a b s t r a c tSediment erosion of the hydropower turbine components is one of the key challenges due to the constituent of hard particles in the rivers of Himalayas and Andes. In the case of Francis turbines, previous studies show that the erosion is mostly observed around stay vanes, guide vanes and runner blades. Depending upon the type of flow phenomena in particular regions and operating conditions, the sediment particles having certain geometric and material properties create distinct erosion patterns on those regions. The flow phenomena in Francis turbines are highly unsteady, especially around guide vanes and runner. The unsteadiness arises in the form of leakage through clearance gap, horseshoe vortex, rotorstator-interaction and turbulences supported by high velocity and acceleration. The erosion on the other hand deteriorates the surface morphology, aggravating the flow. Based on a thorough literature survey, this paper explains the simultaneous nature of the two effects, which in combined, contributes to more losses, vibrations, fatigue problems and failure of the turbine. It also discusses some of the research endeavors to minimize the combined effect by controlling either the erosion or the secondary flow in the turbine. This review paper emphasizes the need of understanding the relationship between the two phenomena and techniques of how the combined effect can be predicted as well as minimized.

Clearance gaps between guide vanes and cover plates of Francis turbines tend to increase in size due to simultaneous effect of secondary flow and erosion in sediment affected hydropower plants. The pressure difference between the two sides of the guide vane induces leakage flow through the gap. This flow enters into the suction side with high acceleration, disturbing the primary flow and causing more erosion and losses in downstream turbine components. A cascade rig containing a single guide vane passage has been built to study the effect of the clearance gap using pressure sensors and PIV (Particle Image Velocimetry) technique. This study focuses on developing a numerical model of the test rig, validating the results with experiments and investigating the behavior of leakage flow numerically. It was observed from both CFD and experiment that the leakage flow forms a passage vortex, which shifts away from the wall while travelling downstream. The streamlines contributing to the formation of this vortex have been discussed. Furthermore, the reference guide vane with symmetrical hydrofoil has been compared with four cambered profiles, in terms of the guide vane loading and the consequent effect on the leakage flow. A dimensionless term called Leakage Flow Factor (Lff) has been introduced to compare the performances of hydrofoils. It is shown that the leakage flow and its effect on increasing losses and erosion can be minimized by changing the pressure distribution over the guide vane. Highlights Use of the computational tool (CFD) to predict the flow phenomena in hydropower application. Validation of the computationally obtained pressure with pressure sensors and velocity with PIV experiment. Use of the validated numerical model to compare the guide vane profiles for erosion-friendly design of the turbines.

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