Numerical and experimental analysis of rapid solidification considering undercooling effect during water droplet impact on a substrate

Freezing of water droplets on a cold surface can involve rapid solidification and undercooling. In this numerical and experimental study, the role of rapid solidification considering the undercooling effect during the impact and spreading of a water droplet onto a cold substrate is investigated. Various attendant physical phenomena, such as free surface evolution, fluid flow, heat transfer, rapid solidification and undercooling at the moving solid-liquid front are accounted in the computational model. In the coupled thermo-fluidic-rapid solidification model, the free surface of the liquid droplet is tracked with the help of volume of fluid method, while the solid-liquid front is tracked with the help of rapid solidification kinetics. The rapid solidification model lets freezing to occur at nucleation temperature which is lower than the freezing point. Results of the rapid solidification model are compared with the conventional solidification model where freezing occurs as soon as the temperature reaches the equilibrium freezing point. A realtime imaging setup is used to experimentally measure droplet spreading and freezing behaviour. The numerical predictions are validated with the experimental results. The model successfully captures undercooling and recalescence. Interfacial heat transfer is analyzed with the help of droplet-substrate heat flux. Undercooling and rapid solidification significantly influence the heat flux evolution. In the end, the effect of substrate temperature and droplet impact velocity is described. The understanding developed in this study regarding the role rapid solidification on interfacial heat flux evolution can be useful for research on ice accretion in aircraft.