Numerical study and design of a two-stage ejector for subzero refrigeration

The supersonic ejector-diffuser systems are usually used for cooling by utilizing low-grade thermal energy, but few can be used in subzero refrigeration applications. In this paper, a methodology was proposed and applied to design a two-stage ejector for subzero refrigeration. The two-stage ejector, using R134a as the working fluid, was operated at 63-74 °C generator temperature and −24 °C to 0 °C evaporator temperature. The performance (entrainment ratio) and flow phenomena of the two-stage ejector were predicted by numerical studies using computational fluid dynamics (CFD) to find the best design parameters for a range of operating conditions. Simulation results showed that (1) when secondary flow pressure is constant, the critical back pressure increases with the increase of primary flow pressure, while the maximum entrainment ratio decreases with the increase of primary flow pressure; (2) when primary flow pressure is constant, both maximum entrainment ratio and critical back pressure increases with the increase of secondary flow pressure; and (3) when secondary flow pressure and outflow pressure are constant, the two-stage ejector has an optimum entrainment ratio value with the increase of primary flow pressure, and that the optimum entrainment ratios of 0.0736, 0.0761 and 0.0813 can be achieved when saturated temperature is at −22 °C, −20 °C, −18 °C, respectively. Numerical study results showed that it is feasible that the proposed two-stage ejector is used for subzero refrigeration applications which can benefit for cold chain logistics systems.