CFD Estimation of the Unsteady Fluid Force Along a Fuel Rod Downstream a Mixing Grid

Unsteady flow loading of fuel assemblies in Pressurised Water Reactors power plants is a potential cause of deformation and of fretting wear damage. Inside a fuel assembly, rods are arranged in 17 × 17 bundles. The rod diameter is equal to about 9 mm, and the gap between two rods is equal to about 2 mm. Each rod is several meters long, and mixing grids are arranged every 0.4 m. The axial flow velocity is equal to about 5 m/s, so that the Reynolds number reaches 5 × 10⁵ in the reactor configuration. Due to the complexity of the turbulent flow pattern, an accurate description of the fluid-structure interaction is still a challenging task, and only a few data about this issue are available today in literature. Recent Computational Fluid Dynamics results are revisited from the point of view of classical axial turbulence-induced vibrations. The unsteady pressure force Power Spectral Densities are determined, a convective velocity is derived, and an estimation of the axial correlation length for the pressure force is given. The results agree reasonably well with the scientific literature.