The transition toward renewable-dominated power systems is increasingly constrained by the shortage of flexible regulation resources. Hydropower, with its rapid response and strong load-adjustment capability, remains a cornerstone for enabling large-scale integration of intermittent wind and solar energy. Splitter-blade runners are widely employed in medium- and high-head conventional hydropower plants and pumped-storage stations due to their broad high-efficiency operating range and superior stability. In this study, based on a runner replacement project at an existing hydropower station, refined computational fluid dynamics (CFD) simulations were carried out to design a splitter-blade runner under strict dimensional constraints. The optimized runner expanded the unit’s stable operating range from 50–100% to 0–100% rated power, while also improving overall efficiency and reducing pressure pulsations. The optimized splitter-blade runner improved efficiency by 1–2%, reduced pressure pulsations in the draft tube by ≈25%, and decreased the runner radial force by ≈12% compared with the baseline configuration. Importantly, this work demonstrates for the first time that splitter-blade runners can be successfully applied at head ranges below 100 m, thereby extending their applicability beyond traditional limits. The results provide both theoretical and practical guidance for flexibility retrofits of existing Francis turbine units in China, offering a feasible pathway to support the adaptability of future renewable energy systems.

The hub ratio of a bulb turbine has an important influence on the unit’s flow capacity, efficiency, and operating stability and is one of the key parameters governing the hydraulic performance and structural safety of the turbine. To select the optimal hub ratio matching the bulb turbine of a hydropower station after local thickening retrofitting of its runner blades, a full three-dimensional physical model of the unit was established, and numerical simulations were carried out to investigate the influence of hub ratio on turbine performance. Four hub-ratio schemes, namely 0.32, 0.34, 0.36, and 0.38, were selected for full flow passage simulations. The key hydraulic performance indicators of the four schemes were compared, revealing a nonlinear relationship between hub ratio and turbine performance: as the hub ratio increases, the overall hydraulic performance of the turbine first improves significantly and then declines continuously. The simulation results show that the turbine achieves the best overall performance when the hub ratio is 0.34. To verify the structural reliability of the hydraulically optimal scheme, stiffness and strength calculations of the runner were performed under typical operating and extreme conditions. The results indicate that, at a hub ratio of 0.34, both the stress distribution and deformation displacement of the runner are within the allowable limits specified by the design requirements, and the structural safety and reliability of the runner meet the operating requirements. This study provides a theoretical basis and engineering reference for parameter optimization, performance improvement, efficiency enhancement, and safe and stable operation of similar low-head tubular units.