Three-dimensional simulation of unsteady flows in a pump-turbine during start-up transient up to speed no-load condition in generating mode

Three-dimensional numerical simulation for a prototype pump-turbine has been performed to investigate the transient characteristics and flow behaviors in the startup process. The rotational speed, which varies with runner torque and inertial of the rotating systems, is obtained by hydraulic-force coupling method. To simulate the guide vane motions, dynamic overset mesh technique is applied with good maintenance of mesh quality. Comparisons with the existing experimental results verify the validity of the current method in predicting the startup transient characteristics of a pump-turbine with good accuracy. During the startup transient, the guide vanes open according to a prescribed sequence, which brings in increasing flow rate to the unit. The runner accelerates due to growing water power energy exerted on the runner blades. When approaching the speed no-load condition, a ring-shaped flow enlacing the entire vaneless space, that we term water-ring, is found to block the through-flow along the circumference, and the pump-turbine features a prominent partial pump flow state in the runner, which equalizes the torque to no load. Thus, the runner rotational speed stops increasing and stabilizes toward rated speed. The prediction shows that the pressure fluctuation amplitude of the speed no-load can be 3.8 to 8 times that of the full load, presenting evidently stronger instability at the speed no-load.