Rotating stall mechanism of pump-turbine in hump region: An insight into vortex evolution

Reversible Pump Turbines (RPTs) are primarily used in pumped-storage hydropower plants, enabling the facility to function in both pump and turbine modes. Under pump mode, the "hump region" refers to a characteristic range of flow conditions that are marked by large flow instabilities. These can eventually jeopardize the performance and operational safety of the plant. This study, through numerical simulation and model testing methods, examined the flow separation and rotating stall evolution mechanisms in the vaneless space and guide vane regions, for an RPT whose operating conditions are within the hump region. Applying the entropy production theory and Rortex method as energy loss and vortex identification methods, study results have shown that energy losses are concentrated at the guide vane's leading edge and vicinal zones, as well as within the draft tube. The energy loss within inter-guide vane flow passages is found to mainly take source from the experienced rigid water rotation at the same zones. Moreover, radial velocity differences, which are in turn linked to a nonuniform flow distribution among inter-guide vane passages, are believed to have considerably contributed the emergence of the rotating stall. Finally, the analysis of vortex flow evolution revealed the importance of flow separation-linked changes in entropy generation term in identifying the vortex evolution mechanism.