Instability Analysis Of The "S" Region Of A Pump-turbine Based On Nonlinear PANS Model

For peak shaving and valley filling of power systems, a pumped storage power station needs to start and stop at a very high frequency. The design for a pump-turbine is based on the pump mode and modified according to the turbine mode, which causing the existence of "S" characteristics."S" characteristics are unstable in theory, which leading to lot of accidence. Focusing on the instability problem due to the "S" characteristics, numerical simulation and model test of a pump-turbine are performed.This thesis develops a new partially-averaged Navier-Stokes (PANS) method based on the filtering method of RNG k-ε turbulence model. Then, this new PANS method combined with nonlinear model for resolving the sheer stress develops a nonlinear PANS model. Using this nonlinear PANS model, the flow fields of two dimensional NACA0015hydrofoil, three dimensional curved rectangular duct and a low specific centrifugal pump are investigated. The computational results agree well with the experimental data, which prove that this nonlinear PANS model is available in the simulation of high pressure gradient flow, secondary flow and complex stall phenomenon.To study the mechanism the improvement of "S" characteristics by misaligned guide vanes (MGV), energy characteristics and pressure fluctuations of a pump-turbine with MGV are experimented and compared with the results with synchronized guide vane (SyV). Different operating points in "S" region are calculated by the nonlinear PANS model, and the computational results are in accordance with model test results. The MGV destroys the high-speed water ring in the vaneless space and improves the "S" characteristics, but aggravates the pressure fluctuation. The amplitude of pressure fluctuation increases as the pre-opening of guide vane increases. The used of MGV will not change the dominant frequency in the vanelss space, and the dominant frequency is the blade passing frequency (BPF).To investigate the similarity relation of pressure fluctuation between model and prototype pump-turbine, six operating points in the "S" region at no-load opening of both model and prototype pump-turbines are studied using the nonlinear PANS model. The amplitudes of pressure fluctuations of a model pump-turbine increase firstly then decrease with the decrease of the flow rate. Pressure fluctuation is the most intense when the pump-turbine runs at the runaway condition. The law of amplitude of pressure fluctuation in prototype is the same with model pump-turbine, but the pressure fluctuation in prototype at zero flow rate condition is more intense compared with model results. Similarity is hardly to be found due to the similarity criterion is not applicable at the same operating point. In the domain of stay vanes and guide vanes, the amplitude of pressure fluctuation decreases as the reduction of radius of monitor points, and the dominant frequency gradually changes from2times of BPF to1times of BPF along the flow direction.Instability at the vaneless space of the model pump-turbine is analyzed based on turbulence kinetic energy equation, and the results of the pump-turbine with MGV are compared with results of SyV. The interference of the runner to the instability of vaneless space is obvious at the downstream of trailing edge of guide vanes, and the starting interference location is0.3times of chord length of guide vanes far away from the trailing edge. The change of normal stress production term has great influence on the flow stability at the vaneless space. The increase of amplitude of pressure fluctuation has a direct relationship with the increase of production term of turbulence kinetic.