Hysteresis Characteristic in the Hump Region of a Pump-Turbine Model
The hump feature is one of the major instabilities in pump-turbines. When pump-turbines operate in the hump region, strong noise and serious fluctuations can be observed, which are harmful to their safe and stable operation and can even destroy the whole unit as well as water conveyance system. In this paper, a low specific speed ( n q = 36.1 min −1 ) pump-turbine model was experimentally investigated. Firstly, the hump characteristic was obtained under 19 mm guide vane opening conditions. More interestingly, when the hump characteristic was measured in two directions (increasing and decreasing the discharge), characteristic hysteresis was found in the hump region. The analysis of performance characteristics reveals that the hump instability is the result of Euler momentum and hydraulic losses, and different Euler momentum and hydraulic losses in the two development processes lead to the hysteresis phenomenon. Then, 12 pressure sensors were mounted in the different parts of the pump-turbine model to obtain the time and frequency characteristics. The analysis of the corresponding fast Fourier transform confirms that the hump characteristic is related to low-frequency (0.04–0.15 times rotational frequency) vortices. The occurrence and cessation of vortices depend on the operating condition and measurement direction, which contribute to the hysteresis feature. Finally, the type of the low-frequency vortices was analyzed through the cross power spectrum.
Influence of the clearance flow on the load rejection process in a pump-turbine
The clearance flow between the runner and the stationary parts greatly effects the efficiency and the hydraulic thrust of pump-turbines. Especially in the load rejection process, the clearance flow of the runner is more important to accurately predict the hydraulic torque on the runner and the variation of the rotational speed. However, the clearance flow was generally neglected in the most previous studies. To investigate the influence of the clearance flow on the simulation of load rejection process, three dimensional simulations were carried out in this paper with and without considering the clearance flow of the runner. To simulate transient characteristic of the boundary conditions, the unsteady total pressure and static pressure boundary conditions were determined at the spiral casing inlet and draft tube outlet using the user defined function. The method of coupling the rigid body motion with the water flow in the pump-turbine was adopted to predict the variation of the rotational speed. Dynamic mesh technology was used to simulate the dynamic closing process of the guide vanes. Based on the available experimental data, the numerical simulation results with and without considering the clearance flow of the runner were compared and analyzed. The results indicate that the accurate simulations of the fluctuating performance characteristics and the unsteady vortex flows in the pump-turbine are significantly influenced by the clearance flow between the runner and the stationary parts during the load rejection transient process. Moreover, the closer to the small discharge conditions, the influence of the clearance flow is more severe. Therefore, the shunt and impact action of the clearance flow to unsteady main flow in pump-turbines should be considered during the simulation of the load rejection transient process.
Numerical and experimental investigations on the cavitation characteristics of a high-speed centrifugal pump with a splitter-blade inducer
A high-speed centrifugal pump with a splitter-blade inducer is investigated in this work. The flow with rotating cavitation is numerically simulated, external characteristics are subjected to experimental tests, and the internal flow is visualized. These procedures are conducted to obtain the pressure, velocity, and vapor volume fraction distribution in the inducer and the impeller of the centrifugal pump. Bubble occurrence, development, and collapse are also observed. The predicted H-Q and η-Q curves agree with the experimental results of external characteristics. The calculated vapor volume fraction also agrees with the experimental results obtained from the visualization system. The mechanism of bubble evolution and the anti-cavitation performance of the high-speed centrifugal pump with a splitter-blade inducer are clearly elucidated.