Analysis of Goertler Instabilities in the IFMIF Li-Target Flow

The International Fusion Materials Facility (IFMIF) uses a high speed Lithium (Li) free surface flow as a target, which is hit by two deuteron beams having each a power of 5 MW. The major function of the Li target is to provide a stable Li surface layer for the production of an intense neutron flux, to remove by forced convection the beam energy deposited in the Li layer and to avoid an overheat of the back wall structure beyond material sustainable limits. The target flow is conditioned by sophisticated shaped nozzle. Downstream the nozzle the liquid metal flows along a concave shaped back-wall in order to generate an increased pressure within the Lithium layer, which elevates the boiling temperature of the liquid. This article describes an analytical and numerical fluid dynamic study of the secondary liquid motions potentially occurring due to the concave shaped wall as well as its impact on the temperature distribution within the liquid lithium. First, the secondary flow distributions appearing in a turbulent flow along a curved wall are analyzed, where the major attention is paid to Goertler vortices, which emerge exceeding a critical flow velocity as a convective instability due to centrifugal forces. Further, a large-eddy simulation (LES) of the turbulent flow along the back-wall is conducted and the obtained data are compared to experiments. The results show that both the formation of the Goertler vortices as well as their decay are reasonably well predicted by the simulation. After the validation of the LES method the simulation has been transferred to the lithium target geometry at IFMIF standard operation conditions assuming decreased lithium layer thicknesses down to 23 mm. In this context besides the LES a conventional fluid dynamic computations based on the Reynolds-averaged turbulence model have been performed. The results show that the Reynolds-averaged models are not capable to predict the occurrence of Goertler vortices as the LES does. The formation of the Goertler vortices, however, yields to hot spots at the back wall which can reach temperature rises of more than 100°C compared to the nominal case.