supercritical C02 cycle, two-phase ejector, phase change model, interfacial area density, low GWP refrigerants
The reduction of global warming potential gases emissions in industrial refrigeration devices requires the development of efficient cycles using low GWP gases. The supercritical C02 cycle assisted by a transcritical two phase ejector is a nice example of such cycles. The design of such ejectors is not obvious and an intense development of performance prediction tools is observed in literature especially regarding two-phase CFD modeling. The two phase flow CFD models are limited today by their lack of flexibility and difficulties to empirically calibrate them. The main drawback of classical Hertz-Knudsen or HRM phase change models is their difficulty to simultaneously predict the mass flow rate and the outlet velocity of the primary flow of the ejector. These two parameters are responsible of the entrainment ratio an ejector at given operating conditions and geometry. This paper presents a thermal phase change model that introduces a non symmetric interfacial area density formulation for two-phase liquid-gas CO2 ejectors. This formulation describes the gas-liquid interface as bubbles at low void fractions and as droplets at high void fractions. The model results are compared to experimental results of literature. It appeared that bubbles number density affects mainly the primary flow rate and the droplets number density affects the outlet velocity of the primary nozzle and thus the mixture pressure and the secondary flow rate.