Abstract: Sediment wear is one of the main causes of Francis turbine failure. For a high-head turbine unit, its sediment erosion rate can be greatly enhanced due to relatively high velocities of its internal flow. To study the sediment erosion mechanism of Francis turbines, in this paper we first obtain the average concentration data and particle size data for the sediment flows in a high-head turbine at a hydropower station based on field measurements, and simulate its characteristics of two-phase flows in the conditions of optimal openings and small openings of its guide vanes. Then, we calculate the trajectories of sediment particles using a Lagrangian method and numerically predict the runner erosion patterns using the Oka model. The erosion mechanism is explored through an analysis on flow characteristics, particle trajectories, and wall impact characteristics; simulation results are verified by comparing with previous studies in the literature. Results show the Eulerian-Lagrangian method gives satisfactory qualitative predictions of the erosion patterns. Under the condition of small guide vane openings, the vortex core developed in the runner passage leads to sediment particles conglomerating, thus intensifying the erosion around the corresponding locations.

Key words: Francis turbine, solid-liquid two-phase flow, erosion prediction, impact properties, CFD