Evolution mechanism of unsteady internal flow of an ultra-high head pump-turbine in pump mode

In this study, the unsteady flow of an ultra-high-head model pump turbine in pump mode is numerically investigated, revealing the evolution mechanism of unsteady flow diffusion and propagation. Under high-flow-rate and high-efficiency conditions, stable flow separation occurs in the channel of the stay vanes. As the flow rate decreases, the flow separation gradually spreads in the radial direction and expands from the stay vanes to the guide vanes. This causes a partial blockage in the channel of the guide vanes. Meanwhile, the flow separation gradually switches from stable to unstable with a low-frequency rotation (about 4.8 % of the impeller rotational frequency) in the circumferential direction. In addition, the correlation between the hump characteristics and unsteady flow is discussed. For an ultra-high-head pump turbine, the unstable flow initiates at static operating conditions from the pump performance curve with negative slope, which is significantly different from medium and low-head pump turbines.