Experimental and modeling studies of thermally-driven subcritical and transcritical ejector refrigeration systems

•Sub- and transcritical ejector refrigeration systems were experimentally studied.•An ejector model of transcritical pressure working fluid was established.•Transcritical system performance is better than subcritical one at most conditions.•Sub- and transcritical Freon ejector performances were compared.•Oscillations were observed in transcritical ejector refrigeration system. This paper presents an experimental and modeling investigation on thermally-driven subcritical and transcritical ejector refrigeration systems using refrigerant R32 as the working fluid. The performances of the ejector and its corresponding refrigeration system are studied under different ejector primary flow inlet temperatures, primary flow inlet pressures, and ejector outlet pressures. A theoretical model of the ejector is established and validated by experimental data. The results show that the physical properties of the working fluid significantly affect the performance of such ejectors. The mass flow rate of the primary flow exhibits a nonlinear change when the ejector inlet pressure or temperature is near the pseudo-critical state. At high primary flow inlet temperatures and pressures, the mass flow rate of the secondary flow of the transcritical ejector no longer increases with increasing primary flow inlet parameters, showing characteristics like the subcritical ejector. The optimal primary flow inlet pressure of the ejector refrigeration system increases with increasing primary flow inlet temperature, but remains unchanged after reaching about 7.1 MPa. The transcritical ejector refrigeration system exhibits a higher coefficient of performance than its subcritical counterpart at most primary flow inlet temperatures. The transcritical ejector has a better pressure lift performance, while the subcritical ejector exhibits a higher entrainment ratio only at low pressure lift ratios. Moreover, the transcritical system exhibits a unique oscillation phenomenon under some conditions, with the oscillation amplitudes in both the mass flow rates reaching up to 60% of their averages.