Effect of leakage flow from guide vanes on erosion of the runner of a Francis turbine

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Abstract
For Francis turbines, the importance of guide vanes in efficient operation is to provide optimum flow conditions to the runner. This requires a clearance gap to be provided at the ends of the guide vanes. Previous studies have shown that these clearance gaps are the source of erosion in the guide vane and the runner. This study presents a numerical investigation of the effects of the clearance gap’s enlargement in terms of flow and erosion. Three guide vane openings are considered, each with and without a clearance gap, to isolate the effect of the clearance gap on the openings. The secondary flow due to the clearance gap is found to contribute to vortex formation, increased turbulence, and erosion, and is called the leakage flow. The erosion locations and normalized erosion arising from the leakage flow are predicted and compared with experimental results. It is found that the erosion at the leading edge is increased by the presence of the clearance gap. Compared with the case without a clearance gap, the erosion at the leading edge is ten times greater under the part load opening condition. Such erosion is concentrated near the hub and shroud for the design condition, but is spread more along the guide vane’s height for other conditions. Leakage flow further contributes to the erosion on the pressure side of the runner in the presence of a clearance gap. This study shows that the vortex developed from the leakage flow contributes to the erosion of the runner significantly at various locations depending upon the vortex’s trajectory.

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