Hydrodynamic Mechanism Analysis Of The Pump Hump District For A Pump-turbine

Nowadays, lots of pumped-storage power stations have been developed in order to adjust the difference between peak load and valley load of electrical power system in China. The stability of unit operation is getting more and more attention. Hump characteristic of pump mode can influence the stability of pump-turbine operation. Therefore, it is necessary to conduct deep research on hump characteristic of pump mode of the pump-turbine.This paper will simulate to make a detailed study of the hydrodynamic mechanism for the formation of the hump region.Firstly, the SST turbulence model was adopted to perform steady simulation of the pump mode for three different guide vane openings. The head, the efficiency and the power characteristic curves of different pump modes were predicted. The simulation results were compared with the same-condition experimental results. Then thepaper analyzed the energy loss of different domains and internal flow characteristics. The results show that the energy loss changing characteristics of five components towards discharge are similar for three different guide vane openings. In addition, the pump input total energy increases towards smaller energy for three different guide vane openings. Meanwhile, steady numerical simulation of pump mode cavitation was conducted in order to simply analyze the hydraulic loss from the cavitation aspect. The results show that the cavitation is very weak and have little influence to the energy loss.It has little effect on the unit performance. Cavitation is not a main reason for the hydraulic loss which contributing to the hump region.Then, in the optimal guide vane opening, unsteady numerical simulation was performed for the chosen five operating conditions. The unsteady simulation results are closer to the experiment than steady simulation results. The internal flow characteristic under different operating conditions was analyzed combined with energy loss, relative peak-to-peak amplitudes and pressure amplitude spectra. It is found that the complex internal condition is always accompanied with large energy loss, high relative peak-to-peak amplitudes and low dominant frequencies. As the discharge decreases, the energy loss inside the draft tube, runner and guide vanes increasesrapidly. As the discharge decreases, there occurs helical backflow inside the draft tube, vortex and backflow inside the runner and many vortexes inside the guide vanes, which contribute to the continuously increasing energy loss inside the draft tube, runner and guide vanes. The relative peak-to-peak amplitudes also increase towards smaller discharge and the low dominant frequencies become more obvious. All above are closely related to the formation of hump region. Besides, the vanishing and regenerating, the strengthening and weakening of the vortex and the spreading of the pressure fluctuation are all concerned with the low dominant frequencies.