Optimized air admission strategy for mitigating hydrodynamic instability in a large Francis turbine

The increasing integration of renewable energy into new-type power systems accentuates the critical role of hydroelectric power in grid stabilization and baseload support. Nevertheless, inherent flow instabilities in hydraulic turbines operating under off-design conditions induce power oscillations. These oscillations constrain the flexible regulation capabilities of hydroelectric power systems. This study investigates a large Francis turbine, employing a multiphase flow model to numerically simulate flow characteristics across a comprehensive range of operating conditions. Pressure distribution on runner blades is quantitatively analyzed utilizing a streamline coordinate system. A third-generation vortex method identifies the complex vortex structures within the flow channel. Building upon these analyses, this study delineates conditions predisposing the turbine to power oscillations, elucidates the mechanism of these vortex-induced oscillations, and proposes an optimal air admission strategy for their mitigation. Results show that the turbine is susceptible to power oscillations in below-rated head and overload conditions, with a characteristic frequency of 4.93 Hz, corresponding to flow separation vortices. These vortices generate negative torque on the blades, with the resulting torque fluctuations directly causing power oscillations. An optimal air admission strategy—admission volume of 1 m3/s—significantly reduces the amplitude of power oscillations by up to 81.3%. The conclusions presented herein offer valuable recommendations for optimizing the stable operating range of hydraulic turbines, enabling timely mitigation of output power fluctuations, and enhancing the flexible control capability of hydraulic systems.