Thermal-hydraulic study on rib and dimple structures for cooling the First Wall of DEMO

•Rib-structuring causes high heat transfer coefficients required for the helium cooled FW.•Semi-detaching the ribs increases the heat transfer and the cooling performance.•V-shaped ribs provide higher heat transfer and cooling performance than dimples. The thermal hydraulics of helium-gas cooled channels for the First Wall of DEMO structured by fully attached and semi-detached upward directed V-shaped ribs and spherical dimples in a staggered array were investigated. Large-Eddy-Simulations with the dynamic Smagorinsky sub-grid scale model were performed. The global and local heat transfer coefficient, pressure drop and cooling performance were analyzed. Compared to smooth channel flows, the global heat transfer coefficient was increased by a factor of 2.6–2.9 and the corresponding pressure drop was increased by a factor of 5.3–5.7 for the V-shaped ribs. Here, the semi-detached ribs provided the highest heat transfer and the best cooling performance. The dimpled structures increased the heat transfer by the factor of 1.1–1.4 without a significant pressure drop rise when compared to smooth channel flows, but regions of local heat transfer deterioration occur over each upstream dimple half.