Generalisation and Evaluation of Macroscopic Models for Microwave Susceptors in Contact With Heated Foods

Abstract

Introducing a thin conductive layer into a finite-mesh (as inherent in e.g. finite difference time domain (FDTD) and finite element (FEM) methods) typically requires a dedicated equivalent macroscopic model allowing for computationally effective and accurate electromagnetic (EM) and thermal simulations. Thin conductive layers, such as microwave susceptors, characterised by their surface resistance (Rs), are adequately represented with a dielectric surrogate layer of higher thickness and proportionally scaled conductivity, maintaining the value of Rs. Systematic evaluation of macroscopic models of microwave susceptors used for enhancing the heating efficiency of microwavable food packages has been reported in [1]. Our studies therein focus on validity, accuracy and practical application limits of the proposed macroscopic models of thin metallic layers, in terms of power dissipated in susceptor placed in free space and irradiated by EM wave, at all angles of incidence. In this work we extend our studies to real-life simulation scenarios, in which microwave susceptor is in contact with food. We first consider a four-layer model as in Fig. 1(left) and conduct both analytical and numerical conformal FDTD calculations. The accuracy and application limit of the macroscopic model are investigated for all incidence angles and both, TE and TM polarisations of the impinging EM wave, for different foods. We aim to determine a range of optimum, in terms of power dissipated in the susceptor, values of the susceptor’s surface resistance in all those cases. The results of our canonical calculations with the four-layer model of Fig.1(left) are validated in the 3D FDTD modelling scenario of Fig.1(right), representative of a real-life domestic oven. While for normal incidence our results are in overall agreement with some of the previously published observations [2], they are formalised and generalised to constitute reliable guidelines for microwave oven and food packaging designers and manufacturers. We also show cases where some of the earlier rule-of-the-thumb guidelines fail.