As the reversible pump-turbines operate in the S-shaped region, instability problems including backflow, vortex formation and rotating stall may appear. Previous researches studied instabilities at different guide vane opening (GVO) on their inception and evolution but few studies explored the effect of the blade lean at the leading edge. In present work, two runners tested by experiments, the runner A with a negative and the runner B with a positive blade lean at leading edge, were studied in CFD mode with a reduced scale model. Six operating points, namely, best efficiency point (OP#1), two points in the normal operating region (OP#2, OP#3), two points near runaway line (OP#4, OP#5) and a low discharge point in turbine brake (OP#6) were calculated for both runners. As the discharge reduces, the flow in the runners loses its symmetry and the efficiency becomes lower and lower. The flow of OP#1, OP#2 and OP#3 is healthy but slight separations locate near the inlet of the passages. At OP#4, obvious vortexes occupy the passages and the visible vortexes prevent the flow from entering the channels. The blockage generates strong backflow near the inlet of the runner. Moreover, the main backflow area locates near the hub for runner A while for runner B it is near the shroud. Unsteady vortex formation and rotating stall respectively exist at the near runaway points (OP#4 and OP#5) and low discharge point (OP#6). At these three points, the pressure fluctuations in the vaneless gap between the runner and guide vanes are very high and the amplitude shows a small difference between the two runners. Dramatic distinction appears on the frequency of the fluctuation. For both of the two runners, a peak corresponding to 70% fn, where fn is the runner rotating frequency, rises in the spectra of OP#4 and OP#5. This peak appears at all the monitors in the vaneless space at the same time standing for the unsteady vortex formation, which does not rotate with the blades. In addition, at OP#6, 40% and 50% fn are detected as the dominant frequencies for runner A and runner B respectively. In addition, the propagation of such two low frequency signal along the annulus in the vaneless space proves the existence of the rotating stalls.

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