spread is reduced and the more disadvantageous to the spread of the liquid droplets,the faster the liquid film retract.When the surface tension of the droplet is 0.01(N/m),the outer edge of the liquid film is splashed and no rebound occurs.The rebound occurs after the droplets impact on the superhydrophobic tube surface is mainly due to the surface energy gradually converts into the gravitational potential energy during the retraction process of the liquid film,which cause the liquid film height continuously raises,and the movement direction of the surrounding air flow are the main driving force for the liquid film to rebound from the surface.The low pressure area inside the liquid film will cause the phenomenon of “necking”.At the same time,the non-uniformity of the velocity makes the interfacial tension in the outer edge area is unstable,the phenomenon of splashing occurs.When the curvature ratio is 0.43,the dynamic characteristic during the droplet eccentric impact on the superhydrophobic tube surface is mainly related to the impact eccentricity.When the eccentricity is 0.5mm and 1mm,the liquid film is fracture due to the air entrainment during the eccentric impact.When the eccentricity increases,the position of the bubble formation moves toward the outside of the tube,and when the eccentricity reaches 1.5 mm,the contact time of the liquid film on the tube surface becomes significantly shorter,and the liquid film entirely rebound from the tube surface.