LIQUID-METAL FLOW IN A SYSTEM OF ELECTRICALLY COUPLED U-BENDS IN A STRONG UNIFORM MAGNETIC-FIELD

Liquid-metal magnetohydrodynamic how in a system of electrically coupl ed U-bends in a strong uniform magnetic field is studied. The ducts co mposing the bends are electrically conducting and have rectangular cro ss-sections. It has been anticipated that very strong global electric currents are induced in the system, which modify the flow pattern and produce a very high pressure drop compared to the flow in a single U-b end. A detailed asymptotic analysis of flow for high values of the Har mann number (in fusion blanket applications of the order of 10(3)-10(4 )) shows that circulation of global currents results in several types of peculiar flow patterns. In ducts parallel to the magnetic field a c ombination of helical and recirculatory flow types may be present and vary from one bend to another. The magnitude of the recirculatory moti on may become very high depending on the flow-rate distribution betwee n the bends in the system. The recirculatory flow may account for abou t 50 % of the flow in all bends. In addition there are equal and oppos ite jets at the walls parallel to the magnetic field, which are common to any two bends. The pressure drop due to three-dimensional effects linearly increases with the number of bends in a system and may signif icantly affect the total pressure drop. To suppress this and some othe r unwelcome tendencies either the ducts perpendicular to the magnetic field should be electrically separated, or the flow direction in the n eighbouring ducts should be made opposite, so that leakage currents ca ncel each other.