Flow-induced fretting in DEMO divertor targets equipped with swirl tapes: Numerical investigation through one-way fluid-structure interaction simulations

The divertor targets in DEMO, sustaining a quasi-steady state heat flux up to 20 MW/m², are made of pure tungsten armoring actively water-cooled CuCrZr pipes. The heat exhaust capability of the divertor targets can be improved by adding a swirl tape in the cooling channel. Analysis from WEST samples and several laboratory studies highlight that under DEMO-like cooling conditions, the insertion of swirl tapes in the pipes triggers surface wear damage. This is due to flow-induced vibrations of the swirl tape within the hosting pipe leading to fretting which takes place whenever two bodies in contact, submitted to normal load, undergo micro-displacement oscillatory motion. Fretting wear damage is a serious problem that may reduce the lifetime of the divertor components and their thermal efficiency. Hence, the objective of this study is to quantify tribological loadings in the divertor inducing fretting through fluid-structure interaction simulations (FSI) to set up dedicated fretting tests in laboratory. One-way FSI simulations are performed using URANS (Unsteady Reynolds-Averaged Navier-Stokes equations) and LES (Large Eddy Simulation) to assess the fluid pressure in order to perform transient structural analysis allowing the estimation of stresses, deformations and tribological loadings (contact forces and sliding amplitude) at the CuCrZr pipe/swirl-tape contact area. Benefits and drawbacks of each modelling method (LES and URANS) are discussed. The numerical outputs highlight that one-way FSI simulation reflects complex contact loading between the swirl and the pipe involving fretting sliding with both impact and fluctuating normal forces. The obtained loads drive the choice of the normal forces and the sliding distance ranges which will be explored in laboratory via fretting wear experiments.