Study On Cavitation Performance And Its Impact On Hump Characteristics Of A Pump-turbine

Pumped Storage Power Plants,using pump-turbines,plays an important role in regulating load and promoting energy saving in power system with its excellent peak clipping and valley filling capability.With the trend of ump-turbines toward higher specific speeds,larger capacities.and higher heads,a higher requirement for the stability of the unit is put forward.Cavitation seriously affects the safe operation of pump turbines.Therefore,it is necessary to further study the cavitation performance of pump mode.The hump area is the main problem that restricts the stable operation of pump turbines.This paper combining with relevant experimental data This paper focuses on the flow characteristics of pump turbines at different cavitation stages to deal with a series of unstable problems caused by cavitation flow of pump turbines.The main contents are as follows:1.Effect of cavitation flow on energy conversion in the runner of pump-turbine.Based on the model test results,through the change law of static pressure,turbulent kinetic energy,bubble distribution in the middle flow surface of the runner,the cavitation flow characteristics of the pump turbine at the design flow were analyzed and calculated.The results show:when the number of cavitation drops below the critical cavitation number,cavitation produces a stronger exclusion on fluids and reduces impeller performance and the efficiency of pump-turbine dropped significantly.At the same time,as the degree of cavitation increases,the distribution of turbulent kinetic energy in the adjacent vane flow channels is significantly different.The turbulent kinetic energy near the suction surface of the blade is significantly higher than that of the pressure surface and more obvious in vaneless space.2.Effect of different flow conditions on cavitation performance of pump-turbine.Selecting 3 kinds of flow conditions,comparison of the appearance and volumetric variation of bubbles in pump runners under different flow conditions.The results show:The cavitation position first appear in the cavitation position of the small flow conditions and design conditions near the leading edge of the suction surface of the blade.The cavitation position of the high flow condition first occurs at the pressure surface near the front edge.At the same time.the anti-cavitation performance of pump turbines decreases as the flow rate increases.As the number of cavitation decreases,The mutual influence of this adjacent blade flow path will continue to be transmitted,the radial trajectory resulting in one week no longer closes,and the radial force peak difference gradually increases,under the same cavitation number.The variation of the axial force is similar to the head coefficient.3.Effect of cavitation performance on hump characteristics.The formation mechanism of the hump phenomenon was analyzed through streamline of the different blade heights and mid-span S1 stream surface of the turbine runner of the runner,and numerical calculation of the hump phenomenon under three different cavitation numbers.The results show:When the pump turbine operates in a small flow condition,the transition of the separation vortex near the shroud side and double circular cascade is related to the hump phenomenon.By comparing and analyzing the hydraulic losses of different over-current components.it is found that the sudden change of flow characteristics in runner produces hump characteristics.At the same time,with the decrease of cavitation number,although the additional hydraulic loss of runner runner is increased.the hump phenomenon is eliminated to a certain extent.This lays the foundation for studying the running condition of pump turbine hump area under the combined action of cavitation instability and unstable flow.