Multi-physics lumped modeling for a MW-class heat flux component equipped with micro-channels.

Nuclear fusion reactors, such as the experimental stellarator Wendelstein 7-X at the Max Planck Institute for Plasma Physics (IPP) – Greifswald represent a promising solution for decarbonizing the power sector in the second half of the century. In pursuit of this objective, the cooling system of the W7-X divertor, specifically developed to handle high heat fluxes, is presently undergoing design improvements. A new concept has been proposed for this cooling system, which involves a network of parallel arrays of sub-millimeter rectangular micro-channels (MCs) on each 0.1mx0.1m tile covering the divertor surface. The micro-channels are obtained using Additive Manufacturing techniques in a galvanized copper heat sink substrate, with tungsten as the plasma-facing material and water used for cooling. To reduce the high computational cost of thermal-hydraulic simulations for cooling analysis components, a lumped modeling approach has been built in previous studies. This involves replacing a group of hydraulic parallel micro-channels (MCs) with a properly calibrated thermal-hydraulic porous strip (PS) through the application of a 5 MW/m^2 heat flux and inlet mass flow rate of 50 l/min for the entire tile. This study was performed in accordance with the constraints of the GLADIS facility at IPP. The aim of this thesis is to verify the PS-model is also suitable for the thermo-mechanical stress evaluation; to do that, results obtained with arrays equipped with MCs and PS are compared in different conditions. Since the heat sink substrate and the plasma-facing tile are bonded, the thermo-mechanical problem in bonded structures is investigated under the free surface edge boundary condition, which is the most critical for structural integrity. In this scenario, the interfacial delamination and shear stresses are analyzed in the elastic regime. First, a simple case with the same layer structure of the tile is modeled, and the results are compared to the literature; then the results are compared to those obtained from arrays with MCs and PS with similar geometry. The evaluation is conducted first by applying a uniform temperature load, and then by using temperature maps extracted from the thermal-hydraulic simulations. The computational fluid dynamics and finite element analyses are performed using the STAR-CCM+ software by Siemens. The PS-model, which estimates higher temperatures, overestimates the delamination and shear stresses at the free edge, highlighting its conservative nature compared to the MCs-model. Moreover, the findings of this study indicate that micro-channels (MCs) and porous blocks, located near the bond interface, contribute to high-stress fluctuations that could lead to the delamination of the bond interface, suggesting that thickness needs to be increased. Results from both models reveal nearly identical interfacial stress predictions at the free surface edge. As a final contribution to the project, the feasibility of hydraulically connecting the tiles in a divertor unit target module, while taking operational constraints into consideration, is also evaluated.