Development of Innovative Design Strategies for the Optimization of the Performances at Partial Loads of Reversible Pump Turbines

Pump-turbines are essential components in pumped-hydro power plants, frequently switching between pumping and generating modes for grid balancing. These machines are often required to operate under deep partial load conditions, which can compromise both their operability and lifespan. Depending on their specific speed, pump-turbines may exhibit an "S-shaped" discharge-speed and torque-speed characteristic curve during generating mode, leading to instability, increased structural vibrations, and pressure perturbations, especially at runaway speed. In this context, stabilizing the runaway operating point and safely synchronizing with the electrical grid may be significant challenge. Despite these pressing issues, new design approaches to prevent the onset of unstable behaviors are still underdeveloped, and control strategies for accelerating start-up and shut-down procedures are not yet fully effective. These strategies are based on semi-empirical methods due to the lack of identified precursors to unstable behavior. This thesis investigates the hydrodynamics of two low-specific-speed Francis-type reversible pump-turbine reduced-scale models under deep off-design conditions. Numerical simulations were conducted using Ansys CFX software, applying the SAS-SST turbulence model, to analyze fluid field evolution and its effects on the characteristic curve slope. The machines' behavior was simulated under transient conditions with time-varying boundary conditions, capturing the transition from safe partial load to zero-discharge. The combined fluid flow and spectral analysis allowed for the determination of the three-dimensional topology evolution of the main unsteady turbulent structures developing in the unstable operating region, as well as the correlation between the characteristic curve slope and the development of flow unsteadiness. Rotor-stator interaction was found to play a key role in the onset and development of unsteady structures in both the guide vanes and the runner. Additionally, large backflows at the runner outlet section were observed to significantly affect pressure distribution and radial force imbalances in the runner. Additionally, this thesis explores the application of the 3D Inverse Design Method (IDM) as a tool for optimizing blade loading distribution in reversible pump-turbines. Recognized as an effective technique in modern hydraulic machine design, IDM has been investigated for its potential to mitigate the complex three-dimensional flow characteristics that arise in reversible turbines when operating in unstable regions. This thesis presents and discusses key design solutions documented in the field of reversible pump-turbines, focusing on the influence of blade stacking and load distribution on hydrodynamic performance. Optimized blade load distributions are also reported, offering useful design guidelines for the development of the next generation of hydraulic turbines. According to the literature, adopting a suitable blade lean can reduce pressure fluctuations during both pumping and turbine modes. However, the correlation between such design parameters and the "S-shaped" characteristic has not yet been fully investigated. Additionally, previous IDM applications have not considered the typical transient phenomena characterizing fluid flow in hydraulic turbines. In this context, this work explores the impact of runner blade lean angle on flow unsteadiness during the transition to zero-discharge conditions. Four distinct blade lean configurations were numerically examined and compared with the original pump-turbine model test geometry. The results show a strong correlation between hydrodynamic instability and blade lean. While all configurations exhibited rotating stall at some point, the runner with a high linear positive lean distribution demonstrated superior resilience to pressure and force perturbations, improving machine performance in unstable regions.