Unsteady numerical investigations of the effect of guide vane openings on the hydrodynamic characteristics under stall conditions in a pump-turbine pump mode

The stall is extremely prone to surge of hydraulic loss, excessive pressure fluctuation and radial force of the pump-turbine in pump mode. These can pose a great threat to the safety operation of the unit. Current research focuses more on the manifestations and propagation mechanisms of the stall, which has limitations and effective methods for passive stall suppression cannot be directly identified. In this paper, unsteady simulations based on the SST-SAS turbulence model are conducted to discuss the effects of guide vane openings on the hydrodynamic characteristics under stall conditions of the pump-turbine from the perspectives of flow pattern, energy loss, pressure fluctuation and radial force. Firstly, the performance test results show that the positive slope of the flowhead curves is the largest at small guide vane openings and smallest at optimal guide vane openings. Then, energy loss analysis based on the energy balance equation indicates that the energy loss in the stay vane is most influenced by guide vane openings, with maximum turbulent kinetic energy production at large guide vane openings. Furthermore, the analysis of the stall characteristics in the impeller illustrates that at large guide vane openings, the axial meridional velocity in the impeller is negative under deep stall conditions, and the flow separation and vortex induce a great local pressure drop in the suction surface of impeller blades near the shroud. Subsequently, the stall characteristics analysis in the diffuser reveals that large-scale vortices in the vaneless regions and stay vane channels result in more severe flow blockage and higher energy loss at large guide vane openings. As the increased guide vane openings, the Rotor-stator interaction effect is weakened by the unstable flow pattern in the diffuser, and the low frequency and near blade frequency signals gradually become the dominant frequencies in the diffuser. Finally, the simulation results in the volute show that the unstable flow pattern in the diffuser provides poor inlet conditions for the volute, which significantly increases the energy loss and pressure fluctuations in the volute. The increase of guide vane openings can also make the flow inhomogeneity in the volute worse. The radial forces on the impeller and diffuser under deep stall conditions both increase with increasing guide vane openings due to the unstable flow patterns.