A study of the reverse cycle defrosting performance on a multi-circuit outdoor coil unit in an air source heat pump – Part II: Modeling analysis

► In this study, we develop a semi-empirical model of the airside defrosting in the four-circuit outdoor coil unit of an ASHP unit. ► We validate the model by comparing the predicted defrosting duration and the temperature variation of the collected melted frost. ► We quantitatively analyze the effects of the downwards flowing melted frost along the outdoor coil surface on defrosting performance. ► The effects includes taking away heat and prolonging the defrost duration. The experimental part of the study of the reverse cycle defrosting performance on a four-circuit outdoor coil unit in an experimental 6.5 kW heating capacity residential ASHP has been reported in Part I of a two-part series. The experimental results indicated that downwards flowing of the melted frost along a multi-circuit outdoor coil surface would have negative effects on defrosting performance. In order to quantitatively analyze the effects, this paper, the second part of the two-part series, reports on a modeling analysis where a semi-empirical model of the airside defrosting in the four-circuit outdoor coil unit was developed. The model was validated by comparing the predicted defrosting duration and temperature variation of collected melted frost with the corresponding experimental data presented in Part I of the two-part series. Using the validated model, the frost-melting rate and the temperature of melted frost on the surface of each circuit were predicted. The effects of the downwards flowing melted frost along the entire outdoor coil surface on defrosting performance were quantitatively analyzed.