Modeling and measurement of frost characteristics on heat exchanger surfaces

Frost growing on heat exchanger fins is a common problem for refrigeration and air-conditioning cooling equipment. Frost growth on heat exchanger fins causes the performance of heat exchangers to decline because frost accumulation blocks air flow path and causes an increase of the air flow pressure drop and decrease of air flow rate across heat exchangers. A test facility was developed to characterize frost growing on heat exchanger flat plates and finned surfaces where the cold surface and the air supply conditions were similar to those experienced in freezers. Several new measurement techniques were used to measure frost height and mass accumulation and heat flux. A numerical model for predicting frost growth on flat plates and plate-fin exchangers was developed. The model treats the frost as a transient, one-dimensional porous media and includes transients two-dimensional heat conduction in the fins. The numerical model was validated using experimental data for frost height, frost density, air pressure drop across the fins and heat rate through the cold base. The agreement between the experimental data and simulation results were within the uncertainty bounds for most of the data. It is found that for a 'constant' air-flow rate through the heat exchanger over a four hour period, the blockage of air-flow resulting from the frost growth increases the pressure drop across the heat exchanger by up to a factor of 10, while heat transfer rate decreases by only 21%