Refrigerant mass distribution in an invertible air-to-water heat pump: effect of the airflow velocity

This paper presents the results of refrigerant mass distribution measurements carried out in an invertible air-to-water heat pump. The unit works with refrigerant R32 and produces a nominal cooling capacity equal to 60 kW. Experimental tests have been performed during both cooling and heating operations using the quick-closing valves technique to isolate the charge within the components of the system. A mathematical model, developed to predict the refrigerant charge in the heat exchangers of the heat pump and its performance, is described and validated. The model includes a physical description of the heat transfer mechanisms in the condenser and in the evaporator. The model employs void fraction correlations to calculate the refrigerant mass in the two-phase regions of the heat exchangers. In the finned coil heat exchanger, the model can account for the effects of the airflow maldistribution, which has been also experimentally measured. When the not uniform air velocity is considered, the refrigerant charge in the heat exchangers is modelled within ±10%. The results of the model showed that the airflow maldistribution can increase the total refrigerant charge in chiller mode by 12.7% and decrease the performance of the system by 11%, compared to the case of uniform air velocity.