A study on the evolution of the instability in two model pump-turbine runners with large blade leans

Abstract

Instabilities usually happen in the reversible pump-turbine in generation mode, especially in the S-shaped region. Flow separations, vortex formations and rotating stalls can exist in the runners and other components. The evolution of the instabilities in two model pump-turbine runners with large blade leans, runner A with a negative lean and runner B with a positive lean, is studied by means of numerical simulations in the present work. The guide vanes opening (GVO) is fixed as 15mm. Six operation conditions from the normal operation region to the turbine brake region are chosen to be simulated. In addition, processes with a decreasing discharge at the inlet of the volute are also calculated to trace the evolution of the instability. The pressure fluctuations in the vaneless gap are monitored in the whole annulus. Different flow structures can be detected from the power spectrum of the pressure fluctuations. As the discharge reduced, the amplitudes of the pressure fluctuations increase visibly from the normal operation region to the runaway conditions and slightly decrease if further reduce the flow rate. Rotor-stator interaction frequency and its harmonics dominate the flow field from normal operating region to turbine brake region with the frequency corresponding to 7fn, 14fn, where fn is the runner rotating frequency, and etc. Low frequency components gradually develop near the runaway points and the wavelet method detects the inception of the rotating stalls in the vaneless gap in the turbine brake region. Different blade leans give arise to an obvious distinction of the flow distributions near the inlet of the runners and in the vaneless gap. The separation generates a swirl stripe extending to opposite direction dependent on the blade lean direction. In addition, due to the different location of the swirl stripes, the pressure fluctuation distributions are opposite for the runners. Higher fluctuations locate near the hub and near the shroud for runner A and runner B respectively