Unsteady flow and pressure pulse analysis of a Francis pump-turbine on reverse pump zone with small guide vane opening

The S-curve characteristic is widespread in pump-turbine. When the unit starts running under turbine-mode, the operating condition usually cannot be changed from no-load to full-load directly, which inevitably make the unit to enter reverse pump zone. This phenomenon brings difficulties for grid connections. In this paper, a model pump-turbine was chosen for analysis. The three-dimensional numerical calculation of the overall flow channel was carried out for the reverse pump mode under an extreme small guide vane opening. The simulations were performed with the SAS SST-CC turbulence model. The steady simulated external characteristic parameter curves were in good agreement with the model test data for five operating points. The operating point where the flow rate is relatively small was chosen for long-term unsteady flow calculations, where eight different points on nine runner blade to blade flow channel were chosen to monitor the pressure fluctuations. The results show that there exists a noteworthy difference of pressure peak-to-peak amplitudes and pulsation frequencies in each rotating flow passage. The largest peak-to-peak difference is up to 4.2%, which means that the flow distribution in the runners is not uniform for this condition. From points rva to rvh with those points closer to the guide vane, the main frequency of the pulsation in the low frequency region is gradually increased from 0.19fn to 1.07fn; the pulsation amplitude caused by rotate and static interference (RSI) will gradually increase, but not for points rvg. The flow pat- tern shows that this points was located on outflow and the inflow transition zone of the runner blade out- flow channels. Flow pattern analysis indicates that the runner flow fields are asymmetrical in different blade passages under this revise pump condition, with significant flow separation and vortexes. © 2018, China Water Power Press. All right reserved.