Large eddy simulation analysis of Francis turbine: A comparison with experimental data and investigation of vortex rope dynamics in the draft tube

The rapid deployment of renewable energy is pushing for an increased use of hydro turbines operating at part load conditions. This is done to enable a quick response to shifts in demand and to compensate for fluctuations in supply from other electricity sources. In these conditions, pressure pulsations in the draft tube are generated which affects the structural behaviour of the machine as well as the hydraulic circuit. This research aims to improve the prediction accuracy of pressure pulsations of Francis turbines in the partial load operation using computational fluid dynamics (CFD) simulations. The obtained numerical approach will contribute the decrease of the pressure pulsations in the hydraulic power stations. In this study, the unsteady LES using the FrontFlow/Blue (FFB) solver is adopted and validated. Results for the case with guide vane openings (GVO) set at 50% reveal the presence of a vortex rope in the draft tube, validated by experimental imagery. Fast Fourier transform analysis highlights a frequency corresponding to the vortex rope's precession speed.