When a liquid droplet freezes on a cooled substrate, the portion of latent heat released by ice formation that is not immediately absorbed by the supercooled liquid droplet is transferred to the solid substrate below the droplet and the surrounding air. It is important to quantify heat dissipation through these two pathways because they govern the propagation of frost between multiple droplets. In this paper, infrared (IR) thermography measurements of the surface of a freezing droplet are used to quantify the fraction of latent heat released to the substrate and the ambient air. These IR measurements also show that the crystallization dynamics are related to the size of the droplet, as the freezing front moves slower in larger droplets. Numerical simulations of the solidification process are performed using the IR temperature data at the contact line of the droplet as a boundary condition. These simulations, which have good agreement with experimentally measured freezing times, reveal that the heat transferred to the substrate through the base contact area of the droplet is best described by a time-dependent temperature boundary condition, contrary to the constant values of base temperature and rates of heat transfer assumed in previous numerical simulations reported in the literature. In further contrast to the highly simplified descriptions of the interaction between a droplet and its surrounding used in previous models, the model developed in the current work accounts for heat conduction, convection, and evaporative cooling at the droplet-air interface. The simulation results indicate that only a small fraction of heat is lost through the droplet-air interface via conduction and evaporative cooling. The heat transfer rate to the substrate of the droplet is shown to be at least one order of magnitude greater than the heat transferred to the ambient air.
droplet freezing; infrared thermography; water vapor distribution; recalescence; solidification
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