THERMAL CONVECTION IN A DUCT WITH A STRONG AXIAL MAGNETIC FIELD

Thermal convection caused by non-uniform internal heating in a horizontal duct with an axial magnetic field is studied numerically. The magnetic field is strong enough to make the flow purely or nearly two-dimensional (uniform in the axial direction). The goal is to understand the effect of convection in toroidal ducts of a conceptual liquid metal blanket of a fusion reactor. A model of a separately cooled blanket with walls maintained at constant temperature and with no significant mean flow along the duct is considered. Turbulent convection appears at a high Grashof number when the magnetic field is purely axial. Adding a much weaker (5% in amplitude) vertical magnetic field suppresses the turbulence, elongates convection structures, and forms strong shear layers in the flow and near-wall jets. Stability analysis reveals that the transformation makes the flow more unstable to three-dimensional perturbations growing in the internal zones of strong shear. The unstable mode takes the form of axially oriented rolls varying slowly along the axis.