Numerical simulation of hysteresis characteristic in the hump region of a pump-turbine model

Pumped-storage technology is one of the cleanest methods of energy conversion. The stable and safe operation of pump-turbines as key parts of pumped-storage power plants are becoming more and more important with the rapid increase in the capacities, specific speeds and heads of pump-turbines. The hump characteristic is one of the unique unstable features that significantly influences the stability of pump-turbines. Recently, an interesting phenomenon hysteresis characteristic was found in the hump region, which enlarges the unstable region and results in a more unstable characteristic of the pump-turbine. In the present study, based on experimental validation, steady simulations were performed to predict the hump characteristic as well as the hysteresis phenomenon. An analysis of the hydraulic loss and flow field shows that the hump characteristic and the hysteresis phenomenon are caused by the decrease in the work of the runner and the increase in hydraulic loss, which are induced from vortices in the guide/stay vanes and backflow near the shroud at the runner inlet. For the same operating condition, owing to the difference in the input direction, the states of vortices and backflow are different, resulting in the hysteresis characteristic.