Hydraulic performance and flow oscillations in a variable speed pump-turbine during a quick power regulation at pump mode

The present study focuses on the transient response of a variable speed pump-turbine during the quick power regulation from input power P 0 at the design condition to around 50% P 0 at the target operation point and the reverse process. Numerical simulation for unsteady turbulent flows has been conducted based on a modified shear stress transport k - ω partially averaged Navier-Stokes model (MSST PANS) to investigate hydraulic performance variations, flow structure oscillations and pressure fluctuations in the pump-turbine during the power regulation. During entire period of power regulation, there are remarkable oscillations of flow discharge and input power, especially when the pump-turbine is operated at the target point with input power of around 50% P 0 , and the acceleration period to adjust the input power from around 50% P 0 to P 0 . There is complicated flow evolution during the power regulation, such as the reverse flow near runner band, vortex at the vaneless region, separation flow groups in stay vanes and guide vanes, etc., which result in the remarkable hydraulic performance oscillation. Further, the unstable flow oscillations during power regulation cause more severe pressure fluctuations in the pump-turbine, where the strongest pressure fluctuations with very low frequency occur at vaneless region. The results are helpful to guide the application of reversible speed pump-turbine.