Role of metallic and composite (ceramic-metallic) supports on microwave heating of porous dielectrics

The heating of porous dielectrics, beef-air (b/a) and beef-oil (b/o), attached with ceramic, metallic or ceramic-metallic composite supports, has been studied analytically using microwaves. Three test porosities were considered, 0.3, 0.45 and 0.6. A preliminary understanding of enhanced heating and power absorption within the material was obtained from the average power vs slab depth plots. The maxima in power, also termed as 'resonances', are observed for specific sample thicknesses and the two consecutive resonances are termed as R-1 and R-2 modes. It is observed that, for both b/a and b/o samples, higher intensity of resonances for power absorption occur in the presence of metallic and alumina-metallic composite supports. The detailed spatial distribution of power illustrates that the sample attached with the metallic support attains lower power distribution for both b/a and b/o samples. The low power absorption near the unexposed face has been enhanced by ceramic-metallic composite support. The overall heating efficiency is also enhanced with ceramic-metallic composite supports. The optimal heating strategy has been derived based on large heating rate with small thermal runaway. Metallic support may be recommended as optimal heating strategy for higher porosities (phi >= 0.45) whereas alumina-metallic composite support may be suitable for smaller porosities (phi <= 0.45) for b/a samples. Metallic or alumina-metallic composite supports give optimal heating effects for b/o samples. For both b/a and b/o samples, the thermal runaway is larger at higher porosities. (C) 2006 Elsevier Ltd. All rights reserved.