Research On The Load Rejection Transient Process Of Pump-turbine

During the actual operation,pumped storage generating units frequently change their working conditions.And often exist serious dynamic instability problems.However,there are fewer three-dimensional studies on the transient flow in the load rejection process.In this paper,the transient load rejection process of a pump-turbine is studied through three-dimensional turbulent numerical simulation.In order to reflect the unsteady characteristics of transient flow in pump-turbine during load rejection process,for the FLUENT numerical simulation,the user-defined function(UDF)is used to specify the pressure boundary conditions at inlet and outlet of pump-turbine.According to the angular momentum balance equation of rotor dynamics and the internal flow field,the unit speed is calculated during the load rejection process.Dynamic mesh method is used to simulate the dynamic closing process of guide vanes during load rejection process.According to the above boundary conditions,the transient flow in the pump-turbine during load rejection process are simulated respectively considering the flow in runner 's clearance and without the runner 's clearance.According to the numerical calculation results of transient flow during load rejection process without considering the flow in runner 's clearance,the dynamic instability of the external characteristics and pressure p ulsation and its internal flow mechanism are analyzed by means of comparison analysis,time frequency joint analysis and internal flow field analysis;According to the numerical calculation results of transient flow during load rejection process considerin g the flow in runner 's clearance,based on entropy production theory and internal flow field,the energy transfer process,loss distribution and its flow mechanism in pump-turbine are analyzed by means of comparative analysis.The analysis results show that the transient flow in pump-turbine during load rejection process can be simulated accurately through three-dimensional turbulent numerical simulation method.Moreover,the flow in the clearance between the runner and the cover and the bottom ring should be taken into account when calculating the actual flow field;During the load rejection process,the external characteristics of the pump-turbine show dynamic instability.The severe pressure pulsation in pump-turbine directly cause the dynamic instability of the external characteristic.The root cause of the dynamic instability of the external characteristics and the pressure fluctuation is the complex vortex flow in the vane-less space and the draft tube during the load rejection process;From an energy point of view,the load rejection process of pump-turbine is a process of energy dissipation,and the energy is from the flow,which is transformed back and forth between the various energy forms and eventually the process tends to be stable.In the process of runaway and braking process before entering the reverse pump conditions after the load rejection,the energy losses in the pump-turbine are mainly distributed in the elbow section of the draft tube,the vane-less space and the runner.During the reverse pump process after load rejection,the energy loss is mainly distributed in the double cascade(guide vane and stay vane)and runner.The root cause of the energy loss in the pump-turbine during the load rejection process is also the vortex flow at the c orresponding location in the pump-turbine.