Mass flow characteristics of CO2 operating in a transcritical cycle flowing through a needle expansion valve in a direct-expansion solar assisted heat pump

An expansion device is one of the key components of heat pump systems. Although in the literature there are several studies that analyze the effect of opening the expansion valve on heat pumps, none of them reported this effect for heat pumps that operate with solar energy. In this study, the mass flow rate characteristics through an expansion device in a direct-expansion solar assisted heat pump (DX-SAHP) operating with CO2 is investigated. With experimental data, a new correlation is proposed. A statistical method is used to determine the dimensionless.. groups that most influences the mass flow rate. The parameters selected to compose the new correlation are: outlet and inlet pressures of the expansion device, geometric (cross-section area) and fluid viscosity. The proposed correlation shows good agreement with the measured data with average and standard deviations of 0.77% and 10.4%, respectively. The maximum relative deviation of all predictions of mass flow rate is less than +/- 30% and approximately 90% of mass flow rate experimental data are within +/- 20% of the predictions. In addition, for a same needle expansion valve opening, the mass flow rate increases with the augment in the solar radiation flux.