Theoretical analysis on the role of annular metallic shapes for microwave processing of food dielectric cylinders with various irradiations

A theoretical analysis has been carried out to analyze efficient microwave assisted heating processes of food cylinders with various shapes of metallic annuli in presence of lateral and radial irradiation. Lateral irradiation represents the sample incident at one direction with source at infinity whereas the radial irradiation represents the situation where the sample is incident with microwave radiations from co-axial cylindrical cavity at infinity. A generic finite element method has been used to obtain computational mesh for all shapes of metallic annulus and Galerkin finite element method has been used to solve electric field and energy balance equations. Influence of metallic annulus has been illustrated for three representative sample lengths or regimes (I, II and III) as a function of wave number (Nw). Sample radius with regime I is smallest and largest sample radius corresponds to regime III. In general, power absorption due to lateral irradiation is larger for regime I whereas identical power with lateral or radial irradiation or larger power absorption with radial irradiation corresponds to regime II in absence of metallic annulus. The metallic annuli are found to enhance power absorption or heating rate for specific aspect ratios of both bread and beef samples. The aspect ratios are defined as the ratio of the dimension of metallic annulus and outer radius of the food sample. Lateral irradiation is found to give larger heating rates especially for regimes II and III for bread samples (low dielectric loss), but radial irradiation gives smaller degree of thermal runaway. On the other hand, radial irradiation gives significantly large heating rates with minimal thermal runaway for beef samples (high dielectric loss) with all regimes. It is interesting to observe that the metallic annuli enhance two to three times microwave power for beef samples involving regimes II and III. Various shapes of metallic annuli are also found to play dominant role either to focus or to optimize heating effects.