Influence of ceramic supports on efficient microwave heating of porous dielectrics

A detailed theoretical analysis has been carried out to study efficient heating of ID porous dielectric samples due to microwaves. The heating effects are analyzed for two types of porous material: beef-air (b/a) and beef-oil (b/o) with and without ceramic supports (Al2O3 and SiC). Three test cases for porosity (Φ) 0.3, 0.45 and 0.6, are considered. The effective dielectric properties are obtained using Fricke's complex conductivity model with air and oil being the discontinuous media. The maxima in average power corresponding to resonances occur at various sample thicknesses for all porous materials with and without supports and two dominant resonance modes R 1 and at R2 are considered where the average power at R1 is larger than that at R2. It is observed that the maxima at R1 mode decreases significantly with porosity for samples with and without supports. It is interesting to observe that average power absorption is enhanced for samples (b/a and b/o) in presence of Al 2O3 support whereas the average power is smaller with SiC support. The detailed analysis on spatial distribution of electric field, power and temperature illustrate that runaway heating is observed at the face which is not attached with support for b/a samples, and the intensity of thermal runaway is increased with porosity whereas the less thermal runaway is observed for b/o samples with all porosity regimes. The thermal runaway is less pronounced for SiC supports. The thermal runaway is also represented by ΔTb (temperature difference) vs time plot which illustrates that greater magnitudes of ΔTb occur with Φ = 0.6 for b/a samples. In contrast, ΔTb vs time distribution is almost identical with porosity for b/o samples. Based on the analysis, it may be recommended that b/o samples with all porosity regimes would exhibit uniform heating rates with Al 2O3 and SiC supports, whereas b/a samples may be heated with minimal thermal runaway heating corresponding to Al2O 3 support at the left face. Copyright © 2005 by ASME.