密聚米粒在微波加熱中極化電荷效應之物理探討

Abstract

本論文探討微波加熱中橢球型介電質系統（以米粒為代表）產生加熱不均的物理機制。透過電磁與熱傳模擬，本研究指出兩個主要因素：與排列方向有關的極化電荷遮蔽效應（polarization-charge shielding），以及米粒間的近場增強效應（near-field enhancement）。 在單一米粒中，其內部電場強度與加熱速率高度依賴外部電場與米粒主軸方向的對齊程度。當米粒緊密排列時，彼此誘發的極化電荷會相互作用，進一步在接觸區域產生局部電場增強或抑制，導致加熱呈現額外的方向依賴性，尤其在橢圓形米粒中尤為明顯。 在電磁模擬中，線極化波會產生顯著的場強不均，圓極化波則可同時抑制排列依賴性與粒子間的近場交互作用，提升加熱速率的均勻性。雖然熱傳導與對流有助於緩和溫度差異，使溫度場的非均勻性不如場強明顯，但圓極化波在各種排列與場條件下，仍能一致地表現出最佳的加熱均勻性。 本研究結果顯示，透過調整外部微波場的極化型態，可有效改善介電米粒系統中的加熱均勻性。其中，圓極化波在各種排列結構與對齊條件下皆展現穩定的效果，具有應用於食品加工、材料工程等產業之潛力。

This thesis investigates the mechanisms responsible for microwave heating non-uniformity in dielectric grain systems, using rice grains as a representative model. Through electromagnetic and thermal simulations, two key contributors are identified: alignment-dependent polarization-charge shielding and inter-grain near-field enhancement. In isolated grains, the internal electric field strength, i.e. the heating rate, depends strongly on the alignment between the grain's major axis and the external field direction. When grains are packed closely, their induced polarization charges interact, causing local field intensification or suppression. This near-field enhancement leads to additional directional dependence in the heating pattern, especially in ellipsoidal grains. Electromagnetic simulations show that linearly polarized waves produce significant non-uniformity in the electric field strength. In contrast, circularly polarized waves effectively suppress both alignment-dependent effects and near-field interactions between grains, leading to improved uniformity in heating rates. Although thermal conduction and convection help reduce temperature differences, circular polarization still provides the most uniform heating performance among all tested conditions. These findings suggest that microwave heating uniformity can be substantially improved by optimizing the polarization of the external field. Circular polarization, in particular, offers a robust solution applicable across various packing geometries and alignment conditions, with implications for food processing, materials engineering, and other industrial applications.