Effect of Stokes number on erosion on Pelton buckets for sediment-laden flows
Aron Dagur Beck, Elena Vagnoni
TL;DR
This work addresses erosion in Pelton buckets caused by sediment-laden jets, using a 3D Eulerian–Lagrangian CFD framework with Volume of Fluid (VOF) for the air–water interface and the Discrete Phase Model (DPM) for particles to study how the Stokes number $Stk$ controls jet structure and erosion patterns. It shows that at $Stk \lesssim 0.1$ the jet particle distribution is approximately axisymmetric and can be imposed radially, while at higher $Stk$ gravity-induced asymmetry requires coupling injector and bucket simulations for accurate predictions. The study also demonstrates that assuming a uniform mean-diameter particle distribution underestimates erosion in critical regions such as the tip and splitter compared with a realistic size and spatial distribution, highlighting the importance of realistic jet modeling for erosion forecasts. These results advance erosion forecasting and sediment management strategies in hydropower plants by informing when to couple injector and turbine simulations and how to design flushing protocols to protect Pelton buckets.
Abstract
With increased glacial melting and the need to maintain sediment continuity for ecosystem health, sediment-laden flows through hydropower plants are becoming increasingly problematic, particularly due to erosion on runner blades and buckets. A widely used mitigation strategy is the use of filters to protect Pelton turbines. However, these filters lead to rapid sediment accumulation in reservoirs, which must be drained frequently to maintain storage capacity. The high cost of such drainage operations calls for longer intervals between them, without compromising runner integrity, as erosion-induced stress concentrations may cause bucket rupture. A better understanding of the causes of runner erosion under varying operational and sediment conditions is essential to allow more sediment to pass through the hydraulic machine safely. This study investigates how sediment-laden flow through the nozzle affects particle distribution in the jet for different Stokes numbers. Furthermore, it analyses how a realistic particle size and spatial distribution in the impacting jet compares to the assumption of a uniform particle distribution with particles of mean size when simulating bucket erosion in Pelton wheels. The results show that the particle distribution in the jet follows a similar axisymmetric shape for low Stokes numbers whereas at higher Stokes numbers it becomes asymmetric. Additionally, it is shown that imposing uniform particle distribution with particles of mean diameter under-predicts tip- and splitter erosion when simulating the erosion on the bucket but is captured by imposing the realistic size and spatial distribution.