The Research On Dynamic Characteristics Of Droplet Impacting A Spherical Surface With Different Wettability

The impact of droplets on objects is widely encountered in a variety of industrial processes,nature and agricultural production,such as spray droplets impacting onto the horizontal tube wall in seawater desalination device,rain impacting on the ground and the wing in the nature,and the pesticide ejected by spraying device impacting on crops,and so on.The morphology of droplet impacting onto surface has effect on heat exchange between the droplets and surface,so it has important theoretical and engineering significance to research the process of droplet impacting on surface.In this paper,the contact angles which were respectively 70°,106° and 106° were made by chemical dislocation etching and surface activity material modified methods.The high speed photography is used to record the dynamic characteristics of droplet impacting onto different wettability and curvature ratio spherical surfaces with different impact velocities.The results show that under the same impact velocity and curvature ratio,with the contact angle increasing,droplet maximum spreading coefficient decreases,and the dimensionless time to reach the maximum spreading coefficient reduces accordingly.With the increase of the surface contact angle,spreading coefficient of stable liquid film decreases,and the dimensionless time to reach stable liquid film spreading coefficient and maximum dimensionless center liquid film height both increases.Under the same curvature ratio and surface wettability,with the increase of the impact velocity,droplet maximum spreading coefficient and dimensionless time to reach maximum spreading coefficient both increases,and the dimensionless time to reach stable liquid film spreading coefficient also increases.With the increase of the impact velocity,oscillation frequency of the dimensionless center liquid film height becomes smaller,and the maximum dimensionless center liquid film height for the first time reached decreases.Under the same surface wettability and impact velocity,with the increase of the curvature ratio,droplet maximum spreading coefficient increases,and the maximum dimensionless center liquid film height for the first time reached decreases.The CLSVOF method is applied to carry out three dimension numerical simulation on the droplet dynamic characteristics during oblique impacting onto spherical surface with different wettability,and the air entrainment phenomenon and droplet breakup phenomenon when it impacting on super hydrophobic sphere are mainly studied.On different wettability spherical surface(30°?70°?106°?133° and 160°),droplet spreading rate and maximum spreading coefficient decrease with the contact angle increasing,and the dimensionless time to reach the maximum spreading coefficient reduces accordingly.With the increase of the surface contact angle,liquid film retraction degree and maximum dimensionless center liquid film height both increase,and liquid film fluctuation range is also greater.Under different impact angles(0°?15°?30°?45° and 60°),the droplet all happens to spread,retract and rebound.Droplet maximum spreading coefficient decreases with impact angle increasing,and dimensionless time to reach maximum spreading coefficient nearly the same.With the increase of the surface contact angle,the maximum air film height decreases,and the time to reach maximum air film increases.The change of entrainment maximum bubble diameter and height before liquid film fracture over surface contact angle is less volatile and highly volatile occurs after liquid film fracture.With the increase of the impact angle,the maximum air film diameter and height both decrease,and the time to reach maximum air film increases.When the impact angle is 30° ~ 60°,with the increase of the impact angle,entrainment maximum bubble diameter before liquid film fracture decreases and entrainment maximum bubble height before liquid film fracture increases firstly and then decreases.When droplet impacting onto super hydrophobic spherical surface with contact angle 160°,the number of droplet breakup increases with We increasing,and with the increase of impact angle,the number of droplet breakup decreases.The critical conditions of droplet state changing after impacting onto the super hydrophobic spherical surface with different impact angles(0 ~ 45°)are given.