Entropy generation analysis of coaxial diffuser effects on energy dissipation and pressure fluctuations in a Francis turbine draft tube

Under part-load conditions, turbine vibrations are predominantly induced by rotating vortex ropes (RVRs) and associated pressure fluctuations in the draft tube. This study proposes a novel coaxial diffuser to simultaneously suppress RVRs and mitigate pressure pulsations. Numerical simulations compare the prototype and modified Francis turbine, analyzing pressure fluctuations and flow dynamics while evaluating the diffuser’s geometric parameters (angle, diameter, height). Results show that the coaxial diffuser leads to a maximum efficiency reduction of only 0.0157%, which indicates that this installation does not significantly affect the turbine’s efficiency. The coaxial diffuser effectively reduces pressure fluctuations in the draft tube, with peak pressure fluctuations decreasing by up to 77.48%. The optimal angle α = 0.5 β ° provides the most significant overall reduction in pressure fluctuations, with an average decrease of 57.96%. Moreover, the coaxial diffuser increases the axial velocity of the water and alters the flow pattern within the draft tube. Constrained by the diffuser walls, the vortex rope cannot move radially. Ultimately, the device separates the dead-water zone from the mainstream, preventing the vortex rope from obtaining velocity compensation, thereby reducing the circumferential velocity and eccentricity. These findings provide a theoretical basis for the stable and safe operation of Francis turbines under partial load conditions.