Developing Empirical Correlations for Frost Thickness and Air Face Velocity Degradation for MicroChannel Heat Exchangers Used in Heat Pump Applications under Frosting Condition

摘要

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. 7 fin samples with various fin width, fin height and fin density were tested in controlled laboratory conditions that replicated those of actual heat pump systems in winter season. The fin surface temperature was experimentally estimated 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 the frosting time and the frost growth rates were depended mainly on the local fin surface temperature. A set of empirical correlations were developed to predict the frost thickness on fin leading edge and the reduction of air face velocity due to air pressure drop across the frosted coil during frosting operation. The correlations aid to calculate 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.