Study of the simultaneous effects of secondary flow and sediment erosion in Francis turbines

Sediment 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, rotor-stator-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.