Numerical simulation of MHD flows in a coupled U-turn rectangular duct with different wall conductance ratios

Abstract

Liquid metal flow in a conducting coupled U-turn rectangular duct subjected to an external uniform magnetic field is numerically analyzed using our in-house MHD solver developed in OpenFOAM software. The coupled U-turn duct is the structure used in self-cooled liquid-metal blankets, which includes the duct wall, partition wall, inflow channel, outflow channel, and connection channel. The characteristics of the fluid velocity, induced electric current, electric potential, and pressure distribution depending on the wall conductance ratio of the duct wall and the partition wall are investigated. As the partition wall divides the inflow and outflow, the three-dimensional induced electric currents close loops at the end of the partition wall in the connection channel. When the duct wall is insulating and the partition wall is weak conducting, the three-dimensional electric current is significant in the connection channel and the start of the outflow channel. The three-dimensional electric current results in the pressure fluctuation. The flow forms a visible primary vortex at the end of the partition wall because of a strong adverse pressure gradient. The pressure along the flow direction has a significant fluctuation near the connection channel. When the duct wall is weak conducting, high-velocity jets form in the side layers near the partition wall and the duct wall. The velocity distribution is a strong asymmetrical M-type profile in the U-turn rectangular duct with conducting duct walls. The normalized electric potential distribution in the different walls along the magnetic field direction is independent of the Reynolds number when the Reynolds number and the Hartmann number is moderate. When the Hartmann number and Reynolds number is high, the normalized electric potential varies with the Reynolds number. The conductance ratio of the duct wall has a decisive effect on the pressure gradient. The pressure drop with conducting duct wall is two orders of magnitude higher than that of cases with an insulating duct wall. As the duct wall is insulating, the wall conductance ratio of the partition wall does not affect the pressure gradient remarkably. While the duct wall is conducting, the increase of the wall conductance ratio of the partition wall results in an increase of the pressure drop significantly.