MAGNETOHYDRODYNAMIC FLOW IN A RIGHT-ANGLE BEND IN A STRONG MAGNETIC-FIELD

The magnetohydrodynamic (MHD) flow through sharp 90 degrees bends of r ectangular cross-section, in which the flow turns from a direction alm ost perpendicular to the magnetic held to a direction almost aligned w ith the magnetic field, is investigated experimentally for high values of the Hartmann number M and of the interaction parameter N. The bend flow is characterized by strong three-dimensional effects causing a l arge pressure drop and large deformations in the velocity profile. Sin ce such bends are basic elements of fusion reactors, the scaling laws of magnetohydrodynamic bends flows with the main flow parameters such as M and N as well as the sensitivity to small magnetic field inclinat ions are of major importance. The obtained experimental results are co mpared to those of an asymptotic theory. In the case where one branch of the bend is perfectly aligned with the magnetic held good agreement between the results obtained by the asymptotic model and by the exper iments was found at high M approximate to 8 x 10(3) and N approximate to 10(5) for pressure as well as for electric potentials on the duct s urface. At lower values of N a significant influence of inertia has be en detected. The pressure drop due to inertial effects was found to sc ale with N--1/3. The same -1/3-power dependency on N has been found in the vicinity of the bend for the electric potentials at walls aligned with the magnetic held. At walls with a significant normal component of the field an influence neither of the Hartmann number nor of the in teraction parameter has been found. This suggests that the inertial pa rt of the pressure drop arises from inertial side layers, whereas the core how remains inertialess and inviscid. A variation of the Hartmann number is of negligible influence compared to inertia effects with re spect to pressure drop and surface potential distribution. The viscous part of the pressure drop scales with M(-1/2). Changes of the magneti c field orientation with respect to the bend lead in general to differ ent flow patterns in the duct, because the electric current paths are changed. The inertia-electromagnetic interaction determines the magnit ude of the inertial part of the pressure drop, which scales with N--1/ 3 for any magnetic field orientation. The dependence of the pressure d rop on M remains proportional to M(-1/2). With increasing M and N the measured data tend to those predicted by the asymptotic model. Local m easurements within the liquid metal exhibit discrepancies with the mod el predictions for which no adequate explanation has been found. But t hey show that below a critical interaction parameter flow regions exis t in which the how is time dependent. These regions are highly localiz ed, whereas the flow in the rest of the bend remains steady.