AN IMPROVED MODEL FOR DROPLET SOLIDIFICATION ON A FLAT SURFACE

An existing model of the deformation and solidification of a single dr oplet impinging on a cold surface has been revised and improved. The o riginal model is based on a two-dimensional axisymmetric flow approxim ation of the velocity field, the Neumann solution to the one-dimension al Stefan solidification problem, and an integral mechanical energy ba lance. The improved model features a more appropriate velocity field w hich satisfies the no-shear boundary condition at the free surface, an d an accurate derivation of the dissipation term from the mechanical e nergy equation. This equation has been solved numerically. Comparisons of the original and the improved models have been performed. Results show that the original model over-estimates the final splat size by ab out 10%. The discrepancy is more pronounced at larger Weber numbers, w here viscous effects dominate. The effects of the Weber number, We, th e Reynolds numbers, Re, and the solidification parameter have been inv estigated through detailed numerical calculations. Two regimes of spre ading/solidification have been identified. If Re/We is small, the proc ess is one of dissipation of the incident droplet kinetic energy; wher eas for large values of Re/We the process can rather be characterized as a transfer between kinetic and potential energy. In the latter case , the variations of the final splat size versus the solidification con stant exhibit a non-monotonic behaviour. This indicates that, for a gi ven material, the deposition process can be optimized. Correlations re lating the final splat size to the process parameters are given.