Enhancing operational stability of a reversible pump-turbine through blade lean design

To provide the flexibility required by the evolving electrical grid, reversible pump-turbines (RPT) nowadays are required to rapidly switch operating mode and to operate at deep partial loads currently affected by strong fluiddynamic instabilities. Despite these urgent needs, the design strategy of RPTs is still lacking of an effective way to mitigate the unstable behaviour in these zones, which are avoided for the negative influence on the machine operability and longevity. This paper is aimed at defining design guidelines for facing this challenge and it presents an in-depth investigation of the influence of the blades lean angle on the RPT unstable behaviour. Considering performances, pressure and force pulsations, five runners with different blade leaning distribution (high negative, positive, curved negative, curved positive) were numerically analysed and compared with the original geometry (no lean) during the transition from a partial load to the zero-discharge condition. As a results, the close relation between the hydrodynamic instability and runner blades lean is demonstrate. All configurations showed the development of a rotating stall at a certain point during the transition but highlighted different intensities of the phenomenon and different onset points. In particular, among the five geometries, the runner with the positive lean distribution proved to be particularly resilient to pressure and force perturbations associated with the full development of the rotating stall, significantly improving the machine behaviour in the unstable region of the characteristic curve.