Numerical and experimental study on flow instabilities of a pump-turbine in pump operating condition at low flow rates

Pumped storage units play an increasingly important role in peak regulation and frequency modulation. Pump-turbines, as the core equipment of pumped storage power stations, frequently operate at off-design conditions especially low-flow pump mode, seriously threatening unit safety and operational stability. Pump-turbines often suffer from severe flow instabilities under low-flow pump conditions, which restrict the operational safety and efficiency of pumped storage units. In this work, experimental tests and full-passage unsteady CFD simulations are jointly applied to explore the unstable flow characteristics of a Francis pump-turbine at part-load pump mode. Four typical flow rates are selected（0.5Qd, 0.55Qd, 0.6Qd and 1.0Qd）,the SST k-ω turbulence model and Q-criterion are employed to capture internal flow fields and vortex structures. The evolution of flow separation, jet-wake, rotor-stator interaction and rotating stall are analyzed at design flow rate and two low-flow conditions. Pressure pulsation signals of key components are processed via FFT to obtain frequency-domain characteristics. The results indicate that flow separation is aggravated with the decrease of flow rate and eventually induces rotating stall. Four stall cells uniformly distribute in the guide vane region, and their dominant frequency is 0.26 fn. Rotating stall propagates circumferentially and interacts with other unstable flows to intensify flow disorder. The numerical results are well consistent with experimental data within a relative error of 5%. This work clarifies the coupling mechanism of various unstable flows under low-flow pump mode, and offers technical support for the stable operation of pump-turbines.