Numerical and asymptotic analysis of liquid metal flows in rectangular sudden expansions

The MHD flow in geometries with varying cross-section is of special interest for applications in fusion technology since in the breeding zone of a reactor the electrically conducting fluid is distributed into the large breeder units through pipes, manifolds and gaps of smaller size. Flows in expansions can lead to the formation of stagnation zones and back flow regions, whose presence can result in the appearance of hot spots or in the accumulation of tritium. The 3D MHD phenomena that occur in these geometries strongly influence flow and pressure distribution. Three-dimensional MHD flows in sudden expansions of rectangular channels have been investigated both numerically by an extended version of the commercial code CFX where MHD equations have been implemented, and by an asymptotic- numeric approach that works at best for creeping flows and high Hartmann numbers. The effects of an externally applied magnetic field both on the flow and the current distribution in the duct have been investigated by means of the developed numerical tool up to a Hartmann number equal to 1000 and a wide range of the interaction parameter. The asymptotic analysis covers the range of Hartmann numbers beyond Ha = 1000. Asymptotic scaling laws have been found that allow drawing conclusions regarding the flow features for very strong applied magnetic fields. The results of the study show that by increasing the imposed magnetic field the recirculation that form behind the expansion reduce in size and the spiraling motion present for smaller magnetic fields is progressively damped out. For sufficiently high magnetic fields the vortices are suppressed but a reverse flow is still observed close to the corners of the duct, near the side walls. The additional pressure drop caused by the presence of the expansion has been evaluated and quantified. (author)