Analysis of vortex evolution in the stall region of pump-turbines in pump mode: Based on model testing and CFD

As the capacity of renewable energy power plants increases, their ability to adapt to fluctuations in the power grid improves significantly. To meet the adjustment demands of the power system, pumped storage plants frequently alternate between pumping and generating modes, inevitably passing through the pump operating stall region during these transitions. The flow dynamics in this stall region are complex, and the unit experiences substantial vibrations, both of which significantly affect the operational stability of pump-turbines. This study employs computational fluid dynamics (CFD) and visualization experiments to assess energy loss characteristics and flow behavior in the vaneless space and draft tube of the stall region. Numerical simulations indicate that energy losses in the vaneless space and draft tube account for the majority of total energy losses, primarily due to large-scale vortex structures. The vortices in the draft tube consist mainly of a near-wall skirt vortex and a central vortex rope, with the skirt vortex being more prominent at lower flow rates. Vortices in the vaneless space arise from rotating stall in the guide vane passage, and the circumferential distribution of this stall leads to low-frequency pressure fluctuations, which are analyzed using the time-averaged enstrophy transport equation.