Characteristics of Microwave Heating Associated Phase Change Using A Rectangular Wave Guide

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Generally, a high-frequency electric field causes polarization in a dielectric material. The motion of polarized particles lags behind the reversals of the electric field and leads to a power dissipation. For water having dipole structure, the contribution of polarization is extremely large and heat is rapidly generated by the rotation and vibration of molecules. This is why microwave heating is highly efficient as well as rapid and direct.The drying and melting due to microwave heating have been investigated theoretically and experimentally, using a rectangular wave guide. In the drying, the heating rate strongly depends on water content in materials and it was found that the drying rate changed with time because of change of electromagnetic fields relating to the moisture content. The calculated results were in agreement with the experimental results. Further, the drying composed of two layers materials due to microwave heating was compared with that due to conventional heating and the characteristics of microwave drying are clarified.The melting was performed for two different layered packed which composed of frozen and unfrozen layers. The direction of melting against incident microwave depended on the structure of layered packed beds because of the difference in the dielectric properties between water and ice in packed beds. Based on a model combined the electromagnetic and temperature fields, the predicted results were in agreement with the experimental results for the melting of frozen packed beds.