# Influence of Cutter Head on Cavitation of Non-Jammed Submerged Grinder Pump
## 4 Numerical Results and Analysis
### Determination of the Number of Cutter Heads
#### 4.2.2 Influence of the Number of Cutter Heads on Cutter Head Cavitation  
For the submersible grinding pump, the higher the number of cutter heads in the moving cutter head, the more times it can cut and grind sundries over the same length of time, and the corresponding grinding effect is better. However, changes in the number of cutter heads have a great influence on the performance of the grinding pump and the cavitation of the cutter head.  
Figure 15 shows the distribution cloud map of the volume fraction of the vapor on the middle section of the water body of the two, three, and four cutter heads, respectively. The area with the higher vapor volume fraction was distributed near the cutter head--this means that cavitation occurred near the cutter head, which is basically consistent with the cavitation area near the cutter head in the grinding effect test. The area with the higher vapor volume fraction was the cavitation area, which is more consistent with the area with lower pressure. There were two main sites where cavitation occurred near the cutter head: The first site was in the rear half of the cutter head, which was caused by the bad shape of the wing; airfoil cavitation was caused by the high-speed rotation of the cutter head, the relatively large flow velocity near the cutter head--resulting in low pressure--and the poor shape of the cutter head. The second site was in the gap between the dynamic and static cutter discs; gap cavitation was caused by drops in pressure due to high flow velocity in the gap.  
It can be seen from the figure that with increases in the number of cutter heads, the airfoil cavitation area in the middle and rear of the cutter head gradually became larger, but the area of clearance cavitation did not change much. This is because when the number of cutter heads increases, the expulsion effect of the cutter head on the water increases,  
Figure 15: Vapor volume fraction of the cutter midsection with different numbers of cutter heads. (a) 2 cutter heads, (b) 3 cutter heads, (c) 4 cutter heads.  
and the relative flow rate increases accordingly, which eventually leads to a larger airfoil cavitation near the cutter head.  
Figure 16 shows the variation trend of the surface pressure of the cutter head from the inlet side to the outlet side when the number of cutter heads was two, three and four, respectively. In all three cases, the pressure near the cutter head x/X = 0.4 and x/X = 0.6 reached the vaporization value of 3574 Pa, which implies that cavitation may have occurred near the two above sites. This corresponds to the cloud map of the vapor volume fraction distribution. It can be seen that with increases in the number of cutter heads, the area where the absolute pressure of the cutter head surface was at the vaporization pressure of 3574 Pa also gradually increased.  
To sum up, with increases in the number of cutter heads, the expulsion effect of the cutter head on water increased, and the relative flow rate near the cutter head increased correspondingly--which eventually led to aggravated cavitation near the cutter head of the grinding pump. Therefore, under the premise of ensuring the cutting and grinding effect, the number of cutter heads should be selected to be as low as possible.