The effect of internal heating by microwave on the drying behavior of a slab was studied. A wet sample of kaolin pressed into a slab was subjected in microwave irradiation of 2.45 GHz. The absorption of microwave energy into a wet slab can be expressed by a function of the moisture content and the pathway length, which is a similar form to Lambert-Beer's law. The drying behavior was compared among three modes: microwave irradiation, hot air heating and radiation heating in an oven. Microwave heating with a constant power resulted in breaking the sample when the internal temperature achieves at 373 K. However, if the power was controlled to maintain the temperature less than the boiling point of water, the drying succeeded without any crack generation until the completion with a significantly faster drying rate than in convective heating or in the oven. It is also noted that the transient behavior of the temperature is quite different from the conventional drying.

The effectiveness of internal heating for enhancing the drying of molded ceramics is evaluated by both modeling and experiments. In the theoretical analysis, three dimensional drying-induced strain-stress are modeled, and the numerical solutions show that the internal heating generates lower internal stress than continuous convective heating or intermittent convective heating. Microwave drying is examined experimentally to study the effect of internal heating on the drying behavior of a wet sample of a kaolin slab. The drying behavior is compared among three modes: microwave heating, hot air heating and radiation heating. The transient behavior of temperatures in microwave drying is quite different from conventional drying by external heating. In particular, the temperature of the slab drops once in the progress of drying. This phenomenon cannot be predicted adequately by a simple model of one-dimensional heat conduction and moisture diffusion accompanied with an internal heat generation rate given as a linear function of the moisture content. It should be noted that the temperature behavior takes place due to the combined interactions with internal evaporation of moisture by rise in internal vapor pressure and shift of impedance or interference in the applicator. Microwave heating with a constant power above 100 W results in sample breakage due to the internal vapor pressure. However, if the power is dynamically controlled so as to maintain the temperature less than the boiling point of water, the drying succeeds without any crack generation until completion with a significantly faster drying rate than drying in convective heating or in the oven.

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A technology to apply a fluidized bed of electrically conductive beads is proposed to improve uniformity of the electric field intensity in microwave dryers, which are required for uniform heating of wet media. The principle of this effectiveness lies in a dynamic random scattering of microwave due to motion of the conductive beads in the bed. The electrically conductive beads were prepared by wrapping aluminum foil around styrene foam balls with cellophane tape. The diameter and density were 13 mm and 123 kg=m 3 , respectively. The minimum fluidizing velocity of these beads agreed with the one predicted by the Wen-Yu equation when the distributor was a porous plate. However, the fluidization of the beads took place at a lower gas rate than the minimum fluidizing velocity due to a jet flow from the small pores for the distributor of a perforate plate. The intensity in the applicator of a commercial microwave oven became the most uniform when the beads were fluidized with a uniform holdup profile along the height of the bed placed in front of the applicator walls. The dynamic effect making the intensity uniform by the fluidization was advanced by increasing the area of apparent reflection by the fluidized bed and the holdup of beads and was superior to a conventional stirring blade.

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