Frosting Behaviors and Thermal Performance of Fin-Tube Heat Exchangers according to the Surface Treatments

Abstract

Microscopic analyses and thermal performance evaluation were conducted to investigate the frosting and defrosting behavior of surface treated heat exchangers. An optimal surface treatment to enhance the thermal performance of the heat exchangers under the frosting condition was determined. Experiments were carried out to investigate frost nucleation behavior on plate fins with different surface coatings. Hydrophilic coated fin showed a film type of frost layer, leading to rapid frost growth. On hydrophobic coated fin, where the Gibbs free energy for nucleation was larger, the frost growth was slower; hence, the condensed water remained liquid for longer time during early stage of the frosting. To evaluate the effect of the surface treatment on the heat exchangers, frosting experiments were carried out on plate fin-tube heat exchangers with hydrophilic, hydrophobic, and hydrophilic-hydrophobic complex surface treatments. Because each surface had a different condensation pattern, and due to the leading-edge effect, the heat exchangers showed different frost growth behavior depending on the location of the surface (i.e., the distance from the tube, and front and rear sides of the heat exchanger). The hydrophilic heat exchanger had the fastest frost growth among the surface treated heat exchangers, with significant blockage concentrated on the F-S (fin-side) of its rear side, resulting in the largest air side pressure drop and a dramatic reduction of the heat transfer rate. While, frost growth was delayed on the hydrophobic surface treated heat exchanger, reducing the air-passage blockage on its rear side. Accordingly, it had the lowest air-side pressure drop and a consistent heat transfer rate over the entire experiment. Each of the hydrophilic-hydrophobic complex surface treated heat exchangers exhibited an air-side pressure drop and heat transfer rate midway between those of the hydrophilic and hydrophobic treated heat exchangers. In surface treated louvered fin-tube heat exchangers, which are commonly used as the outdoor units of heat pumps, air passage blocking by frost occurred at the louvered leading-edge before the fin leading-edge. Also, in the early stages of the experiments, the louvered fin-tube heat exchanger had higher heat transfer rates per unit volume than plate fin-tube heat exchangers. However, due to the increased heat transfer rate, frost growth was faster, and louvered fin-tube heat exchanges showed more rapid performance degradation. During the repeated frosting/defrosting cycles, the hydrophobic surface treated heat exchanger experienced a smaller reduction in the heat transfer rate, with a smaller air-side pressure drop, due to frost retardation. Hence, the hydrophobic heat exchanger provided the best performance under frosting condition. Also, under the wet condition, the hydrophobic and dual type (with a hydrophilic surface on one side of each fin and a hydrophobic surface on the other side) heat exchangers had higher air-side pressure drops than the hydrophilic unit, since the lots of condensed water remained on the fin surfaces.