Test and analysis of ceramic sidewall elements for MHD generators

Busek company has developed two novel sidewall elements for MHD generators which are comprised of ceramic material brazed to copper. This work was funded by a DOE Phase n SBIR. These elements were used to construct a generator module for testing subject to coal fired combustion gas seeded with K₂CO₃. The module was shipped to the CFFF at UTSI for testing. Testing of the module was carried out in conjunction with the DOE sponsored POC test LMF5-J which took place on September 23 - 25, 1993. The LMF5-J test had a cumulative coal fired operation run time of 52 hours. The longest continuous coal fired run time was 47 hours. An external electrical network was used to impose electrical stress on the module during the test. The module performed as designed during the test, even though some unfortunate events occurred during the test which subjected the module to extreme testing conditions. Inspection following the test showed that the module was in excellent condition. Some cracking and material loss was noted from some of the ceramic sidewall elements. This damage is believed to have been caused by overheating of the elements due to cooling water starvation. Analysis of the test data showed that there was a correlation between large heat flux increases and periods in which there was no seed flow. This is believed to have been caused by seed alterations inducing changes in the slag mass and composition. Analysis of the electrical data showed that the near wall voltage differential on the bottom wall was far greater than on the top wall. This difference is believed to have been caused by the current transport through the boundary layers operating in different modes. It is believed that the mode of current transport through the anode boundary layer was diffusion, while the mode of transport through the cathode boundary layer was arcing. Additionally, analysis showed that the calculated conductivity curves agreed with theoretical curves.