AbstractThe use of direct insulations covering the electrically conducting walls is desired when liquid metals serve as coolants in fusion reactor blankets. Perfect insulating coatings restrict the path of induced current to the fluid region and thus minimize the magnetohydrodynamic pressure drop. The subject of the present investigation is the effect of small, local defects in the insulation material on the flow pattern and pressure drop. The case of fully developed magnetohydrodynamic flow conditions in ducts with line cracks aligned with the flow direction is treated analytically, while the three-dimensional flow near small uninsulated spots is investigated numerically.