Development of simplified model for prediction of sediment induced erosion in Francis turbine's sidewall gaps

Sediment induced erosion in exposed components of hydraulic turbines is a major issue in most of the sediment laden hydropower projects. Gradual removal of material can change the components profile which can lead to change in flow characteristics and compromise in the efficiency as well. Among several components in Francis turbine, this study focuses on the runner sidewall gaps to develop a numerical model for erosion prediction. A prototype high head Francis runner with specific speed of 85.4 rpm has been considered as the reference case. A simplified numerical model based on the concept of rotating disc apparatus (RDA) is conceptualized to make analogy with the gap available between runner blades and guide vanes (GVs). Fillet gaps are introduced in the model to emulate the gaps available between runner blade and GV for real case scenario. Four samples with 90° separation of each are mounted on the disc rotating at 750 rpm. Sediment size of 150 μm diameter with particle mass flow rate based on the maximum amount of sediment passing through the reference turbine is used. Numerical calculation is done in ANSYS® with Tabakoff erosion model. Results give an indication of critical zones in the gap region. Erosion pattern, rate density and vorticity are compared with the actual turbine.