Hydrodynamic Mechanism Analysis Of The S–shaped Curve For A Reversible Pump-turbine Model

The S characteristic is widespread in Pump-turbine with a runner speed corresponding to two or three flow rates. When units run in the S area in the process of turbine working condition, flow oscillation appears to make unit connecting with power grid difficult, which affects the safe operation seriously. In this paper, the numerical simulation is compared with the model test to make detailed analysis on the flow mechanism of S-shaped characteristics for the optimal opening and the small guide vane opening respectively.For a model pump turbine as the research object, its performance curve has been analyzed in detail. The performance curve showed obvious S-shaped characteristic. Meanwhile, by analyzing some performance curves of different model pump turbines, the inflection point o f S-shaped can not coincide with the runaway point in the first quadrant. This paper explores the hydrodynamics of some off-design points on S-shaped region and more analysis focuses particularly on the inflection point, very low positive and negative disc harge points with the same unit speed. Firstly, the numerical method is used to calculate and analyze the optimal guide vane opening curve under different working conditions on the basis of model test. And a series of unsteady numerical calculations are a very good prediction for the unit performance curve. Five different conditions are selected to make three-dimensional flow qualitative and quantitative analysis, including the optimal point, the point near runaway and the turbine point, the turbine braking point, the reverse pump point with the same speed, which are distributed in turbine model, turbine braking model and reverse pump model area. Water flows smoothly in the whole flow passage in the optimal point; Comparative analysis between the point near runaway and its near point as the turbine point and the turbine braking point, flow distribution between the guide vanes and the runner determines the three-dimensional flow state of the whole flow passage. The speed in the point near runaway is the largest and water flows in and out along the circumferential periodic. Flow separation and large scale vortex flow appear between blades to block main flow passage, which makes water not work. But the closed “water ring” along the circumferential flow, which is appeared in the vaneless, hinders the flow in and out. Large and small scale vortex flow blocks the rotation of runner, making that the runner can’t be able to maintain the operation of the small discharge condition at a higher speed. Then S-shaped bend is showed near the runaway point in the performance curve.In addition, the small guide vane opening curve under different working conditions on the basis of model test are also calculated and analyzed. To explore Misaligned guide vanes(MGV) on the improvement of the S area, there are four misaligned guide vanes used in a small guide vane opening and the numerical method is used to calculate and analyze performance characteristic curve under different working conditions. Six different conditions are selected to make three-dimensional flow qualitative and quantitative analysis, including inflection points, small discharge points and reverse pump points, which are distributed in S area. Through comparing experiment and numerical performance curves of different openings, analyzing the internal flow behavior of different points and the principle of the attack angle, when MGV is used in the S area, it would increase the vortex flow in stay vane and guide vane, but it can destroy the high speed circulati on in the vaneless in inflection point with more flow running into runner to make rotation speed higher and it can also destroy the closed circulation in the vaneless in the low discharge point with much more flow running into runner to maintain the operat ion of the low discharge condition at a higher speed. Then, with the decrease of discharge, the increasing trend of the unit speed is more obvious to avoid the production of S-shaped bend.