Effect of Needle Opening on Sediment Erosion and Entropy Production in a Pelton Turbine

This study investigates how different needle openings govern the coupled evolution of sediment erosion and entropy production-based hydraulic dissipation in a Pelton turbine. Three representative needle openings (20%, 40%, and 54%) are examined by CFD simulation, and the total entropy production is decomposed into wall entropy production, direct dissipation, and indirect dissipation to quantify the opening-dependent irreversibility budget. The results show that the transition zone and buckets consistently dominate the total entropy production, accounting for 84.48%, 80.78%, and 81.57% of the total at 20%, 40%, and 54% openings, respectively, indicating that the nozzle–runner interaction region is the principal carrier of irreversible loss. Meanwhile, reduced opening intensifies jet contraction and promotes non-uniform sediment redistribution, whereas larger openings improve jet coherence and enhance particle flow-following behavior. The wall-level results further reveal that the correspondence between erosion rate density and wall entropy production becomes progressively more evident with increased opening, especially on the bucket pressure side, while particle incidence statistics indicate a transition from broader-angle, locally triggered impacts at small openings to predominantly grazing delivery at larger openings. Overall, the results demonstrate that needle opening does not merely change the magnitude of loss or erosion, but systematically reorganizes the coupled pathway of jet development, sediment redistribution, near-wall dissipation, and wall damage in a Pelton turbine.