MHD mixed convection flow in the WCLL: heat transfer analysis and cooling system

In the Water-Cooled Lithium Lead (WCLL) blanket, a critical problem faced by the design is to ensure that the breeding zone (BZ) is properly cooled by the refrigeration system to keep the structural materials under the maximum allowed temperature by the design criteria. CFD simulations are performed using ANSYS CFX to assess the cooling system performances accounting for the magnetic field effect in the sub-channel closest to the first wall (FW). Here, intense buoyancy forces (Gr = 10^10) interact with the pressure-driven flow (Re = 10^3) in a MHD mixed convection regime. A constant magnetic field, parallel to the toroidal direction, is assumed with intensity B = 4.4 T. The walls bounding the channel and the water pipes are modeled as perfectly conducting. The magnetic field is found to dampen the velocity fluctuations triggered by the buoyancy forces and the flow is similar to a forced convection regime. The PbLi heat transfer coefficient is reduced to one-third of its ordinary hydrodynamic value and, consequently, hot-spots between the nested pipes and at the FW are observed, where TMax = 1000K. Optimization strategies for the BZ cooling system layout are proposed and implemented in the CFD model, thus fullling the design criterion.