Novel TMPS structures for enhanced heat removal in divertor tiles for W7-X.

To comply with the need for decarbonization of the power and industrial sector, the nuclear fusion Community is developing worldwide new advanced reactor designs to produce heat and/or electricity without emitting CO2. In many of the different reactor designs, heat fluxes greater than 10 MW/m^2 are expected on some components such as the divertor, and must be properly exhausted. The work presented in this thesis is focused on heat removal from the divertor. The aim is to design a new divertor module for the Wendelstein 7-X, equipped with Triply Periodic Minimal Surfaces (TPMS) to maximize heat transfer by promoting the mixing of the coolant in an extended heat transfer surface configuration. TPMS are three-dimensional surfaces that can be mathematically described by sinusoidal and cosinusoidal functions, from these surfaces periodic lattices can be generated. Considering the recent revolution in additive manufacturing, researchers started to investigate TPMS geometries that were impossible to manufacture by conventional manufacturing methods. In the energy field, TPMS find applications as heat sinks in electronic devices or as heat exchangers. The TPMS structures are modeled using the software NTOPOLOGY. They can be chosen between gyroid, which has been intensively studied, and lidinoid which is expected to offer better thermal performance. The parameters of optimization involve variations in the structure type, orientation, and offset. In this case, a 10 millimeters base size with an average wall thickness of 1 millimeter is used. The module is divided into eight identic tiles (100 mm x 300 mm x 5 mm) connected in parallel. Every tile is composed of a copper box plugged by a one-millimeter thick tungsten lid. The TPMS structure is contained inside the box, and the water circulates through it. A one-millimeter soft-copper layer is inserted between the box and the tungsten layer to equalize the differences in the thermal expansion between copper and tungsten. The module can sustain a 15-bar pressure drop at most, with a 1.25 kg/s mass flow rate. The simulations are made using the Computational Fluid Dynamic software STAR-CCM+. As in most of the engineered interest cases, the Reynolds Averaged Navier-Stokes (RANS) equations are used as the turbulence model, with the SST 𝑘 − 𝑤 (Shear Stress Transport) as the closure model. Every material has its own constraints to respect: the temperature of the water must be lower than the saturation temperature, at the respective pressure, to avoid boiling. The copper temperature must be kept under 500 °C while the tungsten below 1200 °C. A parametric study is made increasing the magnitude of the heat flux until one of the constraints is overcome. The results of the analysis, carried out in tight collaboration with the W7-X team at IPP in Greifswald, shows that selected TPMS can reach heat flux of 10 MW/m^2, becoming then an interesting option for the W7-X divertor tiles.