| Fundamental understanding of frost inception and growth on a cold surface shows that the water vapor must be supersaturated for frost to form. This understanding is new, relative to previously published frost growth studies, which assume saturation at the frost surface. The frost nucleation process requires supersaturation of water vapor, and the degree of the supersaturation is strongly dependent on the surface energy. Theoretical analysis of the effect of the surface energy of substrates on frost nucleation was compared to the experimental data. This understanding was applied to a frost growth model. All previously published frost growth models assume that the water vapor is saturated at the frost surface. Using the boundary layer analysis, a procedure to obtain the supersaturation degree at the frost surface was developed, and a simple equation for the supersaturation degree at the frost surface was generated by a regression method using the supersaturation values calculated at the different frosting conditions. This was compared to the experimental results. The saturated interface model results in over-prediction of the mass transfer rate by 4-to-30 times the present experimental values. However, it was found that the present supersaturation model predicts the mass transfer rate within approximately 0∼25% error. It was impossible to compare this finding to the published results because no published data was found for the mass transfer rate at the frost surface from the air stream. Defrosting process by heating the cold surface was analyzed. It was found that the required defrosting time is dependent only on the frost thickness and the temperature of the surface, and that the frost properties (thermal conductive and density) have little affected the defrosting time. Also, it is shown that better time-average evaporator performance will result if the retained water is re-frozen, rather than drying the surface. |
