Investigation of cavitating vortex rope instabilities and its suppression inside a Francis turbine model with Thoma number variation

Cavitating vortex rope at part load (PL) condition at lower values of the Thoma number (⁠$σ$⁠) induces severe pressure fluctuation and efficiency reduction in a Francis turbine, which ultimately hinders continuous energy production. Installation of fins at draft tube (DT) can mitigate these instabilities and can safeguard the turbine operation with lower maintenance costs. The effect of fins on hydraulic performance and internal flow physics at PL condition with the variation of $σ$ is examined in the present numerical investigation. For the two extreme opposite values of $σ$⁠, the flow characteristics are predicted accurately for the turbine with and without fins by conducting transient simulations using ANSYS-CFX. The numerical findings on the structured and unstructured grid points are validated with the experimental results. The turbine's performance remains constant for higher values of Thoma numbers, and as the value decreases, the performance declines. The cavitation vortex rope formation inside the DT with fins is mitigated significantly at the minimum $σ$⁠, while at the maximum value, the vortex rope with bubble generation is restricted. Compared to the without fin case, the swirl intensity is minimized remarkably (68%) with the presence of fins at the lowest $σ$⁠. The maximum cavitation rate is manifested by the DT without fins, which is about 60% higher than the DT with fins. At minimum $σ$⁠, extreme pressure pulsations are induced inside the DT without fins, which are reduced by 43% in the finned draft tube. Therefore, stable energy production is maximized with the installation of fins at both Thoma numbers.