Towards the integration of new-type power systems: Hydraulic stability analysis of pumped storage units in the S-characteristic region based on experimental and CFD studies

As new-type power systems increasingly require pumped storage units to transition frequently between operational states, maintaining hydraulic stability in the S-characteristic region poses a significant challenge during these switching conditions. This study utilizes model testing and computational fluid dynamics (CFD) to investigate the energy loss characteristics and hydraulic instability of pump-turbines operating within the S-characteristic region. The findings demonstrate that energy losses primarily occur in the guide vane passages and the vaneless space. Specifically, energy loss in the flow passage between guide vanes primarily arises from the rigid rotation of water within that region, with these rotating vortices inducing high-amplitude, low-frequency pressure pulsations inside the unit. An analysis based on the time-averaged enstrophy transport equation using the Rortex method indicates that shear effects and the pseudo-Lamb term, induced by velocity gradients, are the primary factors influencing vortex evolution. Additionally, time-averaged RANS equations combined with particle image velocimetry (PIV) experiments reveal that areas near the boundary between positive and negative velocity gradients exhibit significant rigid vorticity, while shear effects reach their maximum strength in regions away from the vortex core along the trajectory of vortex motion.