單模共振腔介電加熱對米象防治之研究

Abstract

本研究應用一單模共振腔之微波介電加熱方式實行於稻米蟲害防制。米象 (Sitophilus oryzae) 對於稻穀侵蝕嚴重，除了造成質與量的降低外甚至影響到食品安全。由於米象與稻米介電性質的差異，在使用電磁波加熱上有選別性加熱的可能。為探討在高頻交變電場下米粒內部的加熱情況，研究使用有限元素分析軟體COMSOL Multiphysics 3.5a之耦合電磁模組與熱傳導模組進行暫態分析求解加熱過程的溫度變化，並在矩形波導管耦合圓柱單模共振腔的幾何結構中尋找合適的進料孔徑尺寸以達到較高加熱效率。模擬中分別探討米粒內不同位置的蟲卵，以及進料管內不同排列方向的米粒所造成的升溫差異。本研究提出之熱致死動力學模型顯示，米象成蟲在51℃僅需3到4秒即可達100 % 致死率。模擬微波加熱結果顯示5秒溫度可達60℃，而實際批次實驗量測加熱5秒所得平均溫度為56.8℃，與理論值接近。另外模擬中發現米粒朝垂直方向加熱與水平方向溫度分佈有顯著差異，前者加熱效率遠高於後者，因此若利用連續流動進料方式所產生的旋轉將有利於溫度均勻度的改進。本研究發展出介電加熱應用於稻米蟲害防治的方法，並以實驗與模擬數據提供將來商用設備設計的基礎。

An application of electromagnetic energy to rice disinfestation was implemented in this study by using a microwave single-mode cavity resonator. Rice weevil (Sitophilus oryzae), the target insect to be controlled, was suggested to show higher characteristic in dielectric loss factor so that differential heating was possible to be achieved in its host medium. To evaluate the intra-kernel temperature raise in the time-varying electric field, the 3D simulations of microwave model were constructed and computed by using commercial finite element method software COMSOL Multiphysics 3.5a with electromagnetic wave module and conduction heat transfer module to simulate the dielectric heating process. Using a single mode resonator to excite high electric field intensity in the center line of the resonator, high efficiency heating without attenuation is achievable. Results of the analysis showed that rice grain infested with a single weevil reached the literature lethal temperature of 60℃ for 5 seconds, having slight selective heating effect in the simulation models. While 56.8℃ was measured for the average experimental value during the same heating time, our thermal-mortality kinetics model found only 3 ~ 4 seconds at 51℃ was necessary for the adult rice weevil to reach 100 % mortality. Other results suggest that rice grain orientation parallel to the resonator axis has better heating efficiency than perpendicular condition; therefore, the heating uniformity might be improved by rotation under continuous feed-in situation. The finding from this study provides information in dielectric heating application to rice weevil control in rice which might help to deign a more energy-saving and time-saving electromagnetic energy treatment.