Numerical investigation of energy dissipation and vortex characteristics of a pump-turbine with splitter blades under the influence of rotating stall in the hump region

Under the current scenario of large-scale renewable energy integration into the grid, pumped storage power stations serve as critical components for peak shaving and frequency regulation. The pump-turbine as a core component of these stations needs frequent operational condition changes during operation. Unstable internal flow phenomena can significantly impact unit stability, particularly the rotating stall issue under off-design conditions. This study employs numerical simulations combined with entropy production analysis and vorticity transport equation analysis to investigate the transient flow characteristics within the hump region of a pump-turbine designed with splitter blades. The results indicate that rotating stall originates from flow separation within the guide vane passages. This propagates circumferentially through interactions between vortex structures, triggering reverse flow, and sudden pressure gradient changes. The development of unsteady vortex structures, such as the separation vortex on the guide vane suction surface and the circumferential vortex in the vaneless region, is closely correlated with a sharp increase in turbulent entropy production rate, constituting the dominant factor in hydraulic losses. Analysis based on the vorticity transport equation reveals the influence of relative vorticity stretching and Coriolis force on the evolution of the stall. This research deepens the understanding of transient flow phenomena within the hump region of splitter-blade pump-turbines and provides valuable insights for enhancing the operational stability of pumped storage units and optimizing internal flow design.