Swirling flow axial injection control in a Francis turbine: An LES study

This study investigates an active control strategy to suppress the precessing vortex core (PVC) and aims to extend the stable operation range of Francis turbine air model. Large-eddy simulation (LES) is used to analyze swirling flow under partial load conditions with axial jet injection within a narrow range of injection flow rates (1 to 5% of the main flow rate). The geometry, flow parameters and control technique are adopted from the experimental work of Litvinovet al. (2023). The effectiveness of the injection is assessed by analyzing the time-averaged velocity and fluctuations, wall pressure pulsations signal and its azimuthal decomposition. Additionally, the influence of axial injection on pressure fluctuations induced by the PVC and instantaneous pressure isosurfaces are examined. The results show that 3% injection flow rate most effectively mitigates the PVC dynamics while not causing other instabilities to occur. On the contrary, jets of 4% and 5% flow rate induce additional perturbations. Proper orthogonal decomposition of the pressure field is applied in this manuscript to reveal coherent structures of the swirling flow in cases without injection and with 3 and 5% jet flow rates.