Electrical performance of a duct with segmented electrodes under various conditions

An MHD duct with 56 electrode pairs has been tested on a 10-MW thermal test facility of Electricité de France. The cross section of the generator is constant and rectangular, the distance between electrodes facing each other is 13cm, and the length of the electrodes 7 cm. The insulating walk are of the peg wall type, and each electrode is made of nine small zirconia praseodymium oxide bricks brazed in a copper holder. Between two zirconia pieces, a thin plate of platinum +40 percent rhodium is able to collect the current from the hot and conducting parts of the ceramic. All of the 112 electrodes are independent, and many load systems are possible. Measurements were made of 1) the transverse open-circuit voltages, 2) Hall generator operation, and 3) the series-connected generator with connection angle corresponding to Ey/Ex= -0.96 (open- and short-circul operations for the last two). The results obtained show that a one-dimensional analysis has sufficient accuracy to predict the performance of this generator, but it is necessary to take into account the voltage drops at the electrodes which are important despite the fact that the electredes are hot (surface temperature ∼ 2000°C). The main limitation of the duct was due to the inability of the insulators to withstand for a long time the voltages developed. Other interesting data obtained during the experiments are the currents collected at the end of the Hall and series-connected generators. It was possible to determine the percentage of the load current collected by the upper electrodes and the percentage collected by the lower ones.