S-shaped characteristics of pump-turbine and improvement on its performance

When a pump-turbine operates in the mode with S-shaped characteristics,its load rejection is difficult and its water pressure takes an unusual increase,causing an intensification of its mechanical vibrations.To explore the origin of such characteristics and corresponding improvement measures,this study conducted a CFD analysis of the full flow and an energy test for a pump-turbine under the conditions of synchronous guide vanes and pre-opening guide vanes.A comparison of the CFD results with experimental data shows that in the low flow conditions,centrifugal force and large incident angle cause the formation of large-scale vortex flows inside the runner and between the guide vanes and runner entrance,and that these vortices should be the major factors causing the S-shaped characteristics.Pre-opening of the guide vanes can increase the flow and reduce the vortices in these two flow regions,and thus it is an effective measure for pump-turbine to avoid the mode with S-shaped characteristics.