## 1 Introduction
[9] conducted an experimental investigation on the pressure profile along the ejector axial direction and concluded that the performance of the ejector was predominately governed by the amount of secondary fluid as well as the momentum of the mixed stream. [PERSON] et al. [12] studied the effects of non-dimensional parameters including the compression and expansion ratios, on the system performance. The results showed that the coefficient of performance is proportional to the expansion ratio and is inversely proportional to the compression ratio. [PERSON] et al. [13] and [PERSON] et al. [14] applied various boundary conditions for inlet flows and investigated their effect on the entrainment ratio. The results suggested that, with the implementation of superheat inlet flow, improvement of entrainment ratio could be achieved, whereas the pressure at the exit remains unchanged. The performance of a steam ejector can be improved by optimizing the geometric configuration of the key components. [PERSON] et al. [15; 16; 17] and [PERSON] et al. [18] developed the relationship between the entrainment ratio and the area ratio. The area ratio is a critical non-dimensional factor that has a significant impact on the performance of the system which is defined as the ratio between the cross-section area of primary nozzle and constant area section. Other experimental [9] and numerical studies [19; 20; 21] have also been conducted on the nozzle exit position. It was reported that positioning the nozzle exit upstream of the mixing chamber entrance gave better performance than pushing it into the mixing chamber. Furthermore, [PERSON] et al. [22] and [PERSON] et al. [23] investigated the correlation between the primary nozzle diameter and system performance. [PERSON] et al. [24] and [PERSON] et al. [25; 26] performed parametric studies on the influence of different configurations of the primary nozzle, and suggested that the expansion angle of the nozzle is also critical to the performance of the steam ejector. Most experimental studies focused on the optimization of geometric parameters and operating conditions as well as the explanation of experimental phenomena. The running cost for the experiment is also too costly to generate a lot of data for comparison. However, the numerical simulation can perform a comprehensive analysis for the flow behavior based on the thermal dynamics and fluid dynamics with the advantages of reducing cost and increasing the efficiency. Figure 2: Characteristic curve of a steam ejector at a fixed temperature [10]. Computational fluid dynamics (CFD) simulation has become an effective tool to analyze the understanding of complex flow behavior due to the rapid advancement of numerical methods and computational power. Previous numerical studies demonstrated that, with the cautiously selected sub-molecular modeling components, CFD simulation could provide detailed information about the flow field that often difficult to assess by experimental means such as gaining more detail about nucleating small particles in the main flow field [27; 28]. During the past few decades, extensive numerical research works based on ideal gas assumption have been performed to analyze the flow field development of the steam ejector. However, the spontaneous condensation phenomena occurring in the nozzle during operation [29] have not been captured under the ideal gas assumption. When the steam passes through the nozzle, the temperature of steam would be much lower than the stagnation temperature due to the inner energy partly transforming into kinetic energy caused by the occurrence of the condensation phenomena. Fluid thermodynamics characteristics of the inlet flows are described in Figure 3a. The state of primary fluid moves from the original state (point A) to point C, which is an isentropic expansion process. However, the real expansion process is line AC', due to energy loss existing in the actual process.