Investigations On The Influence Of The Flow Separation In Guide Vane Channels On The Positive Slope On The Pump Performance Curve In A Pump-turbine

The Pumped hydroelectric storage has diverse working conditions and it is necessary to frequently switch between different conditions.Therefore,operational instabilities leading to startup or shutdown failures are crucial issues to the application of pump-turbines.The positive slope on the pump performance curve of the pump-turbine is a typical unstable characteristic,causing potential startup failures in pump mode accompanied with severe vibration and noise.Thus,the investigation on the positive slope is necessary to improve the stability of pump-turbines.According to the previous studies,the guide vane foil has potential influence on the positive slope of pump-turbines.Therefore,the experimental and numerical investigations on the guide vane foils(F1 and F2 foils)were carried out,to discuss the difference of the flow characteristics of them.The experimental results revealed that the difference of the condition where the stall cell occurs of the two foils leads to the variation of the stall ang le.The analysis based on boundary vortex flux was applied,to discuss the flow mechanism of the occurrence of the stall cell.Then the loss analysis based on the energy equation was carried out to study the loss mechanism of flow separations.The numerical investigation was conducted on the annular cascade of the guide vane foil,to discuss the influence of the vane on the cascade.The flow conditions with the occurrence of the flow separation of the two cascades were different,leading to a potential influence on the flow instability.The flow mechanism of this difference was discussed based on the analysis of boundary vortex flux.The loss coefficient increased suddenly with the occurrence of the stall cell in the channel when the inflow angle is too smal l.High turbulence kinetic energy production term at the region between the straight flow and the separated flow may be responsible for it.The experimental and numerical investigations were conducted on pump-turbines with various guide vane foils.The results revealed insignificant influence of guide vane foil on the pump performance curve out of the positive slope region,while the significant influence was detected in positive slope region.The occurrence of the rotating flow separation with low-frequency pressure fluctuation was also influenced.Then the distribution of the amplitude of the low-frequency fluctuation was discussed.The unsteady loss analysis method based on the energy equation was developed,to discuss the relationship between the positive slope and the rotating flow separation.The variation of the kinetic energy of the mean flow was firstly taken into account so that this method is available for the highly unsteady flow pattern.This method was applied in the present study,to detect that the most of the power loss transforms to the turbulence kinetic energy.The power loss in the pump-turbine mainly owe to the region between the straight flow with high velocity and the separated flow with low velocity.Then the criterion of the occurrence of the positive slope in the view of the energy was developed.In conclusion,the influence mechanism of the flow separation in guide vane channels on the positive slope in the pump-turbine was studied in the present study.The influence of the guide vane foil on the positive slope and the flow separation was also discussed.These results are instructive for the engineering application,to weaken the influence of the positive slope on the pump-turbine.