Geometrical scale down analyses of the EU-DEMO WCLL BZ water U-manifolds

A geometrically scaled-down Test Section representative of the Outboard Segment water manifold of the European DEMO Water-Cooled Lithium Lead Breeding Blanket has been designed for experimental investigation in the Water Loop facility of the W-HYDRA platform, currently under construction at the ENEA Brasimone Research Center. The Test Section is designed to experimentally validate the coolant mass flow rate distribution among parallel Breeding Units (BUs) in the full-scale manifold and to provide experimental confirmation of the numerical tools and modelling assumptions adopted in the blanket design. Building on previous studies addressing the scaling and modelling of a Z-type manifold configuration, the present work focuses on a U-type manifold layout, which ensures a more uniform flow distribution among BUs. A scaling factor of four is applied to the manifold length, cross-sectional areas, number of breeding units and total inlet mass flow rate, while preserving the coolant velocity profiles along the manifolds. This approach allows each BU of the Test Section to be hydraulically representative of four BUs of the full-scale design. The coolant channels within the breeding units are modelled through equivalent porous pipes, enabling a substantial reduction of the computational cost while preserving the hydraulic characteristic. Steady-state single-phase Computational Fluid Dynamics simulations of the Test Section are performed using the same modelling framework adopted for the full-scale configuration, including identical operating conditions, turbulence modelling and numerical settings. The results demonstrate that the mass flow rate distribution within the BUs is preserved with a relative error < 10%, while the distribution among the BUs is preserved within 3% for all units except the inlet one, which is affected by jet entrance effects. The analysis confirms that pressure losses within the BUs dominate the overall pressure drop and can be effectively preserved through a pressure-profile-based scaling. Minor limitations introduced by the scale-down, such as enhanced inlet jet effects and vortex instabilities near the manifold extremities, are identified and discussed, providing guidance for future design optimizations of the experimental Test Section.