Numerical Study Of The Hump Characteristic And S-shaped Characteristics Of A Pump-turbine

Due to the rising productivity and higher living standards, the large-sizedthermal power station and nuclear power station now are developing rapidly. Itrequires that the power grid should possess the greater and more flexible regulationcapability under the condition of security, stability and economy. The pumpedstorage power station which has double peak regulation capability is a good choiceto realise this goal. However, The existence of a positive-slope in the head-dischargecharacter of a pump-turbine in pump mode can lead to the operating instability, andthe S-shaped characteristics of a pump-turbine in generating mode will make itdifficult to connect with power grid. So it is very necessary to analyze regions of thepositive slope and the S-shaped characteristics by experimental investigation andnumerical simulation which are two useful ways to improve the unstable region.To achieve the above-mentioned object, a numerical simulation of thepump-turbine’s internal flow field was taken. Both the pump mode and generatingmode were analyzed by steady CFD simulation., the evolution of regions of thepositive slope and the S-shaped characteristics were observed by changing the guidevane opening. The unsteady flow structure and pressure fluctuation in regions ofpositive slope were obtained through unsteady CFD simulation. The main contentsof the paper are as follows.1. The steady CFD simulation was conducted respectively under three guidevane openings of18mm,22mm and32mm in pump mode, and the character curveswere also obtained. The evolutions of flow field under these three kinds of openingswere analyzed according to the simulation result, as well as the relationship betweeninternal flow structure and character cure shape. This paper preliminarily exploredthe regions of positive slope of18mm guide vane openings.2. Unsteady CFD calculation in pump mode was conducted. Pressurefluctuation pattern on the flow field was studied through FFT analysis. The spectrumwas obtained and the cause of each main frequency was analyzed. Comparingpressure fluctuation under different working conditions, this paper pointed out thepressure fluctuation characteristics in regions of positive slope.3. Steady CFD calculation of turbine mode, runaway mode and braking modewas conducted under three guide vane openings of5mm,10mm and32mm, and thecharacter cures in the first quadrant were obtained. The flow fields of turbine mode,runaway mode and braking mode were analyzed under these three openings, as wellas the relationship between internal flow structure and S-shaped characteristics. The research result shows that the flow structures of the runner, guide vane andstay vane have a great influence on regions of positive slope at off-designedconditions in pump mode, and can also lead to lower efficiency. In generating mode,vaneless zone, runner inlet, runner outlet are full of vortexes which have asignificant impact on S-shaped characteristics.