Numerical simulation of a cavitating draft tube vortex rope in a Francis turbine at part load conditions for different σ-levels

In the present study, numerical investigations of a Francis turbine at model scale are performed. A part load operating point is selected for the analysis with a focus on the cavitating draft tube vortex rope. For three σ-levels, different aspects are investigated. First, the extension of the vapor volume is analyzed and compared to high-speed visualization recordings from the measurements. Furthermore, the pressure fluctuations for different σ-levels are analyzed and compared to experimental data at a measuring point in the draft tube cone. Finally, the impact of the different cavitation volumes on the axial and circumferential velocity distribution is investigated. In addition, the SST and SAS turbulence models are compared for two pressure levels.