# Sediment Erosion Characteristics and Mechanism on Guide Vane End-Clearance of Hydro Turbine
## 4 Results and Discussion  
### Sediment Erosion Mechanism Analysis in End-Clearance Walls of the Guide Vane  
Figure 10 shows a comparison of the wear morphology and flow field on the end-surface guide vane. Figure 10a shows the instantaneous velocity field at 1 s of the numerical simulation. It can be seen in Figure 10a that the sand-laden water flowed through the guide vane at a certain angle. The fluid pressure increased before entering the clearance due to the barriers of the airfoil wall and the head cover wall. Next, a pressure difference was produced on the internal and external clearance. The above situation led to an obvious increased velocity of entry into the clearance. The fluid velocity decreased gradually with the further development of the fluid. Additionally, the flow field at the front of the guide vane shaft was uniform without an obvious vortex structure. Combining Figure 10b-d, it was found that there was basically no accretion of sediment particles and no obvious erosion wear on the end-surface of guide vane before the shaft, which was basically consistent with the actual wear morphology. When the fluid flowed through the shaft, obvious vortex structures appeared on both sides of the shaft due to the disc shear effect. After fluid flowed through the shaft, a symmetrical von Karman vortex street was observed. Again, by combining Figure 10b-d, it was found that there was a high impact quantity of particles at regions where the vortex appeared, which is mainly because the particles had the following behaviors and will be involved in the vortex. Additionally, the erosion wear was serious in regions where large accretion of particles was observed, which mainly appeared behind the shaft and at both end-surfaces of the shaft. The wear pattern was mainly furrow shaped. On the end-surface after the guide vane shaft, the fluid flowed evenly, so only a few particles impacted, and the erosion wear was small. The above situation was mainly because the guide vane adopted a large eccentric shaft structure, and the fluid entered the clearance from one side of the large wing surface without being affected by the shaft. Figure 11 shows a comparison of the wear morphology and flow field on the end-surface of the head cover. As can be seen from Figure 11c, the erosion wear regions on the end-surface of the head cover mainly focused on the corresponding head cover to the leading edge and the suction side (small wing surface) of the guide vane. On the corresponding head cover to the leading edge of  
Figure 10: Comparison of wear morphology and flow field on the end-surface of the guide vane: (**a**) Flow field; (**b**) discrete phase model (DPM) accretion; (**c**) DPM erosion; and (**d**) wear morphology. the guide vane, the wear morphology was obviously pit superimposed by multiple erosion. From Figure 11a,b,d, this is because the fluid entered the clearance in an oblique uprush status, the sand particles continuously impacted on the end-surface of the head cover, and the accretion rate and velocity of sand particles were higher here. It can be seen from the above that high velocity, high concentration, and a large impingement angle of sand particles are the main reasons for serious erosion wear. On the corresponding head cover to the suction side of the guide vane, the wear pattern was furrow shaped. However, there was less erosion wear on the corresponding head cover to the pressure side (large wing surface) of the guide vane. This is because the accumulation area and velocity of sand particles on the former were larger than those on the latter. It can also be seen that although the DPM accretion rate of sand particles was larger before the shaft, there was almost no wear from the actual wear morphology. Combined with Figure 11a,b of the instantaneous velocity field and particle velocity field at 1 s of the numerical simulation, the fluid velocity gradually decreased and the velocity defect increased when fluid entered into the clearance to the front of the shaft.