Modeling experiments on continuous microwave processing of thermal runaway materials

Conference · Thu Dec 31 04:00:00 EST 1998

OSTI ID:343604

Thomas, Jr, J R; Goodson, C C; Jackson, M; Stern, C H ^[1]; Vogt, G J ^[2]

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1. Virginia Polytechnic Inst. and State Univ., Blacksburg, VA (United States)
2. Los Alamos National Lab., NM (United States)

Microwave processing of metal oxide ceramic fibers offers several potential advantages. However, work so far has revealed several difficult problems; typically, these materials have very low dielectric loss coefficients at room temperature which increase rapidly with temperature. Although microwave heating of such low-loss materials can be accomplished by using high field strengths in high-Q applicators, this approach usually fails because of thermal runaway. The authors seek to demonstrate that custom designed microwave applicators can overcome the problem of thermal runaway and produce heating patterns which lead to temperature profiles tailored to specific processing objectives. The first step is to develop a verified model of the electromagnetic and temperature fields in an applicator and load. The authors report results from experiments on well-characterized ceramic rods and a heat transfer model of the experiments which assumes uniform electromagnetic fields. A 2-D thermal model of the experiments was developed using the implicit finite-difference technique with temperature-dependent properties. This model gives results in good qualitative agreement with the measured temperatures. These results lead to three useful conclusions: (1) thermal runaway can be avoided by controlling the power absorbed by the sample; (2) conduction along the length of a microwave-heated rod, along with convection and radiation from the surface, can lead to fairly localized hot spots in a small cavity, even with uniform fields; and (3) the difference between forward and reflected power to the cavity is an overestimate of the power absorbed by the load, suggesting that there were significant losses in the cavity walls.

• OSTI ID: 343604
• Report Number(s): CONF-980601--
• Country of Publication: United States
• Language: English

Related Subjects

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATION
36 MATERIALS SCIENCE
CERAMICS
ELECTROMAGNETIC FIELDS
FINITE DIFFERENCE METHOD
HEAT TRANSFER
MATHEMATICAL MODELS
MICROWAVE HEATING
TEMPERATURE CONTROL
TEMPERATURE DEPENDENCE