CO₂ is regarded as an effective and environmentally friendly refrigerant. Using a CO₂ ejector is a proven method for enhancing the effectiveness of a transcritical CO₂ refrigerant system. However, the complex internal flow of a CO₂ ejector, involving supersonic effects, phase change effects, metastable effects, and so on, makes it difficult to understand. In order to summarize the current state of the technology and knowledge gaps, this work provides a comprehensive literature review on CO₂ ejectors. In the first part, mathematical modelling and simulation calculations of CO₂ ejectors are presented, and an overview and classification of ejector models are given. In the second part, the structural optimization part of the ejector is described in detail, and the nozzle structure, the mixing chamber length, improvements to multi-jet systems, and the impact of these factors on the system performance are analyzed. In the third part, flow visualization is used to study the complex flow phenomenon, and the effect of the shock wave on the entrained rate of the ejector is discussed. Finally, the paper outlines the relationship between all ejector technologies, working fluids, and ejector performance and makes valid recommendations for further research and development of CO₂ ejectors.

This paper establishes a mathematic model of a CO₂ two-phase ejector to investigate flow distribution in the components of a mixing chamber and diffuser. The suction chamber was modeled using the characteristic line method to describe the development process of the supersonic expansion wave, and the mixing chamber, as well as diffuser models, were built based on the double-flow model. The reliability of the model was verified by experimental data. The distributions of flow parameters along the axis of the mixing chamber and diffuser were analyzed under different expansion ratios of the ejector. Structure optimizations of the mixing chamber and diffuser were conducted. The results showed that the primary flow temperature gradually increased along the axis of the mixing chamber and diffuser, but the Mach number distribution decreased for a certain ejector expansion ratio. The temperature and Mach number of the secondary flow showed the opposite trend. Moreover, at the initial stage of mixing, the fluid pressure increased rapidly, and the Mach number of the primary flow decreased rapidly. The gas-phase fraction of primary flow increased gradually in the mixing chamber and was stable in the diffuser. When the length–diameter ratio of the mixing chamber was about 10.8–12, it was beneficial to mix uniformity, and when the expansion angle of the diffuser was 4–6°, the ejector had a better ejector efficiency.

An improved model to calculate the length of the mixing chamber of the ejector was proposed on the basis of the Fano flow model, and a method to optimize the structures of the mixing chamber and diffuser of the ejector was put forward. The accuracy of the model was verified by comparing the theoretical results calculated using the model to experimental data reported in literature. Variations in the length of the mixing chamber L_m and length of the diffuser L_d with respect to variations in the outlet temperature of the ejector T_c, outlet pressure of the ejector p_c, and the expansion ratio of the pressure of the primary flow to that of the secondary flow p_g/p_e were investigated. Moreover, variations in L_m and L_d with respect to variations in the ratio of the diameter of the throat of the motive nozzle to the diameter of the mixing chamber d_g0/d_c3 and ratio of the outlet diameter of the diffuser to the diameter of the mixing chamber d_c/d_c3 were investigated. The distribution of flow fields in the ejector was simulated. Increasing L_m and d_c3 reduced T_c and p_c. Moreover, reducing p_g/p_e or d_g0/d_c3 reduced T_c and p_c. The length of the mixed section L_m2, which was determined on the basis of the Fano flow model, increased as p_g increased and decreased as d_c3 increased. The mixing length L_m1, which was considered the primary flow expansion, showed the opposite trend with that of L_m2. Moreover, L_d increased as p_g/p_e and d_c/d_c3 increased. When the value of d_c was 1.8 to 2.0 times as high as that of d_c3, the semi-cone angle of the diffuser ranged between 6° and 12°. At a constant d_c/d_c3, decreasing T_c and p_c increased L_d.