MICROWAVE-HEATING - AN EVALUATION OF POWER FORMULATIONS

Microwave heating of food systems have conventionally been modeled based on a Lambert law formulation of the absorbed power. However this is strictly valid for semi-infinite samples only. The correct power dissipation must be computed from Maxwell's equations. To determine the conditions of the approximate applicability of Lambert's law for finite slabs, we have compared it with the microwave heating predicted by Maxwell's equations. We have found that the critical slab thickness L(crit) (in cm) above which the Lambert law limit is valid can be estimated from L(crit) = 2.7-beta-1-0.08, where the penetration depth, beta-1, is the distance in cm from the sample surface where the field reduces to 1/e of its incident intensity. Temperature profiles calculated with the Lambert law limit for slabs thicker than L(crit) are within 0.5% of those predicted with the power calculated from Maxwell's equations. Using Maxwell's equations we have developed a general formulation for power absorbed in a homogeneous, isotropic multilayered medium exposed to plane waves from both faces. We report temperature profiles obtained by solving the transient heat conduction equation with the microwave power as a source term. Thermal and dielectric properties are assumed to be temperature-independent.