Effects of temperature dependent properties in electromagnetic heating

摘要

Electromagnetic heating such as microwave processing and radio frequency heating becomes very popular because of its non-contact, pollution free and fast distribution of thermal energy within the object of interest. In electromagnetic heating, the temperature distribution within a sample greatly depends on the dielectric properties which are functions of electromagnetic frequency, temperature and the composition of the object. There are many experimental and numerical investigations on electromagnetic heating because of its widespread use in food and other industries, but only few researchers have looked at this problem from analytic point of view specifically for the temperature dependent properties. In this paper, we developed an analytic expression for temperature distribution in a three dimensional rectangular object under electromagnetic heating and presented a method to incorporate temperature dependent properties of the object in determining the temperature distribution at different times. A simplified Maxwell's equation is solved for plane wave to obtain electric field distribution in the body, and the electromagnetic power absorption is computed from the electric field distribution which was then used as a source term in the energy equation. Next an unsteady, three dimensional, non-homogenous energy equation is solved by integral transform technique to obtain temperature distribution. Finally, this closed form analytical solution is used to study the effects of electromagnetic frequency, dielectric properties, and heat transfer coefficient on temperature distribution in a rectangular salmon fillet. It is found that incident frequency, sample thickness and processing time have significant influence on the heating pattern. For radio frequency heating, the temperature dependent dielectric properties influence the temperature distribution significantly, but the effect of temperature dependent dielectric properties is less dominant for the microwave frequency used in the household microwave oven. Our results also show that microwave heating provides heterogeneous temperature distribution with alternate hot and cold spots. On the other hand, the radio frequency heating allows almost uniform temperature distribution within the body, which makes it a better choice for quick and convenient heating process especially for commercial and industrial heating.