Instability Characteristics And Influencing Factors Of Positive Slope On Pump Performance Curves Of A Low-Specific-Speed Pump-Turbine

It is very important for the high head pump-turbine to operate at stable conditions.The mainly unstable operating points in pump mode are in positive slope zone.To research the instability characteristics and influencing factors in positive slope performance curves,the model test,numerical simulation and theoretical analysis have been developed in this thesis.The reduced scale model pump turbine with high head in prototype units has been studied in the model test,with the using of high speed camera system.The numerical simulation was carried out by using a 2?-f turbulence mode.Operating points with different guide vanes,flow rate and cavitation numbers has been chosen to analyze the inner flow field to describe the dynamic of positive slope happened in partial flow rate conditions.An additional simulation based on simplified 2D model was carried out to analyze the generating and development of rotating stall.A empirical formula for rotating stall frequency based on guide vanes opening was drawn by the 2D simulation analysis.The turbulence kinetic energy component in guide vanes channel was calculated to find how the rotating stall influence the flow filed then cause the head drop.By the experimental and simulation results,the main unstable flow characteristics when the positive slope happened are the rotating stall in the guide vanes channel.The frequency of rotating stall in this unit is much lower compared with the other researchers mentioned.The rotating stall frequency at large guide vanes opening is about 5.94 Hz.The frequency got lower with the guide vanes opening decreasing.It is about 1.15 Hz at smallest guide vanes opening.The stall cell number is also related to the guide vanes opening.When the guide vanes opening decreased,the stall cell number also got reduced.By the 2D simulation analysis,the frequency is caused by the stall zone rotating in the channel.When the guide vanes opening decreased,larger reverse flow speed war need to generate the rotating.Larger reverse flow speed means more time for the stall zone rotate from one channel to the next channel,which cause a lower frequency.The frequency was related to the sine value of guide vanes opening angle.The distribution of pressure fluctuations shows that the rotating stall was mainly happened in the downstream position of guide vanes.The experiment and simulation results both showed that the cavitation has different influence on the positive slope when the guide vanes opening is different.The cavitation in the pressure side of runner blade inlet can waken the positive slope at large guide vanes opening.The sheet cavitation in the suction side of runner blade inlet make almost no influence on the positive slope at middle guide vanes opening,while the cavitation vortex in the suction side of runner blade inlet cause the positive slope happened earlier in small guide vanes opening.The distribution of turbulence kinetic energy component shows how the rotating stall influences the flow when the positive slope happened.The rotating stall mainly caused the reverse flow in the main flow direction,then caused the large velocity gradient and then local large velocity.When the positive slope exist,the flow in the guide vanes channel reveal heavy 3D characteristics.To sum up,the instability characteristics and influencing factor of positive slope was studied in this thesis.The results of this thesis can give advice to reduce the positive slope phenomenon in the pump turbine operating.