## 4 Conclusions  
In this study, the CFD method was used to study the Venturi tube's cavitation phenomenon. The effects of convergence and divergence segment length on Venturi tube cavitation characteristics are investigated and discussed. The findings provide useful guidance for designing and studying Venturi structures under various pressure conditions. The present work leads to the following conclusions:  
The cavitation characteristics around the throat region are generally unaffected by changes in the divergence section's length. In addition, the influence of the divergent section length on Venturi cavitation characteristics relies on the pressure conditions provided. With the increase in pressure drop, the Venturi tubes with shorter divergence section lengths have longer low-pressure areas, which can yield greater cavitation zones. The peak turbulence kinetic energy at the divergence section of the Venturi tube shows a two-stage growth. In the first growth stage, the Venturi tubes with shorter divergence sections have a faster growth rate of the turbulence kinetic energy, resulting in a significant enhancement of the bubble collapse strength. In the second growth stage, the growth rate of the turbulence kinetic energy peak slows down due to the limited development space of the straight section, leading to a suppression of the enhancement of bubble collapse strength. Unlike the divergence section, the length of the convergence section has a more profound impact on the cavitation behavior in the Venturi tube. Under different pressure conditions, the length of the convergence section exhibits varied cavitation behavior. The cavitation effect in the Venturi tube is not enhanced by shortening the length of the convergence section when the inlet pressure is low. In contrast, a shorter convergence section length can extend the low-pressure area and promote the formation of longer cavitation clouds when the inlet pressure is high. The amplification of turbulent kinetic energy induced by the inception of a cavity at the throat of the Venturi tube increases as the length of the tube's contraction section grows. However, the primary factor influencing turbulence kinetic energy at the diffuser section is the inlet pressure conditions. With increasing inlet pressure, the effect of changing the contraction section length on turbulence kinetic energy at the diffuser section gradually diminishes. Under the same inlet pressure, the effect of Venturi tube divergence section lengths on the outlet mass flow rate and maximum throat velocity is minor, whereas the length of the convergence section has some discernible impact on these parameters. As the convergent section increases, the outlet mass flow rate and the maximum throat velocity exhibit a decreasing trend. \begin{table}
\begin{tabular}{c c c c c c c} \hline \multirow{2}{*}{**Cases**} & \multirow{2}{*}{\
\begin{tabular}{c} **P\({}_{\text{in}}\)** \\ **(MPa)** \\ \end{tabular} } & \multirow{2}{*}{
\begin{tabular}{c} **\(\sigma\)** \\ **(mm)** \\ \end{tabular} } & **L\({}_{\text{C}}\)** & **Q** & **V\({}_{\text{t}}\)** & **Vol** & **Vap\({}_{\text{m}}\)** \\ \hline
4 & & 0.2480 & 24 & 1.92805647 & 28.05349 & 8058.9 & 0.9013388 \\
2 & 0.4 & 0.2603 & 48 & 1.88341402 & 27.27604 & 19,173.1 & 0.8957157 \\
5 & & 0.2678 & 72 & 1.86739731 & 26.88531 & 19,232.4 & 0.8973735 \\ \hline
9 & & 0.1236 & 24 & 2.72222116 & 39.70596 & 110,433.2 & 0.9740466 \\
7 & 0.8 & 0.1298 & 48 & 2.67167545 & 38.62886 & 85,491.1 & 0.9747809 \\
10 & & 0.1328 & 72 & 2.65792438 & 38.19953 & 77,281.3 & 0.9731250 \\ \hline
14 & & 0.0823 & 24 & 3.34104545 & 48.63226 & 253,014.8 & 0.9919481 \\
12 & & 0.0863 & 48 & 3.28465412 & 47.39365 & 162,272.6 & 0.9926510 \\
15 & & 0.0881 & 72 & 3.26788526 & 46.89446 & 146,933.8 & 0.9918739 \\ \hline \end{tabular} \({\text{V}_{\text{t}}}\) represent the maximum velocity of the Venturi throat; Vol is the volume of the cavity in the Venturi; \({\text{Vap}_{\text{m}}}\) is the maximum vapor holdup.  
\end{table}