Evolution of flow structures and pressure fluctuations in the S-shaped region of a pump-turbine

To fully understand the evolution of flow structures and pressure fluctuations in the S-shaped region of pump-turbines, three dimensional (3D) numerical simulations were carried out for a model pump-turbine at four different guide-vane openings (GVOs). We found that special backflow vortex structures (BFVSs) form around the runner inlet. The backflow location shifts from the hub side to the shroud side and then to the midspan, as the operating point moves from the best efficiency point to the zero discharge point along the S-shaped characteristic curve. At large GVO, if the discharge is relatively large, the flow separations at the runner inlet are primarily driven by the local adverse pressure gradient, resulting in hub and shroud side backflows. At small GVO or small discharge, the separations are mainly dominated by the centrifugal force, resulting in backflow at the guide-vane midspan section. The locally distributed BFVSs enhance the local rotor-stator interaction (RSI) in the vaneless space, leading to different pressure fluctuating intensities along the spanwise direction for high frequency components. On the other hand, the rotating stall is the main cause of the uneven distributions of low frequency pressure fluctuations along the vaneless space between runner and guide-vanes.