Characterization of the effect of roof-assisted fluid injection on Francis turbine

With the increasing penetration of renewable energy into power grids, the operational load on hydropower units continues to grow, leading to aggravated flow instabilities under part-load conditions. Traditional mitigation approaches, such as runner blade profiling, natural aeration, and shaft air admission, exhibit certain limitations. In this study, numerical simulations were carried out on a large-scale Francis turbine to investigate the regulation effect of top-cover auxiliary fluid injection (water/air admission) across a 40%–100% load range. Results indicate that the turbine develops distinct vortex structures under different operating regimes—such as inter-blade vortex cavitation at 40% load and draft tube vortex ropes at higher loads—which can trigger severe hydraulic vibrations. The proposed auxiliary injection significantly improves the runner flow field, effectively reducing the cavitation area on the blade suction side, with stronger effects at deep part load. Furthermore, its suppression of pressure pulsations exhibits load dependency: notable attenuation of the 0.2fn component is observed at 40% and 80% load, whereas effects are minor at 60% and 100%. Finally, comparative analysis reveals that air admission is generally superior to water injection in mitigating unsteady pressure pulsations, while under high-load cavitation-dominated conditions, water injection performs better. These findings provide new technical guidance for optimizing Francis turbine stability under off-design operation and for the targeted application of auxiliary fluid injection strategies.