Thermodynamic approach for designing the two-phase motive nozzle of the ejector for transcritical CO2 heat pump system

Abstract

In this paper, a one-dimensional thermodynamic analysis is developed to design and evaluate the performance of primary nozzles in two-phase ejector utilized in the transcritical CO2 heat pump system. Two thermodynamic approaches (ideal and non-ideal) are adapted to model the expansion processes for comparison. The operating parameters and correlations were adapted from verified experimental and theoretical studies in open literatures. Mass flow rate, density and quality of the working fluid (CO2) across the primary nozzle are calculated. The isentropic expansion of CO2 through the coverage-diverge (C-D) nozzle is then studied. The supersonic properties of the refrigerant such as Mach number, velocity and the speed of sound are obtained and the relations between them are analysed. Other design parameters related to the nozzle geometrical domain namely the area, the area ratio and the diameters are deduced. The results show that at the nozzle exit section, two phase flow and normal shock wave were detected. There is a similar trend between ideal and non-ideal models in terms of supersonic flow characteristics. The domain, boundary conditions and results of non-ideal model could be further verified by a CFD software in 2D and/or 3D models.