Solidification of impacting droplets on concave and convex spherical cold surfaces

This work numerically examines solidification process of impacting droplets on concave and convex surfaces. A combined VOF and enthalpy-porosity approach is employed to capture the air-droplet interface driven by flow and the liquid-solid interface driven by phase change. The numerical model is validated using experimental and numerical results of tin droplet deposition on flat horizontal surfaces available in the literature. The influence of surface geometry, Weber number, contact angle and Stefan number on droplet deposition is thoroughly examined. For low Weber number dominated by surface tension effect, as the surface becomes more convex (from concave) or more non-wetting (larger contact angle), droplet spreading decreases leading to a smaller contact area for heat transfer and therefore a lower solidification rate and longer solidification time. For higher Weber number, the depositing droplet spreads faster and solidifies more rapidly. Higher Stefan number correlates with a smaller latent heat and accelerate the solidification process. The fully-solidified droplet shapes under various conditions are analyzed and categorized.