The Effect of Mesh Refinement on the Calculation of Cryostat Thermal Loads in DEMO during the Incident Helium Ingress

During and after the ingress of helium into the cryostat with active magnets, the heat is transferred from the cryostat to the cold magnet structures. Due to the high temperature difference between the helium and the cryostat, natural convection occurs that causes a significant local cool-down of the cryostat walls. A steady-state CFX simulation was performed using the updated simplified DEMO tokamak model to assess the thermal loads on passive thermal shields and cryostat walls. The predicted temperature distribution in cryostat walls as well as the estimated HTC depend strongly on the density of numerical mesh in the fluid domain, especially its near wall resolution at the fluid/solid interfaces. Only a sufficiently refined mesh in the fluid near-wall region leads to the mesh-independent prediction of temperatures in the cryostat walls. This however requires a huge computational effort.