## 4 Results and Discussion  
### The Influence of Nozzle Geometry on Internal Steam Condensation  
4.1.1 Nozzle Throat Radius  
As steam flows through a nozzle, it expands rapidly and is accompanied by spontaneous condensation. In the flow process of wet steam's spontaneous condensation, the free  
Figure 5: Validation of numerical simulation: **(a)** Moore A nozzle, **(b)** Moore B nozzle, **(c)** Moore C nozzle, and **(d)** the structures of three nozzles. energy barrier of the molecules does not cause the steam molecules to aggregate and condense immediately, which results in the steam continuing to expand beyond the saturation line and deviating from the equilibrium state. Take a nozzle with the throat radius of 0.05 m as an example, as the steam flows from the convergent section to the divergent section, the temperature falls rapidly in the convergent section but a significant temperature step occurs upstream of the divergent section (see Figure 6a). This can be attributed to the release of the latent heat that has been generated by the steam condensing. At the same time, this latent heat heats the surrounding steam and causes an abrupt increase in pressure (Figure 6b) and the Mach number (Figure 6c) at the same position as the temperature step; this is an effect that is known as the condensation shock. The steam saturation ratio that is described by Equation (11) reflects the transition of the steam from the equilibrium state (Sr = 1) to the non-equilibrium state (Sr = 1). As shown in Figure 6d, the saturation ratio remained almost at 1 in the convergent section, while it suddenly increased to the peak value at the position of condensation shock and then rapidly dropped to 1 in the divergent section. As a result of this, massive droplet cores were formed in the condensation shock position when the saturation ratio was big enough to make the steam molecules overcome their intermolecular rejection of coming together. The droplets' nucleation ratio, in Figure 6e, reflects the fact that the massive droplet cores were generated in the condensation shock region and that, after crossing this condensation shock wave, there was no droplet core appearing in the divergent section. After the droplets' nucleation ratio dropped from the peak value of approximately log10\({}^{22.5}\) to zero, the number of droplets per unit volume was kept constant in the divergent section, as illustrated in Figure 6f. Moreover, the liquid mass fraction started to rise until it reached its maximum at the nozzle outlet in the divergent section, as depicted in Figure 6g. It is worth noting that the curve of the liquid mass fraction can be divided into two parts: the rapid nucleation zone with slope 1 and the droplet growth zone with slope 2, as presented by the schematic diagram in Figure 6g. Obviously, slope 1 is larger than slope 2. In the rapid nucleation zone, the reason for the steepness of slope 1 of the liquid mass fraction curve is that the massive droplets nucleated, which resulted in the liquid mass fraction (Figure 6g) and droplets' average radius (Figure 6h) increasing sharply in this zone. However, in the droplet growth zone, the droplets' nucleation ratio (Figure 6e) was zero and the droplets per unit volume (Figure 6f) also trended to a constant, which led to those steam molecules that were present in a free state condensing gradually on the droplet cores. Since the droplet growth process was much slower than the nucleation process, this made slope 2 of the liquid mass fraction curve smaller than slope 1. It is also not hard to infer that the average droplet radius grows much slower in the droplet growth zone than it does in the rapid nucleation zone, as shown in Figure 6h.