Effect of Transient Heat Transfer of a Condenser on a Cascade Heat Pump Performance

Abstract

Transient heat transfer characteristics of a condenser of a cascade heat pump are investigated by comparing the experimental result and the prediction from a mathematical model. In the experiment part, the cascade heat pump consists of the low-temperature refrigeration cycle using R22 and the high-temperature heat pump cycle using R134a. The transient effect is presented by circulating water to receive the rejection heat from the condensing refrigerant of the cascade heat pump causing the rise of the hot water temperature. On the other hand, a mathematic model of the condensation heat consisting of the heat transfer models in the two-phase mixture region and the superheated vapor region is developed to compare the results. The effect of the hot water flow rate on the hot water temperature and the heat transfer rate is examined. The result indicates that the hot water temperature continuously increases with time. The condensation heat rapidly increases at the beginning and gradually increases with time. The result obtained from the model prediction of the condensation heat is similar to the experimental result except the initial and final period. The discrepancy between these two results is because of the transient behavior at beginning and the existent of the subcooled region.