Load Rejection Process Of A Pump-Turbine With The Ball Valve And Guide Vanes Jointly Closing:CFD Simulation

Pumped storage power plant(PSPP) undertakes the task of peak regulation and frequency regulation in the power grid.Its transition process is frequent and violent.And the development of pump turbine units to high head,large capacity and high speed aggravates the danger of the transition process.As an important measure to ensure the safety of water conveyance system,the joint closure of ball valve and guide vanes can optimize the characteristics of load rejection process.Many achivevements have been obtained,but the three-dimensional analysis is still lack.In this paper,the CFD simulation method of one-dimensional water conveyance system coupled with three-dimensional pump-turbine(1D-3D coupling method)is applied to simulate the load rejection process of a unit in a PSPP.The 3D transient flow is investigated with combined action of ball valve and guide vanes.The purpose is to explore the influence of ball valve on pump-turbine flow patterns,pressure fluctuations and forces on the runner.The main works of this paper are as follows:1.The analysis of 3D flow characteristics of a ball valve during linear closure in a simple pipe.Using CFD numerical simulation method,the characteristics of water hammer and pressure fluctuations of a ball valve under linear closure law in a simple pipe were studied.The results show that at the early stage of ball valve shutdown,there is almost no change of the pressure field.At the middle stage,the pressure before the valve increases,while the pressure behind the valve decreases.In the later period,the pressure before the valve rises to the peak and then decreases,while the pressure behind the valve decreases to the stable value.Through the analysis of the forces on the ball valve,the correlation between the forces and the flow patterns were investigated.The instability of the flow patterns is the main reason for the force fluctuations in the later period.The transformation of kinetic energy and potential energy between the inlet and outlet of the ball valve and the energy consumption continue to occur,but is especially serious at the later stage,which cause the flow instability.In addition,the formation mechanism of multiple vortices in the flow field during closing process of the ball valve was revealed.2.The analysis of 3D flow patterns characteristics during load rejection process with guide vanes linearly closing of a pump turbine.The 1D-3D coupling method was adopted to simulate the load rejection process of a prototype pump turbine.Macro-parameters change dramatically with the variations of working conditions.The unit parameter characteristic curves fluctuate intensively near the no-load operating point and finally converge.The flow patterns at the runner inlet were analyzed.It shows the transformation of lower band reflux to upper crown reflux cause the turning point of flow rate at the early stage.And with the development of the upper crown backflow,the decline rate of discharge is accelerated.The occurrence of rotating stall in the runner increases the pressure fluctuations,and also makes the runner rotates eccentrically.The axial force of the runner has a sudden drop at early stage,which is related to the formation of the central spiral vortex rope in the draft tube.The new Omega method was applied to identify the vortex structures,which proves a better effect than Q criterion.There are mainly three types of vortex in draft-tube during load rejection,namely,central spiral vortex rope,sidewall vortex and upper annular vortex.The evolution laws and formation mechanism of the three vortexs were also analized.3.The analysis of pressure fluctuation characteristics of load rejection process wth ball valve and guide vanes joint closure and the effect of ball valve closure on the flow fields.The 1D-3D coupling method was used to simulate the load rejection process of a prototype pump turbine with ball valve and guide vanes joint closure.Comparing with the load rejection of ball valve non-action condition,the closure of ball valve delays the change trend of macro-parameters,but accelerates the convergence of the trajectory of unit parameters.Comparing the pressure fluctuations with the experimental data,the macrolaws agree well but a phase difference does exists,which is caused by the difference of the closing time of ball valve between the calculation and experimental condition.The closure of the ball valve reduces the working head of the pump turbine,slows down the rise rate of rotational speed and sharply reduces the amplitude of the second peak pressure value.Ball valve has a significant impact on the runner flow patterns,which directly inhibits backwardto-inflow transition of the middle height of runner inlet at later stage of load rejection,restraining the occurrence of rotating stall,and thus suppressing the pressure pulsation.However,load rejection aggravates the instability of the ball valve flow patterns,making the operation of the ball valve risky.The above three works complement each other,reveals the mechanism of instability of load rejection,which lays the foundation for the balance of regulation of PSPP.