*4.2. Cavitation Characteristics of Impeller in Waterjet Propulsion Pump System*

In order to understand the cavitation characteristics of the water jet propulsion pump system more comprehensively, the waterjet propulsion pump equipped with inlet passage was calculated and analyzed. Figure 15 is the comparison between the cavitation characteristic curves of waterjet propulsion system and that of the waterjet propulsion pump at the designed flow rate. The figure shows that the head and efficiency of the pump have decreased after assembling the inlet passage. The flow rate of the pump has not changed under the same inlet condition, so the *NPSHr* of the pump remains unchanged. After assembling the inlet passage, the hydraulic loss and installation height increase, which reduces the *NPSHa* of the pump system and makes the cavitation performance of the pump worse.

**Figure 15.** Cavitation characteristic curve of the waterjet propulsion system.

Figure 16 shows the distribution of the vapor volume fraction on the suction surface of the blade in the waterjet propulsion pumping system. Compared with Figure 10, the cavitation form in the impeller is still airfoil cavitation after assembling the inlet passage. When the inlet passage was assembled, the distribution of cavitation region on the blade surface becomes uneven due to the influence of the

flow pattern of the inlet passage. As shown in Figure 16, under the same *NPSHa* conditions the area of cavitation region on different blade surfaces is different, and the degree of cavitation development is also different. Comparing Figure 16a,b, it can be seen that when the *NPSHa* value is 1.71 m, the cavitation of the No. 3 blade develops the fastest, and the cavitation was first discovered at the rim of the No. 3 blade near the leading edge. This phenomenon is mainly due to the impeller rotation and the uneven distribution of the flow pattern at the outlet of the inlet passage. With the decrease of *NPSHa* value, cavitation began to appear in the rim position near the leading edge of the blade, except for blade No. 3. It can be seen that the cavitation characteristics of impeller blades are changed after assembling the inlet passage due to the influence of the passage. With the decrease of *NPSHa*, the cavitation on the suction surface of the blades gradually extends from the leading edge to the trailing edge. When the *NPSHa* value decreases from 2.03 m to 1.30 m, it can be seen from Figure 16 that cavitation gradually occurs in the impeller blade during this process. When the *NPSHa* value is 1.3 m, 50% of the suction surface of the No. 2 and No. 3 blades are covered by vapors. At the same time, it can be seen from Figure 15 that the head and efficiency of the pump system are declining in this process. However, the head and efficiency of the pump device in Figure 13 have a certain upward trend when the *NPSHa* value decreases from 1.30 m to 1.15 m. Compared with Figure 16d,e, it can be seen that the distribution of vapors on suction surface of the No. 2, No. 3, and No. 4 blades becomes uneven. As can be seen from Figure 16h, when the cavitation is fully developed, the regions with large vapor volume fraction are mainly concentrated in the middle and trailing edges of the blades. Therefore, cavitation erosion is more serious in the middle and trailing edges of suction surfaces of blades.

**Figure 16.** *Cont*.

**Figure 16.** Distribution of vapor volume fraction on suction surface of blade.

The uneven distribution of cavitation region in impeller is related to the flow pattern at the inlet of impeller. The cavitation development in impeller is affected by the change of inlet structure after assembling the inlet passage. Figure 17 shows the velocity distribution contour of the passage outlet with different *NPSHa* values. It can be seen form the figure that the flow velocity distribution at the outlet of the flow passage is not uniform. The flow velocity around the wall of the flow passage is high, while that near the drive shaft is low. According to the law of conservation of energy, the flow velocity around the wall of the passage is high, so the corresponding low pressure region will first occur at the rim of the blade, which results in the initial position of cavitation near the leading edge of the blade. The red color region in Figure 17a represents the high-speed region, so the region where the blade under high-speed impact first produces cavitation in Figure 16d.

**Figure 17.** Velocity distribution contour of the passage outlet with different *NPSHa* values.

The inlet passage has a great influence on the flow pattern of impeller inlet, mainly because the inlet shape of the passage is irregular, especially the lip. The lip curvature changes greatly, so the flow pattern at the flow passage inlet is uneven, and the lip is the most prone part of cavitation in the passage. Moreover, there is a diffusion section between the impeller inlet and the outlet of the runner, and the diffusion angle affects the influent flow pattern. At the same time, the curved section in front of the impeller inlet causes a change in the flow rate, and the curved section is also a potential region where cavitation occurs. It can be seen from Figures 18 and 19 that critical cavitation occurs at the lip location. As *NPSHa* decreases, the cavitation region of the curved connecting section on the lower side of the drive shaft gradually increases, and the cavitation region begins to appear on the upper and lower sides of the drive shaft.

**Figure 18.** Vapor volume distribution in the inlet passage with different *NPSHa* values.

**Figure 19.** Cavitation volume distribution in the inlet passage with different *NPSHa* values.
