Numerical analysis of 60° V-ribs for helium-cooled high heat flux loaded first wall, challenges and contributions

• Complementary use of LES and RANS models was used to address research questions associated with optimizing the rib pattern for the helium cooled FW that can allow efficient operation at heat flux densities up to 1 MW/m². • It was found by a RSM on a long domain that the hydraulic entrance length of a v-ribbed channel was about 38 times the hydraulic diameter. This precludes the reduction of FW engineering related problems to the simulation short segments. Further, it was found that the high temperature gradients in helium FW flows exposed to 1 MW/m² heat fluxes induce variations of 30 – 50 % of the thermophysical properties, thus counter-indicating the use of constant properties models. • A comparative study applying both LES (regarded as reference) and RSM to short channel sections with periodic boundary conditions revealed significant differences between the two modelling approaches, with 25 % lower heat transfer predicted by RSM. This clearly shows a deficiency of RSM to model the absolute level of heat transfer in the studied rib geometries. The study of rib height variations in those models gave a first indication that v-rib height with a height of 0.4 mm would be expected to yield a superior cooling performance (better cooling per pumping power) in comparison to the high profile ribs with 0.9 mm, investigated earlier. This finding was confirmed in a RSM study on long channels, where the rib height was variated over the axial length. • Comparative RSM studies were also applied to investigate the impact of a geometry detail such as a tip chamfer of the v-ribs. It was found that such a detail can have a significant impact (∼ 20 %) both on pressure drop and heat transfer. The HCPB (Helium Cooled Pebble Bed) blanket concept for EU DEMO fusion reactors employs high-pressure (8 MPa) helium gas as coolant for the plasma facing first wall (FW). Up-to-date estimates for the total maximum of heat flux go up to 0.73 MW/m². Higher short-term transient loads are possible. 60°-V-shaped ribs show high heat transfer and are thus the subject of the presented studies. Although Scale-Resolving Simulation (SRS) techniques such as LES (Large Eddy Simulation) are able to calculate heat transfer and pressure drop precisely, their application is limited to an individual ribs or a few mm channel segment due to the high required mesh count. Nevertheless, SRS techniques can be used to compare different ribs and to evaluate the performance of RANS (Reynolds-Averaged Navier-Stokes Simulations). Selected RANS models can be used to evaluate the development of secondary flow structures along the channel and compare complete channels with different rib configurations for thermohydraulic performance. The objectives of the present paper are (i) to offer guidance on the range and limits of applicability of numerical methods when dealing with cooling flows in structured channels (sections 1–4) and (ii) to provide results on specific design features of surface structures (sections 5–7) that help designers in implementing thermal-hydraulic efficient yet fabrication friendly structured channels Challenges like long entrance length and high material properties gradients are shown. Strategies for ribs height reduction with increasing heat transfer and reduction of pressure drop resulting in higher Cooling Performance Numbers (CPN) are found. Thermohydraulic performance of fabrication friendly ribs is calculated along the channel.