Analysis of Industrial Microwave Ovens

Doktorsavhandling, 1998

This thesis deals with the analysis and design of microwave ovens used in the food industry. Such analysis is desirable in order to optimize the performance of the ovens and to reduce the time and expenses of developing new or improved ovens. The initial electromagnetic analysis is performed using the Finite Difference Time Domain method (FDTD) and it is shown that even relatively large installations, such as multi-applicator tunnel ovens, can be readily analyzed on inexpensive PCs. To enable thermal simulations, used to find the temperature distribution within the food products, a simple but efficient magnetron model is presented. Using this model the magnetron output power can be modelled as a function of e.g. the position of the food load within the oven or of permittivity changes in the food. The problem of process optimization is addressed and it is shown that due to the long computation times when using FDTD, more efficient analysis tool are required. For this purpose a moment method (MM) code, based on an integral equation formulation for multilayer structures, is presented for a specific type of oven. This code is able to calculate the field distribution in a multilayer food load in less than a minute, which makes it possible to run hundreds of simulations in a reasonable time in search for the optimal oven design. To be able to classify the quality, or performance, of a specific oven the concept of cost functions is introduced. The cost functions assign a figure of merit to the field distribution within the oven, and serve as a measure of how close the field generated by the oven comes to the desired field distribution. We use two different cost functions, one that will produce a maximally uniform field, by minimizing the difference between the hottest and coldest part of the food, and one that will suppress the horizontal component of the electric field in order to minimize overheating of the edges of the food. Throughout this thesis, a reoccurring topic is that of edge overheating, i.e. the fact that sharp edges of high-permittivity food products often tend to be excessively heated when using microwaves. The edge overheating effect is analyzed in detail using the Multiple Multipole Program (MMP). We show that the main factors governing edge overheating are the polarization of the incident field, the curvature and angle of the edges, and the presence of other scatterers in the vicinity of the edge.