Numerical study on the spreading and solidification dynamics of ellipsoidal metal droplet impacting on curved solid surfaces

Spreading and solidification of non-spherical metal droplets impacting on curved surfaces is a complicated problem that involves the coupling of fluid flow, heat transfer, and phase change combined with complex wetting behaviors at the contact line. Unveiling the physics behind the intricate process is of utmost importance for applications in metal droplet-based manufacturing. In this work, the volume-of-fluid method coupled with the Solidification/Melting model is used to investigate the dynamic behaviors of hot ellipsoidal tin droplets impacting on cold spherical surfaces. The numerical model is validated against experimental and simulation data available in literature, and a proper value of mushy zone constant is selected by matching the spreading factor in prior experimental work. Then, the effects of initial droplet aspect ratio, surface curvature, Weber number, and surface wettability on the temporal evolutions of droplet profile, spreading factor, and solid fraction are examined. The simulation results show that an increase in aspect ratio results in a larger maximum spreading factor at a later time for prolate droplets, while for oblate droplets this happens at an earlier time when decreasing the aspect ratio. For the droplets with a same aspect ratio, an increase in surface curvature reduces the spreading factor, and the solidification process is slowed down accordingly. A larger impact velocity normally results in an extended spreading due to the increased kinetic energy. In addition, a significant alteration in the early spreading factor is observed under a small Weber number revealing the important role of capillary effect. Droplet spreading is promoted on a hydrophilic surface contributing to a larger surface area for heat transfer and thus a higher solidification rate. The effect of surface wettability depends on droplet shape, which is stronger for prolate droplets than oblate ones.