Numerical and experimental modelling of microwave applicators

Abstract

This thesis presents a time domain finite element method for the solution of microwave heating problems. This is the first time that this particular technique has been applied to microwave heating. It is found that the standard frequency domain finite element method is unsuitable for analysing multimode applicators containing food-like materials due to a severe ill-conditioning of the matrix equations. The field distribution in multimode applicators loaded with low loss materials is found to be very sensitive to small frequency changes. Several solutions at different frequencies are therefore required to characterise the behaviour of the loaded applicator. The time domain finite element method is capable of producing multiple solutions at different frequencies when used with Gaussian pulse excitation; it is therefore ideally suited to the analysis of multimode applicators. A brief survey of the methods available for the solution of the linear equations is provided. The performance of these techniques with both the frequency domain and time domain finite element methods is then studied. Single mode applicators are also analysed and it is found that the frequency domain method is superior in these cases. Comparisons are given between the calculated results and experimental data for both single mode and multimode systems. The importance of experimental verification being stressed. The choice of element type is an important consideration for the finite element method. Three basic types of element are considered; nodal, Whitney edge elements and linear edge elements. Comparisons of the errors with these elements show that Whitney elements produce a consistently lower error when post-processing is used to smooth the solution. The coupled thermal-electromagnetic problem is investigated with many difficulties being identified for the application to multimode cavity problems.