EXPERIMENTAL AND THEORETICAL MHD PERFORMANCE OF A ROUND PIPE WITH AN NAK-COMPATIBLE AL2O3 COATING

A key feasibility issue for the international thermonuclear experiment al reactor (ITER) vanadium/lithium breeding blanket is the question of insulator coatings. Design calculations show that an electrically ins ulating layer is necessary to maintain an acceptably low magnetohydrod ynamic (MHD) pressure drop. To begin experimental investigations of th e MHD performance of candidate insulator materials and the technology for putting them in place, a new test section was prepared. Aluminum o xide was chosen as the first candidate insulating material because it may be used in combination with NaK in the ITER vacuum vessel and/or t he divertor and MHD performance tests could begin early in Argonne's l iquid metal experiment (ALEX) because NaK was already the working flui d in use. Details on the methods used to produce the aluminum oxide la yer, as well as the microstructures of the coating and the aluminide s ublayer, are presented and discussed. The overall MHD pressure drop, l ocal MHD pressure gradient, local transverse MHD pressure difference a nd surface voltage distributions in both the circumferential and axial directions are reported and discussed. The overall MHD pressure drop, measured at 30 and 85 degrees C, was higher than the perfectly insula ting case, but many times lower than the bare-wall case. It was demons trated that the increase in MHD pressure drop above the theoretical va lues is largely due to the presence of instrumentation penetrations in the test section walls, which provide current paths from the fluid to the walls of the pipe, resulting in local areas of near-bare-wall MHD pressure drop.