# Sediment Erosion Characteristics and Mechanism on Guide Vane End-Clearance of Hydro Turbine
## CFD Predictions
### Computational Domain  
The guide vane used in turbine (HLA351-LJ-180) is a prototype with a net head of 189 m and capacity of 18.4 MW. A small end-clearance was present to adjust the guide vane angle, depending on the load or flow variation. The structure diagram of the end-clearance between the guide vane and the head cover of the turbine is shown in Figure 4.  
The design size of the end-clearance guide vane was 0.22 mm. However, the head cover is raised upward under the high-pressure effect of water in practical operation, making the actual clearance size slightly larger than the designed size, so that it reaches 1 mm. Therefore, the clearance size of 1 mm was used to study the turbulence and sediment erosion characteristics in the end-clearance. To study the influence of a guide vane with a large eccentric shaft on clearance flow (including the flow around the cylindrical shaft) and flow around the airfoil, the geometric structure of the upper part of the guide vane airfoil profiles was used in consideration of the computing resources and computing time. The 3D computational model and relative parameters for the end-clearance of the turbine guide vane are shown in Figure 5.  
Figure 4: Schematic diagram in the end-clearance of a turbine guide vane.  
A Francis turbine with a relatively low guide vane height has good watertight condition. However, the end-clearance of a guide vane is rapidly enlarged because of the high-water pressure, large deformation of the head cover, and sediment erosion. Hence, it is of great necessity to solve the leakage of the end-clearance of the guide vane. The guide vane of a Francis turbine used a large eccentric shaft structure (see Figure 6), that is, a large wing plate was added to the upper and lower pressure surface of the guide vane, and the guide vane shaft was enlarged as much as possible. On one hand, this structure increases the retaining water length in the circumferential direction of the guide vane; on the other hand, it increases the covered area of the guide vane end region at the upper and lower facing plate and the leakage resistance. Thus, the leakage of the guide vane end-surface is reduced and the efficiency of the turbine improves. At the same time, this structure can prevent the direct erosion of the guide vane shaft by a sediment-laden river.  
The computational domain was discretized using a structured grid as shown in Figure 7. Boundary layers were utilized to keep y+ within its accepted limits and accurately consider the wall effects. The first layer thickness of 0.001 mm in the clearance domain was determined to meet the accuracy of the large eddy simulation calculation of the clearance flow field. Then, the y+ values were controlled to be below 1 in the clearance domain. There were 51 grid layers in the clearance of 1 mm to accurately capture the turbulence characteristics. For the purpose of confirming that the grid pattern used was adequate, a grid convergence test was conducted on five different grid numbers. With the increase of grid numbers, the variable quantity of pressure drops between inlet1 and outlet1 is less than 2% as the test standard. Finally, the grid numbers in the computational domain was 7.143 million.  
Figure 5: Computational model and relative parameters (mm).  
Figure 6: Guide vane with a large eccentric shaft.