Unstable flow characteristics in a pump-turbine simulated by a modified Partially-Averaged Navier-Stokes method

Abstract

Positive slope characteristics are very important for the safe and stable operation of a pump-turbine. In this study, the unsteady flows in a pump-turbine at pump mode are investigated numerically. To predict the positive slope characteristics with an improved accuracy, a modified Partially-Averaged Navier-Stokes (MPANS) model is employed to capture the unstable physics in a pump-turbine. It is confirmed that the present numerical method predicts the positive slope characteristics in the pump-turbine fairly well compared with the experimental data. It is noted that at the drooping point of the performance curve (positive slope), there are three sets of rotating stall cells in the flow passages of both the guide vanes and stay vanes. In the guide vane region, the flow is completely shut off by the rotating stall, whereas in the stay vane region, the flow passage is partly blocked at the drooping point. The numerical results also reveal that the remarkable variation and high angle of attack (AOA) values upstream the leading edge of the guide vane contribute to the flow separation at the vane suction side and induce rotating stall in the flow passage within the positive slope region. Furthermore, the propagation of the rotating stall is depicted by both Eulerian and Lagrangian viewpoints: the rotating stall blocks the flow passage between two neighboring guide vanes and pushes the flow toward the leading edge of the subsequent guide vane. The rotating stall cell shifts along the rotational direction with a much lower frequency (0.146fn) compared with the runner rotational frequency, fn.