A technical review on combined effect of cavitation and silt erosion on Francis turbine

Francis turbine, a reaction type and medium head turbine is most widely used in hydropower plants to generate electricity due to its wide range of operation. Cavitation erosion and silt erosion are hydraulic transient phenomena, which are the common problem associated with hydro turbines. The metallic surface gets damaged due to high local stress caused by collapsing of the vapor bubble while silt erosion removes the material from turbine surface due to the dynamic action of silt particles. Cavitation erosion depends on suction height, temperature, and sigma factor while silt erosion depends on silt size, concentration, flow velocity, and impingement angle. The investigations have been carried out by various researchers on cavitation and silt erosion in the hydro turbines individually and combined. It is revealed that the combined effect of cavitation and silt erosion is more severe than the individual effect. In coalesced effect, the cavitation mechanism may be inhibited or promoted by effect of silt erosion. In this study, a comprehensive review has been carried out to evaluate the effect of cavitation erosion, silt erosion, and combined erosion on the performance of Francis turbine. Due to combined effect, various factors like surface properties, erosion parameters, and flow characteristics are responsible for efficiency loss and material degradation. At present few studies are only marked on the combined effect in hydro turbines. It is therefore required to carry the extensive work for understanding and develop the correlation between the material erosion and performance loss by numerically and experimentally.