Study On Cavitation Performance Of The "S" Characteristics Of The Pump-turbine

Pumped storage power stations are at an important stage of large-scale construction and rapid development.Pumped storage units are gradually becoming the main units for peak shaving and valley filling in the power system due to their operating characteristics.The main principle of its work is: during the day and night when the power consumption is high,the pump-turbine unit works as a turbine,and the upstream reservoir drains to generate electricity,and during the low energy consumption time in the middle of the night,the unit works as a pump Use excess electrical energy to pump water from the downstream to the upstream reservoir,so that the water can obtain potential energy,which is equivalent to storing excess electrical energy.Due to the relatively large number of operating conditions of pump turbines and the frequent conversion between operating conditions,there are Two unstable areas,namely the hump area and the "S" characteristic area,these two unstable operation areas will cause unstable unit startup process under pump operating conditions and grid connection difficulties under turbine operating conditions,resulting in the unit The running stability problem is more prominent.Cavitation is a kind of hydrodynamic phenomenon that is unavoidable in the operation of fluid machinery.During the operation of the pump-turbine,the occurrence and development of cavitation will not only lead to a decrease in hydraulic performance and an increase in energy loss,but in severe cases,it will also produce strong vibration and noise,which will affect the safety and operation of the turbine.Stability has a very adverse effect.In order to study the influence of the cavitation performance of the pump turbine on the formation and development of the "S" characteristic zone,this paper selects three typical operating points in the "S" characteristic zone,adopts the SST k-? turbulence model and Zwart-Gerber-Belamri The cavitation model is used for the numerical simulation of singlephase flow and cavitation flow for a pump-turbine model of a pumped-storage power station.Three typical operating points in the "S" characteristic area are selected for calculation and the "S" is analyzed.The flow pattern and pressure distribution inside the characteristic zone explore the characteristics of the "S" characteristic zone.In the calculation of cavitation flow,the cavitation state is controlled by changing the cavitation coefficient at the calculated operating point,and the change rules of hydraulic performance and internal flow pattern of the turbine under different cavitation coefficients are analyzed.The results show that in the case of single-phase flow,as the flow rate in the "S" characteristic zone decreases,the flow state in the guide vane zone gradually deteriorates from smooth to smooth,and the gradient of turbulent kinetic energy also gradually increases.A ring that impedes the development of the water flow appears gradually between the wheels;the circumferential velocity component of the water flow from the runner to the draft tube gradually increases,and the resulting vortex structure becomes more and more serious,making the flow velocity and speed unable to match More contradictory;in the case of twophase flow,the occurrence of cavitation increases the hydraulic loss in the flow channel and the hydraulic efficiency of the unit decreases;with the gradual development of the cavitation phenomenon,the blade load gradually decreases,reflecting the blade's ability to transform water flow energy,Leading to a decrease in the power of the unit;with the gradual development of the cavitation phenomenon in the selected operating point,the cavitation gradually develops in the direction of the blade,the area of the low pressure area expands outward from the center,the flow state in the runner further deteriorates,and part of the blade flow path There are multiple backflow vortices that hinder the flow,resulting in the rotation speed of the runner not matching the flow rate,which is an important reason for promoting the development of the "S" characteristic phenomenon.