Three-dimensional MHD flow in sudden expansions

Magnetohydrodynamic (MHD) phenomena caused by the interaction of electrically conducting fluids with a magnetic field are exploited in many metallurgical processes to control and manipulate metals. The knowledge of MHD effects is also a key issue for the development of fusion reactors where a plasma is confined by a strong magnetic field and liquid metals are used to produce tritium that fuels the reactor. A numerical tool, based on an extension of the commercial code CFX, has been developed to study MHD flows in arbitrary geometries and for any orientation of the imposed magnetic field. As an example of application a detailed analysis of the MHD flow in sudden expansions has been performed, focusing on the effects of the magnetic field and inertia forces on flow and current distribution and on pressure drops caused by induced three dimensional electric currents. The results show that by increasing the applied magnetic field the recirculations that form behind the expansion reduce in size and the spiralling motion is progressively damped out. For sufficiently high magnetic fields the vortices are suppressed but a reverse flow is still observed close to the comers of the duct, near the side walls. Complex current paths have been found and special emphasis has been placed on the analysis of the evolution of 3D current loops that form in the core region of the duct. A parametric study has been performed for a constant applied magnetic field varying the relative strength of inertia effects compared to that of electromagnetic forces. When Lorentz forces are much larger than inertia forces, no vortices occur. By increasing inertial effects, vortical structures form behind the cross-section enlargement. The results show that both the pressure drop and the size of the recirculations are strongly affected by inertia forces. The numerical results have been compared with experimental data for surface potential and pressure distribution. A very good agreement has been found, confirming the reliability of this computational approach.