## 3 Results and Discussion
### Effects of the Convergent Length on Cavitation Characteristics  
#### 3.3.1 Analysis of Pressure and Vapor Volume Fraction  
Figures 12 and 13 disclose the influence of the convergence section length on the absolute pressure and vapor volume fraction at various inlet pressures. Figure 12 indicates that the impact of the convergence section length on the absolute pressure of the flow field depends on the inlet pressure. Specifically, when the inlet pressure is 0.4 MPa, a slower pressure recovery rate is observed for the Venturi with a 24 mm convergence section length. These results suggest that a contraction section length of 24 mm results in a higher throat velocity and slower velocity reduction, potentially impacting the intensity of cavitation. However, for inlet pressures of 0.8 or 1.2 MPa, a shorter convergence section length leads to a longer low-pressure region, with greater differences observed as the inlet pressure increases. Furthermore, changes in the absolute pressure can affect the volume fraction of vapor, as depicted in Figure 13. The coverage and amplitude of the vapor volume fraction decrease as the length of the convergence section decreases when the inlet pressure is 0.4 MPa. Conversely, when the inlet pressure is 1.2 MPa, the coverage area of the vapor volume fraction increases with decreasing convergence section lengths. These findings indicate that the impact of the convergence section length on the cavitation characteristics of the Venturi tube is non-monotonic and also dependent on the inlet pressure conditions.  
Figure 14 compares the effect of the length of the convergence section on the distribution of the vapor phase under different pressure conditions in a more visible way. It can be observed that the cavitation cloud within the Venturi tube, characterized by a 24 mm  
Figure 12: Absolute pressure variation along Venturi wall with different convergence lengths under varying inlet pressure: (**a**) 0.4 MPa, (**b**) 0.8 MPa, and (**c**) 1.2 MPa.  
Figure 13: Vapor volume fraction variation in Venturi tube with different convergence lengths under varying inlet pressure: (**a**) 0.4 MPa, (**b**) 0.8 MPa, and (**c**) 1.2 MPa.  
convergence section length, exhibits a distinct \"jet\" shape under an inlet pressure of 0.4 MPa. Furthermore, it is noteworthy to mention that the length of the cavitation cloud is comparatively shorter compared to other convergence length scenarios. The observed phenomenon can be ascribed to the direct impact of the length of the convergence section on the cross-sectional area of the inlet. The Venturi tube with a small cross-sectional area of the inlet has a higher inflow velocity under the same inlet pressure. Cavities in the throat may have a shorter residence time due to the higher flow velocity, and they are carried into the divergence section by the flow before they fully develop and are pushed to both sides of the tube axis by the flow. As a result, cavitation clouds develop into the shape of jets. However, when the inlet pressure is 0.8 or 1.2 MPa, the shorter convergence length of the Venturi tube has a longer cavitation cloud. There are two potential causes for this. On the one hand, as depicted in Figure 12, a bigger pressure difference causes the low-pressure area with a shorter contraction section length to be longer, which is beneficial to the development of the cavity. Furthermore, fluid impinging on the wall of the convergence section will result in a significant flow loss at high-pressure differentials. Shorter convergence structures reduce the flow loss in the convergence section, improve the throat velocity, and extend the cavitation cloud's coverage area.  
Figure 14: _Cont._