Abstract. The performance of a reversible pump turbine with S-shaped (dQ11/dn11>0) characteristics is of great harmful to the transient processes such as start-up and load rejection. When working point transition is required, then process in turbine mode must be fast, efficient and reliable. Especially during load rejection, the pressure increasing at spiral case and negative pressure taking place at draft tube. This phenomenon has great relation with the characteristic curve with pronounced S shape. In this paper, the characteristic curve is improved with runner optimization. And hydraulic performance results are compared with runner geometry parameters changing. The runner optimization is an effective method to solve this problem.
IntroductionThe performance of a reversible pump turbine with S-shaped characteristics is of great crucial to the transient processes such as start-up and load rejection in figure 1. When working point transition is required, the process in turbine mode must be fast, efficient and reliable. Especially the S-characteristic curve takes place at large guide vane opening. When the single unite or mul-tiple unites reject full load at rated power at rated guide vane opening all units load rejection at the same time or in certain intervals such as delayed load rejection, it's very dangerous to engineering. The negative pressure will take place if the one uinit rejects the full load and after another unit is load rejected several seconds later. The transient study should find at which interval the delayed load rejection is the worst case and calculate the extreme pressure in the water passage to avoid any potential damage.In recent years, there have been several severe accidents reported for the pumped storage projects (PSP) such as HuiZhou PSP(8x300MW) [1] and XiLongChi PSP(4x300MW). The safe operation during the transient is of significant importance for high head pumped storage projects.The load case list should cover all the most critical load cases to ensure the safe operation of the whole plant. During the load rejection, the pressure in the spiral case is rising and the pressure after guide vane is dropping down after guide vane is closing, and the rotation speed of the machine is increasing or oscillating [2] . To ensure the safe operation of the whole plant, the maximum pressure in the water passage head water side, and the minimum pressure in draft tube, maximum over-speed should not exceed a certain limit, which is usually given by the customer.
In this paper, to study the effect of dynamic and static interference of clearance flow in fluid machinery caused by changes in rotational speed, the model was simplified to a rotor-stator system cavity flow. Investigating the flow characteristics in the cavity by changing the rotor speed of the rotor-stator system is of considerable significance. ANSYS-CFX was applied to numerically simulate the test model and the results were compared with the experimental results of the windage torque of the rotor-stator system. The inlet flow rate and geometric model remained unchanged. With an increase in the rotating Reynolds number, the shear stress on the rotor wall gradually increased, and the maximum gradient was within l* < 0.15. In addition to the shear stress, the tangential Reynolds stress Rrθ contributed partly to the torque on the rotor wall. The swirling vortex formed by entrainment in the cavity of the rotor-stator system tended to separate at ReΦ= 3.53 × 106. As the rotating Reynolds number continued to increase, the secondary vortex finally separated completely. The strength of the vortex in the rotor turbulent boundary layer decreased with an increase in the rotating speed, but the number of vortex cores increased with the increase of speed. Depending on the application of the fluid machine, controlling the rotating speed within a reasonable range can effectively improve the characteristics of the clearance flow.
Stall is a common phenomenon in centrifugal pumps under low-flow conditions; it has a significant impact on fatigue and can even damage mechanical structural components. Computational fluid dynamics was used to perform high-precision numerical calculations to describe multiple operating conditions in the computational domain. The accuracy of these numerical simulations was verified by comparing the results with the single-flow channel flow patterns captured by time-resolved particle image velocimetry and the external characteristics of the centrifugal pump. On this basis, the unsteady spatiotemporal evolution of the vortex structure under stall conditions and the kinetic energy conversion relationship were determined. The stall vortex under the rotating stall condition has a relative motion with the impeller in the circumferential direction between channels, with the characteristic propagation frequency fcs = 0.71 Hz. For stationary stall conditions, the critical stall condition has a greater kinetic energy dissipation compared with the deep stall condition, with energy differences being more than three times larger at the blade leading edge, where the stall vortex is formed.
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