# Numerical Investigation of Methodologies for Cavitation Suppression Inside Globe Valves
## 3 Results and Discussion
### Comparison of Cavitation in Globe Valves with and without Cages  
When there is a large pressure drop, local pressure is very likely to drop under the saturated vapor pressure. Here, the globe valve without a cage, the globe valve with one cage and the globe valve with two cages are simply compared to find out the effects of the cage. The outlet pressure is chosen as 0.1 MPa and the working temperature is 80 \({}^{\circ}\)C.  
The velocity and pressure distributions in the globe valve with a different number of cages are shown in Figure 4, and an obvious difference in the velocity and pressure distributions can be found between different globe valves. The Reynolds number is around \(7\times 10^{5}\), which means the flow regime is turbulent. At the bottom of the valve body, there is a high-pressure and low-velocity zone, which results from the sudden change of the flow direction. When there is no cage, most water flows directly toward the valve downstream, resulting in a small high-pressure zone at the bottom of the valve core closing to the valve outlet. While cages are added, the valve resistance increases, and the pressure distribution at the valve core bottom becomes more even. From Figure 4, it can also be found that the cage number has little influence on the streamline distribution once the cage is added.  
Figure 4: Velocity and pressure distributions for the globe valve with a different number of cages.  
When water vapor volume fraction is above 0.1, the water vapor volume distribution in different globe valves is compared qualitatively [4; 37] and shown in Figure 5. It is clear that cavitation caused water vapor mainly congregates around the valve seat and the bottom of the valve core if there is no cage, but the water vapor only appears at the location near the valve outlet, which is because of the high velocity and low pressure as shown in Figure 4. Besides, a relatively large water vapor volume can be found at the location where the valve body and the downstream pipe connect. When a cage is added, the region where water vapor appears varies and is limited close to the cage itself, which protects the valve body from being damaged. When two cages are added, the region where cavitation appears reduces further but is still near the two cages.  
To compare cavitation intensity in a globe valve with a different number of cages, dimensionless cavitation number \(\sigma\) is used. The expression of the cavitation number is shown below.  
\[\sigma=\frac{p_{o}-p_{v}}{1/2\rho v^{2}} \tag{4}\]  
here \(p_{o}\) is the pressure at valve outlet, \(p_{v}\) is the saturated vapor pressure, \(\rho\) is the density of water, \(v\) is the velocity at the valve outlet.  
The loss coefficient \(\xi\) is also calculated to analyze the flow characteristics of different globe valves. The expression of the loss coefficient is shown below.  
\[\xi=\frac{p_{i}-p_{o}}{1/2\rho v^{2}} \tag{5}\]  
here \(p_{i}\) is the pressure at valve inlet.  
The comparison of the cavitation number and the loss coefficient between different globe valves is shown in Figure 6. It can be found that the cavitation number and the loss coefficient increase with the increase in the valve cage number, which means that cavitation intensity can be suppressed by adding a valve cage in a globe valve. In Figure 5, it is noticed that the water vapor volume in the globe valve with two cages is higher than other globe valves. The reason is that when the total pressure in the valve inlet is the same, the velocity inside the globe valve with two cages is lower due to high resistance, which leads to a higher static pressure at valve inlet. Thus, the actual water vapor volume is higher in the globe valve with two cages. Together with Figures 5 and 6, it can also be found that the cavitation number is useful when deciding the cavitation intensity in valves.  
Figure 5: Water vapor volume distribution for globe valve with a different number of cages: (**a**) no cage; (**b**) 1 cage; (**c**) 2 cages.