Heat Transfer and Velocity Distribution in Liquid Metal Magneto-convective Flows Around Cooling Pipes

In the water-cooled lead lithium (WCLL) breeding blanket, the released thermal power is extracted by water-cooled pipes around which the PbLi circulates. The magnetohydrodynamic (MHD) flow that establishes in the blanket is determined mainly by the action of driving buoyancy forces, due to density gradients, and braking effects caused by viscosity and electromagnetic forces. The latter are induced by the interaction of the electrically conducting PbLi with the plasma-confining magnetic field. The characteristics of the magneto-convective flow depend on thermal conditions, such as differential or volumetric heating, geometry, electrical properties of the structural material, and reciprocal orientation of gravity and magnetic field. In the present study, we investigate numerically MHD-convective flows in closed geometries with differential heating and internal cylindrical obstacles that represent the cooling pipes, as present in the breeding zone of the WCLL blanket. The effects of applied temperature gradient, strength, and orientation of imposed magnetic field are studied via 3-D numerical simulations. The aim of the analysis is getting an overview of flow features and heat transfer properties depending on operating conditions and geometrical setups. It is found that the most intense convective flow occurs when the magnetic field is aligned with the cooling pipes since this configuration allows for quasi-2-D flow patterns with only moderate magnetic damping. The strongest reduction of convection is observed when the magnetic field is perpendicular to the pipes. In the latter case, the core velocity is significantly reduced, while the convective heat transport is confined to thin layers aligned with the magnetic field parallel to external walls or tangent to the pipes.