Local frosting behavior of a plated-fin and tube heat exchanger according to the refrigerant flow direction and surface treatment

Abstract

Heat pumps have been promoted as energy-efficient heating and cooling systems. However, in the heating mode, heat pump performance is often significantly degraded by the frost layer that forms on the fin surface of the heat exchanger due to the low temperature of the outdoor unit. In this research, we experimentally investigated the thermal performance and frosting behavior at various locations on a plate-fin and tube heat exchanger with respect to refrigerant flow direction (counter-flow and parallel-flow) and fin surface treatment (bare, hydrophilic, hydrophobic, and hybrid). On the bare heat exchanger, the frost growth was more uniform with a counter-flow than with a parallel-flow, and thus the overall heat transfer rate was higher under counter-flow condition. The performance difference between the counter-flow and parallel-flow conditions became more significant when the heat exchanger was operated at lower refrigerant temperature and air velocity. Although the initial heat transfer rate of the hydrophilic unit was the highest among the surface-treated heat exchangers, this unit also suffered a dramatic reduction in the heat transfer rate because of the fast frost growth and non-uniform frosting behavior on its rear side. On the other hand, although the hydrophobic heat exchanger had a slightly lower initial heat transfer rate than the hydrophilic unit, it exhibited a consistent heat transfer rate throughout the experiment, owing to the frost retardation effect and the relatively uniform frosting behavior on its rear side. Each of the hybrid hydrophilic-hydrophobic surface-treated heat exchangers showed a lesser reduction of the heat transfer rate than the hydrophilic unit, and a higher initial heat transfer rate than the hydrophobic unit.