Developing empirical correlations for frost thickness and air face velocity degradation for microchannel heat exchangers used in heat pump applications under frosting conditions

Abstract

This study experimentally investigated the frost growth on louvered folded fins in outdoor microchannel heat exchangers used in air-source heat pump systems. The effects of surface temperature and fin geometries on the performance of the microchannel heat exchangers under frosting condition were studied. Seven fin samples with various fin widths, fin heights, and fin densities and a louver angle of approximately 30° were tested in controlled laboratory conditions that replicated those of actual heat pump systems in the winter season. The fin surface temperature was experimentally measured with the novel methodology developed in the present study. Experimental data of local frost thickness, air pressure drop across the coils, time of frost–defrost cycles, and heat transfer rates were recorded for heat exchangers operating in actual transient frosting conditions. Data showed that frosting time and frost growth rates depended mainly on the local fin surface temperature. A set of empirical correlations were developed to predict the frost thickness on the fin leading edge and the reduction of air face velocity due to air pressure drop across the frosted coil during frosting operation. The correlations were compared against experimental data of frost collected in the present study, and satisfactory agreement was achieved. The frost thickness correlation and air face velocity correlations aid in the calculation of the instantaneous air-side Reynolds numbers during frosting operation of the fin samples. These are critical for predicting the heat transfer rates of the microchannel coils in quasi-steady-state frosting operating conditions.