Effect of Blade Number on the Performance of a Small Francis Turbine for Rural Local Power Generation

The increasing integration of distributed photovoltaics and wind power in rural grids necessitates enhanced peak-regulation flexibility, for which small Francis turbines offer a promising solution. This study investigates the effect of long–short blade number on the hydraulic performance and energy losses of a representative rural small Francis turbine under constant runner material usage. Five configurations (N = 13–17) are evaluated using SST-DES-based CFD simulations and entropy production theory. At the rated condition, the N = 15 configuration achieves the highest overall efficiency of 93.65%, exceeding N = 17 by 0.19 percentage points and N = 16 by 0.61 percentage points; at high-flow conditions, it maintains a similar advantage of approximately 0.26 percentage points over N = 17. In the low-flow region, the N = 17 scheme exhibits slightly higher efficiencies, with advantages of 0.85 percentage points over N = 15. The N = 15 scheme demonstrates more uniform velocity streamlines, gentler pressure gradients, and smaller high-entropy-production regions across all flow components, particularly at the rated point. Overall, N = 15 provides the best balance between rated-point performance and off-design stability and is recommended for engineering applications.