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

通常，高频电场在介电材料中引起极化。极化粒子的运动滞后于电场的反转并导致功率耗散。对于具有偶极结构的水，极化的贡献非常大，并且通过分子的旋转和振动迅速产生热量。微波加热是一种高效、快速、直接的加热方式。本文利用矩形波导管对微波加热的干燥和熔化进行了理论和实验研究。在干燥中，加热速率强烈地依赖于材料中的含水量，并且发现干燥速率随着时间而变化，因为与含水量有关的电磁场的变化。计算结果与实验结果吻合较好。同时，对两层物料的微波干燥与常规干燥进行了比较，明确了微波干燥的特点。由于填充床中水和冰的介电性质的差异，微波作用下的融化方向取决于填充床的结构。基于电磁场和温度场相结合的模型，预测结果与实验结果相一致的冷冻填充床的融化。

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.