Numerical study on two-phase supersonic expansion refrigeration in novel CO2 refrigeration technology

•Propose an innovative approach to CO2 refrigeration using a novel two-phase expansion cycle.•Develop an advanced CFD model to predict the supersonic expansion and condensation of pure CO2.•Explore the effects of inlet conditions on pure CO2 supersonic condensation and liquefaction.•The maximum COP is 2.50 and the maximum Relative Carnot Efficiency reaches 0.826 of the ideal cycle. The serious environmental problems have promoted the development of CO2 natural working fluid refrigerants. The supersonic two-phase expander with supersonic nozzle as the key component is proposed. A novel CO2 supersonic two-phase expansion refrigeration cycle is constructed and its cycle performance is also calculated. Then, the numerical simulation of the flow condensation and liquefaction process of CO2 in the nozzle is carried out. The results show that the inlet parameters of the supersonic two-phase expander have an important effect on the cycle performance. With the increase of inlet pressure, COP first increases and then decreases. The optimal inlet pressure of the system is 5.3 MPa with the corresponding maximum COP by 2.50, and the Relative Carnot Efficiency decreases from 0.809 to 0.755. With the inlet temperature increase, the maximum COP and the Relative Carnot Efficiency are 2.49 and 0.826, respectively. The established model can reasonably describe the supersonic expansion refrigeration characteristics and condensation phase transition characteristics of CO2. With the increase of nozzle inlet pressure and the decrease of temperature, the inlet degree of supercooling increases, while the initial nucleation position moves forward further and the peak nucleation rate decreases. Meanwhile, the droplet number decreases, both the droplet radius and the liquid mass fraction increases. The ideal cycle performs well, which provides a valuable reference for CO2 utilization.