Microwave thawing examinations of a frozen material (tuna) in a microwave oven

Abstract

In this paper, temperature distribution of heated material (frozen tuna) in a microwave oven is calculated by the method which combines FDTD for electromagnetic field calculation with Heat Transport Equation (HTE) considering melting heat and the nonlinear and rapid change of complex relative permittivity and heat conductivity during defrosting. Meanwhile, temperature distribution is measured by heating experiment using a microwave oven of commercial model and thermo-graphy is compared with the calculated one. As a result, the calculated temperature distribution of heated material agrees well with the measured one, and the validity of the combined method considering melting heat and the change of complex relative permittivity and heat conductivity is confirmed. I. INTRODUCTION It is well known empirically that material in a microwave oven is heated unevenly. Conventionally, this problem has been examined by trial and error by microwave oven manufacturers. In such a background, temperature distribution of the heated material in a microwave oven was calculated by a coupled method of FDTD method for electromagnetic field analysis and Heat Transport Equation (HTE). (It is called FDTD-HTE method). And the calculated result and the measured result by using thermo-graphy were compared. As a result, both results agreed well and the validity of FDTD-HTE method was confirmed(1). When the heated material is frozen, it is considered that the uneven heating occurs more often in such a case as defrosting frozen meats. The reason is that the complex permittivity of the heated material extremely rises while shifting from the state of freezing to defreezing, and the electromagnetic field concentrates at the defreezing potion in the material. As a result, the very part which is defreezing in the heated material is heated more, while the frozen part stays intact. On this kind of problem, for the fundamental examination, electromagnetic field analysis was coupled with heat transport analysis considering melting heat and the change of complex relative permittivity and heat conductivity of the ice in the two dimensional waveguide. The calculated results on the boundary between ice and water were compared with the measured results and the validity of FDTD-HTE method was confirmed(2). However, to the authors' knowledge, FDTD-HTE method considering the changes of complex relative permittivity has not been applied to a three dimensional analytical model such as an actual microwave oven. The method has also not been examined by comparing the calculated temperature distribution with the measured one. In this paper, the temperature analysis of defrosting heated material in a microwave oven considering melting heat and the nonlinear and rapid change of complex relative permittivity and heat conductivity is examined using FDTD-HTE method. First, a commercial microwave oven is expressed to be used in FDTD method including a door, a turn table and complicated structures in the microwave oven. Next, elec- tromagnetic distribution in the microwave oven is calculated. Then, temperature distribution of the heated material is calcu- lated using a heat transport equation considering melting heat and the change of the complex relative permittivity and heat conductivity. Meanwhile, the temperature distribution of the heated mate- rial is measured using thermo-graphy, and the calculated and measured temperature distributions are compared. NOMENCLATURE