Analysis of Rotor-Stator Interaction in Turbine Mode of a Pump-Turbine Model

The highest-level fluctuations in large pump-turbines are usually originated from rotor-stator interaction (RSI) in the vaneless region. Hence, the studies of RSI phenomenon and corresponding unsteady effects are significantly important to reduce the pressure fluctuations. In this paper, firstly, RSI in a pump-turbine, featuring 20 stay vanes, 20 guide vanes and 9 runner blades, is analyzed through diameter mode theory, which has been used widely. Then, 3-D unsteady numerical simulations are performed under six guide vane openings in turbine mode. The comparison including performance and pressure characteristics between numerical and experimental results shows a good agreement. Finally, best guide vane opening 21° is chosen to analyze the distribution of pressure fluctuations. The detailed investigation of numerical results shows that frequencies in the vaneless region at best guide vane opening are mainly blade passing frequency (BPF) and its harmonic frequencies caused from RSI. The variation of BPF and its harmonic frequencies is confirmed by diameter mode theory. For this type of the pump-turbine, the amplitude of 2BPF (18fn) shows the highest corresponding diameter mode k2=-2, which indicates two high pressure regions caused by the component of 18fn in the vaneless region. Furthermore, the two high-pressure regions rotate in the counterclockwise direction with rotational speed of the runner blades. This research could provide a basic understanding of RSI to have a further study for pressure fluctuations in pump-turbines.