An Investigation on Heat Transfer During the Freezing of Condensate Droplets

Abstract

The heat transfer and the freezing process of condensate droplets resting on a cold metal plate have been studied both experimentally and numerically. The experimental part of the investigation dealt with the measurements of the thermal and flow parameters and the visualization of the freezing process. The parameters which were varied include the ambient air velocity, air-to-plate temperature difference, and the droplet size. The tests performed were the steady-state temperature distribution, transient cooling curve at a location within a droplet, freezing time, average air-side convection coefficient on a cold metal plate covered with droplets, and freezing front motion on the freezing of droplets. Two new dimensionless numbers have been introduced to account for the freezing time and the air-side convection coefficient. Two empirical correlations, one for freezing time and the other for the average air-side convection coefficient, were developed.

A mathematical model based on control volume method was developed to calculate the temperature distributions, heat transfer rates, and phase-change region position during the freezing of hemispherical droplets on a cold metal plate. The model predictions of the freezing of a hemispherical droplet showed that the narrow ring around the droplet perimeter was a good place for continued ice nucleation. The basic structure of a frost layer was composed of two zones: the ice sublayer and the dendritic layer. The results predicted by the mathematical model have been compared with available analytical solutions, previously published results, and experimental data. The good agreements indicated that the model could be used to predict steady-state and transient temperature distribution, temperature history, freezing front velocity, and freezing time. Combined with other softwares, the mathematical model could also be used to graphically simulate the freezing process.