MHD flows through ferromagnetic rectangular ducts in liquid metal blankets

In most designs of liquid metal blankets, the reduced activation ferritic martensitic steel with high relative magnetic permeability is proposed as the structural wall material, which will have an obvious influence on the magnetic field distribution inside the duct and consequently modify the liquid metal magnetohydrodynamics (MHD) flow state. However, the MHD flow state considering the influence of the ferromagnetic wall is lack of systematic investigations especially under the relevant conditions of magnetic confinement fusion reactors. In this work, systematic investigations on the ferromagnetic MHD effect are conducted by experiments and numerical simulations considering the relevant condition of fusion reactors such as high magnetic fields up to 10 T and the actual magnetic permeability of ferromagnetic walls. It is found that magnetic field lines are mainly gathered through the side wall for ferromagnetic rectangular ducts, which will result in the overall magnetic shielding effect. As applied magnetic fields increase, the magnetic shielding effect weakens, increasing the aspect ratio and wall thickness of the duct is benefit to enhance the overall magnetic shielding effect. A slightly magnetic strengthening effect is firstly observed in our experimental and numerical investigations, which is characterized that the average magnetic flux intensity in the fluid region is slightly greater than applied magnetic fields and the pressure drop in ferromagnetic ducts is also higher than that in non-ferromagnetic ducts when the applied magnetic field is bigger than the threshold of transition. The dimensionless pressure gradient in ferromagnetic rectangular ducts usually increases firstly and then decreases with the increase of applied magnetic fields, the pressure drop estimated from the coefficient of the square of the average magnetic flux intensity in the fluid region is generally accurate with exceptions in some extreme conditions. These findings will provide a theoretical guidance for future liquid metal blanket designs.