Validation of CFD models with respect to the thermal-hydraulic design of the ESS target

For the proposed next generation neutron source for Europe ESS (European Spallation Neutron Source), the thermal-hydraulic design of the target is one of the main issues. The design will concentrate on the heat removal capability of the target, flow stability, and active cooling of the target window by the mercury flow. Moreover the behaviour of gas bubbles, which will be necessary to mitigate the pressure pulse inside the target, will be considered in the thermal-hydraulic design. Because Computational Fluid Dynamics (CFD) is one of the main tools to be used for the thermal-hydraulic design, validation of corresponding models in available CFD codes is a very important matter within the ESS project. Up to now, a lot of validation work took place within the European Commission funded ASCHLIM project. From the analyses performed within this project and additional validation work performed by NRG and FZJ, it could be concluded, that CFD codes are in principal able to predict the wall cooling at the target window with engineering accuracy if certain ancillary conditions are considered. In order to quantify the accuracy of the RANS modelling of the turbulent momentum transport for the ESS target concept, an experiment with a prototype target, made of acrylic glass, in a water loop was set up. Velocity field measurements, using Particle Image Velocimetry (PIV), will be performed, and the measured flow field will be compared to CFD results. With respect to the transport of small gas bubbles in a mercury flow, up to now no suitable experiments are available for the validation work needed for the ESS target, and therefore, only some basic investigations took place. (author)