Investigation of the freezing process of water droplets based on average and local initial ice fraction

We presented both experimental and numerical studies on the freezing of impacting water droplets on a cold surface at different surface temperatures. The numerical model consists of two parts. The first one is to determine the temperature evolution of the droplet prior to the occurrence of freezing by solving the heat conduction equation, and the second one is to simulate the freezing process via an extended phase change model. Experiments were conducted to observe and record the freezing process. The droplet profile and the propagation of moving water-ice interface during freezing were obtained from image analysis. Based on the numerical pre-recalescence temperature of the droplet, the average initial ice fraction and local initial ice fraction were obtained. Then, both the two kinds of initial ice fractions were used to figure out the difference that they brought to the predicted freezing process. Through a comparison of the experimental observations and the numerical predictions, the freezing process predicted by using local initial ice fraction showed a better agreement with the experiment than using average initial ice fraction.