Study On Flow Characteristics Of A Pump-turbine During Shutdown Transient Process

Solar energy,wind energy and other renewable energy are significantly affected by the environment and need to be coordinated and adjusted by other energy sources.In addition,the daily variation law of residential electricity consumption presents a typical periodicity.These factors greatly increase the demand of the grid for the pumped storage units with flexible regulation,and the pump turbine needs to be switched frequently under different working conditions.Transient process is a necessary process to balance the power grid load in the operation of pump turbine,especially the start-stop cycle,whose complex dynamic instability seriously restricts the stable operation of pumped storage unit.In this paper,the transient process of pump-turbine shutdown is taken as the research carrier,and the dynamic characteristics of the transient process of pumpturbine shutdown are explored by combining experimental research with numerical simulation.In order to obtain more realistic and reliable flow characteristics,the external mesh reconstruction method is used to simulate the guide vane closing motion.The pressure is used as the inlet and outlet boundary condition for a given test.The rotor rotational momentum balance equation is used to calculate and update the rotational speed of the rotor.The complete three-dimensional numerical simulation strategy of transient process;based on the above scheme,the numerical simulation of the transient process of pump-turbine shutdown considering the flow of runner gap is carried out.Time domain analysis and time-frequency analysis(short-time Fourier transform,etc.)are used to compare the pressure pulsation obtained by experiments and numerical simulations to clarify the complex frequency components of pressure pulsation during transient process and analyze the propagation law.,vortex core distribution,vorticity field analysis,etc.,to explore the evolution history of special vortex flow inside the pump-turbine during the shutdown transient;compare the external characteristic parameters of the shutdown transient process with or without the runner gap flow,using pressure pulsation analysis and internal The combination of flow analysis explores the effect of gap flow on the dynamics of transient processes.The results show that the three-dimensional numerical simulation strategy used in this paper can reasonably predict the dynamic characteristics during the shutdown transient process.In this process,the double-row cascade vortex structure has complex evolution characteristics,with the guide vane closed,the vortex The structure successively undergoes the horseshoe vortex and the flow vortex attached to the abdomen of the movable guide vane,the complex crushing vortex system under the reverse liquid flow impact,and the vortex in the fixed guide vane after the working condition is stabilized;The gap flow,the runner gap flow effectively reduces the axial water thrust,destroys the flow vortex attached to the lower edge of the blade near the inlet,and then delays the growth and movement of the flow vortex,and even blocks the evolution of the blade flow path.As a result,the hysteresis of the external characteristic parameters such as torque and flow rate is 0.724 s.After the flow in the back pump condition is stabilized,the radial velocity component of the gap inlet ring surface is sharply reduced,and the flow damage to the mainstream region is weakened,and the hysteresis disappears.