Modeling Infrared and Combination Infrared-Microwave Heating of Foods in an Oven

Abstract

A quantitative, model-based understanding of heat exchange in infrared and combined infrared-microwave heating of food inside an oven is developed. The research is divided into three parts: measurement of optical properties, radiative heat transfer analysis and combined microwave-radiative heat transfer analysis. Optical properties of reflectance, absorptance and transmittance in a potato tissue are measured as a function of wavelength, using a spectroradiometer. Penetration of energy is higher for halogen lamps that emit in the near- and mid-infrared range, compared to ceramic rods that emit mostly in the far infrared range. Reflectance in the near infrared range increases with moisture content of the food, thus decreasing the energy coupled. Surface structure has significant influence on the optical properties. A 3-D radiative heat exchange model of an oven-food system is developed using a commercial finite-element package. The air in the oven is assumed transparent to the radiation. Heat conduction is assumed in the entire oven (food and air) for the short duration. The wavelength dependence of emissivity (non-gray surface) is found to significantly affect the surface radiative flux and the use of a non-gray model is recommended for such materials, although simplification of the emissivity variation is required to keep the computation time reasonable. Lowering food surface emissivity reduces the radiative flux that is absorbed by the food surface. Reducing oven wall emissivities increase the radiative flux on the food surface. The location of the radiative heat source in the oven as well as placement of the food relative to the heat source were found to have significant influence on the radiative heat flux over the food surface. To add microwave heating, Maxwell's equations of electromagnetics were solved for the same cavity using separate finite element software and the volumetric heat generation, in the food, obtained from this model was input to the radiative heat transfer model, thus coupling them. Using measures such as mean temperature rise and the standard deviation of temperatures, it was demonstrated that combination heating leads to more uniform heating, without compromising the speed of heating.