Effect of turbulence models and cavitation intensity on the motive and suction nozzle mass flow rate prediction during a non-equilibrium expansion process in the CO₂ ejector

A variety of ejector constructional concepts can be effectively examined by advanced numerical tools. The development of the modelling techniques of the two-phase meta-stable expansion process in the CO₂ ejector motive nozzle provides the detailed flow data of the primary stream. However, the complexity of the entrainment processes in the mixing section still requires attention for the accurate prediction of the suction nozzle flow. Hence, in this paper, an analysis of four turbulence models and cavitation phenomena (four intensity levels) in the CO₂ ejector was performed on the basis of the calibrated non-equilibrium expansion model of the ejector motive nozzle. Twenty-three motive nozzle points from the subcritical range (from 50 bar to 70 bar and 8 °C to 23 °C) were coupled with the evaporation temperature of -6 °C and two pressure lift levels (3 bar and 6 bar). Simulations of 460 cases were validated using data from the CO₂ ejector refrigeration unit with a 50-kW cooling capacity. The vapour quality distribution in the motive nozzle with cavitating evaporation was described in terms of the primary pressures and the considered turbulence models. An influence of the turbulence formulation on the secondary stream behaviour in the mixing zone was also investigated regarding the suction nozzle mass flow rate. Finally, the proposed model of the CO₂ ejector was characterised by the mass flow rate prediction discrepancy at an average level of 4.6% and 12.3% for the motive and suction nozzle, respectively.