The FrancisPLUS project set out to extend the operating envelope of the Francis turbine so that it can meet the demands of a highly variable renewable‑energy grid while preserving its high hydraulic efficiency of up to 97 %. The research focused on the interaction of pressure fluctuations, cavitation, and vibrational loads with the turbine’s structural integrity and on the resulting effects on life expectancy and power output. To this end, a comprehensive experimental and numerical programme was carried out. Model turbines of approximately 0.4 m diameter were instrumented with strain gauges and pressure transducers and tested under critical load cases on a dedicated test rig. The measured stress distributions and pressure swings were used to generate trend curves and to identify key design parameters that influence robustness. The experimental data were then validated against full‑scale turbines up to 8 m in diameter, confirming that the scaling laws derived from the model tests hold for commercial machines. Computational fluid dynamics simulations provided detailed pressure and velocity fields, from which stresses were calculated and compared with the experimental results. This dual approach enabled the team to pinpoint the most critical regions of the runner and to propose design modifications that reduce peak stresses without compromising efficiency.

In parallel, the project investigated ventilation as a supplementary mitigation strategy. A series of tests evaluated the effect of controlled air injection on cavitation inception and on pressure fluctuation amplitudes. The economic assessment of adding a ventilation system showed that the additional capital and operating costs could be offset by the extended service life and by the increased flexibility in grid support. Material science studies examined the suitability of advanced alloys and surface treatments for the runner and guide vanes, aiming to improve fatigue resistance under transient loading. Rotor dynamics simulations were performed to assess the impact of altered mass distribution and stiffness on the natural frequencies and to ensure that the modified design remains free of resonant excitation during start‑stop cycles.

The technical outcomes of FrancisPLUS culminated in an optimized runner geometry that maintains near‑optimal efficiency across a broader range of flow rates, reduces cavitation risk, and improves fatigue life under frequent transient operations. The project also produced a validated methodology for scaling laboratory results to full‑size turbines, a set of design guidelines for pressure‑fluctuation mitigation, and a cost‑benefit framework for ventilation systems. These deliverables provide a clear pathway for manufacturers to upgrade existing Francis turbines or to design new units that can reliably support the integration of solar and wind power into the grid.

Collaboration was central to the project’s success. VOITH Hydro Holding GmbH & Co KG acted as the consortium leader and technical coordinator, ensuring that all partners delivered their tasks on time and that the overall project schedule remained on track. EPFL (École Polytechnique Fédérale de Lausanne) prepared the project summary and contributed expertise in fluid‑structure interaction. The Institute of Fluid Mechanics and Hydraulic Flow Machines at the University of Stuttgart performed cavitation simulations for various operating points as part of the EU HYPERBOLE programme. Université Laval in Québec, Canada, served as the academic partner for the Tr‑FRANCIS sub‑project, focusing on transient fluid‑structure interaction and rotor dynamics. The project was funded by the German Federal Ministry of Education and Research under the grant code 03EE4004A, with a duration that spanned from the initial design phase through to the final validation and reporting in December 2023. Regular steering meetings, technical workshops, and continuous evaluation ensured that the consortium maintained a cohesive approach and that the scientific objectives were met within the agreed timeframe.