Effect of fin types of outdoor fan-supplied finned-tube heat exchanger on periodic frosting and defrosting performance of a residential air-source heat pump

Abstract

The effect of fin types—flat, wavy and louver fins—of outdoor fan-supplied heat exchanger on the periodic frosting and defrosting performance of a residential air-source heat pump was experimentally compared. Six consecutive frosting/defrosting cycles were performed for each fin type, and the frosting/defrosting durations were almost equal, about 46.6 min and 3.9 min for each fin type, due to the same defrosting control schemes. The time-average values during the third, fourth and fifth frosting/defrosting cycles were used to compare the overall frosting/defrosting performance among three fin types, and the real-time data during the fourth cycle were analyzed to compare their dynamic characteristics. The experimental results showed the flat fin had the best thermal performance of the air-source heat pump, followed by wavy and louver fins. The time-average heating capacity, COP and input power for flat fin during frosting cycle were more by 17.1%, 9.0% and 7.6%, respectively than those for louver fin. The frosting curves for flat fin increased slightly and then decreased gradually, while the curves for wavy and louver fin types increased slightly at first, then decreased rapidly and gradually at last. Meanwhile, the curves for louver fin decreased considerably earlier than those for wavy fin. The initial increases of the frosting curves were ascribed to that the positive frost effect of increased surface roughness surpassed both the negative frost effects of increased frost layer thermal resistance and reduced fan-supplied airflow rate due to frost blockage. The frosting curves for wavy and louver fin types decreased rapidly because the outdoor axial fan experienced the stall zone caused by more frost accumulation and greater air pressure drop, and decreased slightly at last after the stall zone was went through. The frosting curves for flat fin decreased always slightly because the outdoor axial fan did not experience the stall zone during the frosting duration of 46.6 min. The frosted finned-tube heat exchanger and fan characteristic should be coupled together for the optimal design.