Numerical investigation on the nozzle opening influence to the Pelton bucket outflow power loss and runner dynamic stress

As a type of hydraulic turbine suitable for high water head hydropower exploitation, the Pelton turbine is subjected to repetitive impacts from high-speed water jets. The nozzle opening controls the Pelton turbine flow rate, affecting both the turbine hydraulic performance and the structural response. Considering single nozzle jet, this study numerically investigated the influence of nozzle opening to the hydraulic performance and the bucket outflow characteristics of a prototype Pelton turbine via the combination of RANS method and SST turbulence model. Based on the predicted high-resolution flow field, one-way FSI numerical method was used to analyze the stress variation at runner bucket key locations with the nozzle opening. Quantitative analyses on the bucket outflow spatiotemporal distribution revealed one core outflow strip area and two outflow power loss peaks. The percentage of outflow loss in overall turbine power input increased with the nozzle opening, occupying 2.5% absolute loss and 25% of total loss of the turbine at the rated opening. FSI simulation showed the bucket root, cutout edge, and the rear surface tip as vulnerable locations with stress concentration. The increase from 0.60 to 1.09 times of the rated discharge impacted the most on equivalent stress at the monitoring point locating at the middle of the cutout edge, with the stress amplitude and maximum stress surging up by 92% and 85%, respectively. The study illustrated the risk for both the higher outflow loss and the potential structural failure of the Pelton turbine runner with the prolonged operation at large nozzle opening conditions, providing quantitative evaluation indicators for the efficient and stable operation of the Pelton turbine.