Numerical Simulation of Natural Convection in Freezing Water Droplets Using OpenFOAM

Droplet freezing on cold surfaces plays a critical role in icing phenomena and thermal management systems. In this study, a numerical model is developed to investigate the freezing of a single water droplet, with emphasis on the influence of natural convection on internal flow dynamics. A two-phase (water–ice) solver is implemented in OpenFOAM by incorporating an enthalpy–porosity solidification model and a buoyancy model into an existing framework. The solver is verified against the analytical solution of the one-dimensional Stefan problem and validated using benchmark cases of natural convection and solidification in a cavity. Using the validated model, we examine the effects of natural convection and water density inversion on the internal flow behavior during droplet freezing. Simulations are performed for a rigid axisymmetric droplet configuration. By accounting for density inversion in the buoyancy source term, the model successfully captures the experimentally observed reversal of internal flow during freezing. The results indicate that the flow reversal occurs when the maximum droplet temperature approaches the density inversion temperature of water. While early-stage freezing follows the classical Stefan solution, comparisons with experimental data indicate that incorporating droplet impact and heat transfer to the surroundings would further enhance the model’s predictive capability.