## 1 Introduction
[1; 17] conducted different experiments under three nozzle ejectors with a different cross-section area. A certain structure of a steam ejector was found to enable the ejector refrigeration system work in a heat source temperature between 40 \({}^{\circ}\)Cand 70 \({}^{\circ}\)C. [PERSON] et al. [18] established an experimental ejector refrigeration system driven by low-grade energy of 55 \({}^{\circ}\)C using HFO-1234 ze(E), as the working fluid. Computational fluid dynamics (CFD) method is widely used in the study of the steam ejector owing to its comprehensive analytical potential benefit by the visualization [19], as well as the advantages of prediction for the flow dynamic behavior [20]. It not only can predict and reflect the flow characteristics inside the ejector but also can estimate the pumping efficiency of the ejector. Besides, the complex and non-linear turbulent effect on the fluid structure can be characterized to evaluate the mixing process and the wall shearing behaviours [21; 22; 23]. The simulation study of geometric parameters were mainly in terms of nozzle structure [24], the area ratio of nozzle throat to constant-area section [25; 26], the NXP (nozzle exit position) [27; 28; 29], diffusion angle [30] and the structure of mixing chamber [31]. The influence of five different nozzle geometry on the flow characteristics inside the ejector was studied by [PERSON] et al. [32]. The results showed that the increase of ER can be achieved by increasing the exit perimeter and the special geometric structure. [PERSON] et al. [33] discussed the effect of eight different primary nozzle structure on the ejector refrigeration performance. [PERSON] and [PERSON] [34] analysed the influence of the NXP on the performance of the ejector and obtained the optimal geometry of the ejector using the methanol as the working fluid. The variation of pumping performance with a variable primary nozzle geometry was discussed by [PERSON] et al. [11]. [PERSON] et al. [35; 36], [PERSON] et al. [26] and [PERSON] et al. [37] comprehensively analysed and discussed the variation of the pumping performance under different geometric and operating parameters. A better understanding of the mixing process of the working steam and the pumped steam under different back pressure was studies by [PERSON] and [PERSON] [38]. According to the work of [PERSON] et al. [27], a higher COP can be obtained by decreasing the temperature of primary steam and raising the temperature of secondary steam. In Part 1, the flow structure characteristics of the entire fluid in the steam ejector were analysed meanwhile, the correctness and applicability of the CFD model were verified. Since the primary fluid stream drives additional flows via the secondary inlet, it significantly improves the overall momentum across the ejector and outlet thrust. With the utilization of pressurized steam as the primary fluid, such a system can be potentially applied for automobile cooling system. Nonetheless, the optimization of the pumping efficiency of the steam ejector has always been the major consideration in the design of ejector refrigeration systems. Different from other devices with one converging-diverging structure, there were two converging-diverging structures in a typical steam ejector, one was in the nozzle, and the other was in the subsonic diffuser. The primary fluid is accelerated into the supersonic fluid in the primary nozzle. The primary fluid is mixed with the suction steam and discharged into the condenser through the pressure difference in the subsonic diffuser. Due to the unique, plain structure of the ejector, the flow structure of the fluid in the ejector is also complex, such as transonic flow [15], choking flow [39], shock train [40], boundary layer separation [41], shock-mixing layer [42] and the pseudo shock [40; 43], etc. Shock waves are generated during the transonic fluid flow. When a shock wave contacts with the boundary layer, a shock-mixing layer structure will be formed.