Droplet impact and solidification in a thermal spray process

A numerical model was developed on the basis of SOLA-VOF algorithm to study the impact and solidification of a liquid droplet upon its impingement on a substrate. The model, in general, is applicable to transient fluid flows and heat transfer including two moving boundaries: a liquid-gas free-surface boundary and a liquid-solid interphase. The model, in particular, was used to analyze the formation of a coating layer made from one droplet impact as a function of processing parameters in a thermal spray process. The numerical model was developed step by step by first considering an isothermal droplet impact and then modifying the flow dynamic model to include heat transfer and simultaneous solidification. The modification of the fluid dynamic equations in the presence of solidification was based on the improved 'fixed velocity' technique. The first solidification model used was a 1D model well suited for plasma spray operations. A 2D axisymmetric ' enthalpy' model was finally employed for heat transfer and simultaneous solidification in the droplet and conduction ion heat transfer in the substrate. Previous models of droplet impact either neglected or used simplifying assumptions when dealing with: capillary effects, simultaneous solidification, droplet-substrate thermal contact resistance, and heat transfer to the substrate. The model developed in this study, however, considered capillary effects at both liquid-substrate and liquid-solid interfaces, simulated simultaneous solidification and heat transfer to the substrate during the impact dynamics, and considered thermal contact resistance at the surface of the substrate. By comparing detailed numerical predictions with available experimental measurements and by considering analytical models in conjunction with the concept of dimensionless numbers, it was found that: capillary effects during droplet impact is negligible if <math> <f> We&Gt;<rad><rcd>Re</rcd></rad>,</f> </math> and the effect of solidification on droplet impact dynamics is negligible if <math> <f> <rad><rcd>Ste/Pr</rcd></rad>&Lt;1.</f> </math> Numerical predictions were compared and verified with available experimental results for two sets of water and tin droplets impacting a flat stainless steel surface. The model predictions were then obtained for two typical thermal spray processes: RF and DC plasma spray operations. For water and tin droplets, capillaxy effects were important; for typical plasma spray operations, however, capillary effects were negligible, i.e., no knowledge of contact angle is required when studying the impact dynamics in plasma spray conditions. For tin droplets considered in this study, we found that simultaneous solidification considerably affected the impact dynamics and maximum droplet spread. For typical plasma spray cases, however, solidification effects were much lower. When capillaxy and solidification effects are more important the numerical modeling of the problem is more challenging; a fine and uniform computational mesh, therefore, must be used. Verification of the model results for droplet impact cases with high capillary and solidification effects, therefore, verifies its results for the cases where these effects are less important.