Objective Investigation of turbulent flow and heat transfer in structured cooling channels Evaluation of truncated attached and semi-detached upward directed V-shaped ribs and spherical dimples for helium-gas cooled First Wall applications Highlights The thermal hydraulics of helium-gas cooled structured channels were analyzed and evaluated Semi-detached ribs ► highest heat transfer ► best cooling performance ► reduced local heat transfer deterioration Dimples ► heat transfer increase without

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... gnificant pressure drop rise ► large regions of low heat transfer 3. Global thermal hydraulics V-shaped ribs ► HTC ratio is 2.6-2.8, PD ratio is 5.4-5.8 Semi-detached V-shaped ribs ► additional heat transfer ► cooling performance increase Dimples ► heat transfer increase without significant PD (HTC ratio is 1.31-1.38, PD ratio is 1.0-1.11) ► the closer the dimples, the higher the heat transfer 4. Local heat transfer V-shaped ribs ► local heat transfer deterioration at the rear surface and behind the rib reduced Semi-detached V-shaped ribs ► narrowed regions of low heat transfer Dimples ► low HTC ratio in the upstream dimple halves ► large regions of HTC/HTC 0 < 1 ► high wall temperatures expected 2. Simulation Details Turbulent flow and heat transfer were determined in a FW cooling channel model of DEMO Re = 1.05·10 5 (0.0490 kg/s and 0.0407 kg/s) ►inlet pressure of p in = 8 MPa(abs) ► inlet temperature of T in = 340 °C LES with dynamic Smagorinsky SGS-Model ► incompressible helium-gas Periodic boundary conditions in streamwise direction Constant heat flux ► = 0.75 MW/m 2 on the plasma-side-facing FW surface ► = 0.08 MW/m 2 on the breeding-blanket FW surface Fig. 3. Normalized global heat transfer coefficient (HTC) ratio vs normalized pressure drop (PD). *S. Ruck, F. Arbeiter, Detached eddy simulation of turbulent flow and heat transfer in cooling channels roughened by variously shaped ribs on one wall, International Journal of Heat and Mass Transfer 118 (2018) 388-401. Fig. 2. Geometric details of the dimpled channels and the ribroughened channels. Fig. 4. Normalized local heat transfer coefficient of the attached and semidetached rib with the large gap fillet and the dimpled surface with s 1 = 1.2. Gray areas highlight possible 'hot spot' regions where HTC/HTC 0 < 1. Conclusion V-shaped ribs ► highest HTC ► local heat transfer deterioration and the pressure drop increase can be reduced by shape optimization Semi-detached V-shaped ribs ► additional heat transfer provided ► no additional PD compared to attached ones Dimples ► large regions of HTC/HTC 0 < 1 can lead to high wall temperatures Fig. 1 . Sketch of the computational domain for the ribbed channel and the dimpled channel.