Numerical Analysis of Heat Transfer Characteristics in Microwave Heating of Magnetic Dielectrics
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MICROWAVE energy has been employed widely in materials processing for its volumetric heating effects, although the heating mechanism and the heat-transfer characteristics are still not well understood.[1–6] Magnetic dielectrics are known as good microwave absorbers because of their high dielectric and magnetic loss.[7,8] Numerous researchers have shown that the heating rate of such materials under microwave irradiation is much faster than that in conventional thermal heating (e.g., magnetite can be heated by microwave to 1273 K [1000 C] in 1 minute[9–11]). This observation provides a solid foundation that the large-scale extraordinary quick and efficient processing of magnetic dielectrics with microwave could be achieved provided proper heating parameters are applied. Although a large body of literature exists on the modeling of microwave heat transfer, most studies have concentrated on food processing.[12–19] Generally, these papers are restricted on the modeling of heat conduction and convection, and thermal radiation was ignored. Also,
ZHIWEI PENG, PhD Student, and JIANN-YANG HWANG, Professor, are with the Department of Materials Science and Engineering, Michigan Technological University, Houghton, MI 49931. Contact e-mail: [email protected] CHONG-LYUCK PARK, Postdoctoral Research Associate, and BYOUNG-GON KIM, Director, are with the Mineral Processing Division, Korea Institute of Geoscience and Mineral Resources, Daejeon 305-350, Korea. GERALD ONYEDIKA, PhD Student, is with the Department of Chemistry, Federal University of Technology Owerri, Owerri P.M.B. 1526, Nigeria. Manuscript submitted March 18, 2011. Article published online November 30, 2011 1070—VOLUME 43A, MARCH 2012
thermophysical, dielectric, and magnetic properties are assumed normally to be independent of temperature in the literature. For high-temperature microwave processing, however, such assumptions cannot be taken because of the strong radiation from heated materials to surroundings. An extensive literature review indicates that little attention has been paid to the modeling of microwave heat transfer on magnetic dielectrics, especially a detailed analysis of the parameters affecting such heating.[20–23] Magnetite is a typical magnetic dielectric. It is an important raw material in iron and steel industry. Considerable literatures concerning microwave heating of magnetite have been reported and the research demonstrated that a rapid ironmaking could be achieved by heating the mixture of magnetite with common reducing agents (e.g., coal) under microwave irradiation.[9,10,24–29] Nevertheless, these works mainly considered the reduction process under specific experimental conditions and generally omitted theoretical investigation of heat transfer during microwave heating. The objective of this study is to analyze the heattransfer characteristics in microwave heating of magnetite, the representative magnetic dielectric, by predicting the temperature distribution using an explicit finitedifference approach with consideration of radiation effect. Fur
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