Abstract
The positive slope on the pump performance curve of pump-turbines suggested potential operational instabilities in pump mode. In the present study a low specific speed model pump-turbine was experimentally and numerically examined. The results revealed that the positive slope region exists on each Guide Vane Opening (GVO) curve except at the optimum GVO. The positive slope occurred in the discharge coefficient between 80%-85% optimum discharge coefficient at every GVO. Hysteresis was detected on the occurrence of the positive slope between measurements with increasing and decreasing discharge coefficient. FFT (Fast Fourier Transformation) analysis on pressure fluctuations revealed the occurrence of rotating stall with two cells close to the vaneless space between the runner and the guide vanes in the positive slope region. Two-dimensional simulation was then applied for a further understanding of the flow mechanism of the rotating stall.
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