## 1 Introduction
[19] predicted the flow characteristics of a wet steam flow in a nozzle and demonstrated that the steam's condensation behavior could depress the efficiency of the nozzle and reduce the pumping performance of the steam ejector. Moreover, they also studied the effect of primary steam superheating on the steam condensation in a nozzle and the performance of the steam ejector [20]. [PERSON] et al. [21; 22] numerically studied wet steam flow in the nozzle of a steam ejector and found that the steam's condensation behavior reduced the maximum value of the flow Mach number but improved the compression ratio and entrainment ratio of the steam ejector. In order to improve the accuracy of their numerical calculations, [PERSON] et al. [23] implanted a set of codes based on [PERSON]'s classical nucleation rate and droplet growth rate into the CFD  
Figure 1: Schematic diagram of steam ejector and nozzle structure. solver and found that it could finely predict the position of the Wilson point and the wet steam flow properties in a nozzle. Similarly, [PERSON] et al. developed in-house CFD code that was based on a two-fluid model in order to numerically study the supersonic flow of wet steam with a non-equilibrium phase change in a high-pressure nozzle [24] and high-pressure thermo-compressor [25]. A condensing steam flow mode with droplet size distribution in a Laval nozzle was also implemented into in-house CFD code by [PERSON] and [PERSON] [26]. A piece of Euler-Euler two-phase flow model code that was optimized by [PERSON] et al. was validated to be able to capture the complicated condensation characteristics of water vapor in the Laval nozzle [27]. These researchers also found that wet steam flow in a nozzle would reduce the performance of the steam eject more than dry steam. This was reflected in the entrainment ratio that decreased from a score of 0.6973 (which was obtained by the dry steam model) to 0.6242 (which was obtained by the wet steam model) under the given operation condition [28]. Using the multi-objective genetic algorithm method that is based on the modified condensation model, [PERSON] et al. [29] optimized the primary nozzle of a steam eject in order to improve its entrainment ratio. With regard to the aforementioned investigations into steam condensation in the nozzle of a steam eject, these researchers have only paid attention to the impact of the spontaneous condensation behavior of steam on the inner flow and performance of the steam eject and the optimization of numerical algorithms in order to improve the accuracy of CFD simulations. However, there are few studies on the influence of a nozzle's geometric structure on its internal steam condensation behavior. [PERSON] et al. [30] numerically investigated the wet steam flow in nozzles with area ratios of 11, 18 and 25. They observed that all of the nozzles' static pressures that were obtained by the wet steam model were higher than those for the ideal gas model, while no significant difference existed in the nozzles' momentum. [PERSON] et al. [31] also increased the area ratio of the nozzle and found that it reduced the steam condensation's intensity and nozzle's pressure, but increased the liquid mass fraction. In this article, a wet steam model with entropy generation rates that are relevant to three different mechanisms (viscous dissipation, heat transport, and phase change) was innovatively established in order to numerically study the behavior of wet steam condensation in a nozzle. Apart from the nozzles' throat radii, the effects of the divergent section expansion angle and divergent section length of the nozzle on the spontaneous condensation of steam were investigated. Importantly, the distributions of the entropy generation areas in the nozzles with different geometric structures were visualized for the first time. Finally, the relationships between the nozzle's mass flow rate, total entropy production and the liquid mass fraction of the nozzle for three types of geometric structures were established.