紅外線加熱乾燥多孔性圓管之熱質傳分析

Abstract

為了瞭解紅外線輻射加熱乾燥多孔性圓管過程中各重要參數對生產速率及產品品質 之影響，本文利用數值分析來探討多孔性圓管受紅外線輻射加熱之乾燥過程。在熱風與 紅外線輻射之乾燥下，考量多孔性圓管之主要機構包含：1.在孔洞與細長毛細管內流動 之水份；2.藉由對流與擴散的方式通過孔洞之水蒸氣。在物理模式中，本文考慮一有限 長度之多孔性圓管受紅外線加熱乾燥過程中，紅外線熱源均勻照射於圓管之外管壁，內 外兩壁面並存在對流之邊界條件，質量擴散係數為一隨溫度與水份變化之函數，其熱傳 與質傳係數，比容與熱傳導值皆隨水份值變化而改變。我們運用能量方程式、質量擴散 方程式以及紅外線輻射傳遞方程式就各種不同乾燥條件進行理論分析而算出溫度與含水 量之分布，進而知其乾燥速率下降之速度，以避免造成產品之毀壞。在數值計算方面， 本研究利用交替向隱蔽法(A.D.I. method)來解決非線性暫態熱傳導與質傳之問題，並 利用可變鬆弛係數以加快收斂之速度。 由結果可發現，我們增加輻射熱通量，降低熱傳導──熱輻射參數，加大Bi、 Bim、與受熱面積即可有效地加快乾燥速率，縮短乾燥所須之時間。

In order to realize the important variables during a drying process that influences the producing rate and quality, a numerical analysis is performed in this study to examine characteristics in infrared-radiation drying of cylindrical tubes of porous media. When the tubes are dried by hot air and infrared radiation, the major internal moisture transfer mechanisms considered include 1. capillary flow of water in the fine capillaries and voids; and 2. water vapor flowing through the voids of a porous material by convection and diffusion. In this study, we consider the drying of cylindrical tubes with finite length. The infrared radiation is uniformly incident upon the outside surface of the tubes, while at both the inside and outside surfaces there exists the convective boundary conditions. The mass diffusion is a function of temperature and moisture content. The convective heat and mass transfer coefficients, specific heat capacity and thermal conductivity are all dependent upon the water content. In numerical analysis, we solve the species equation, energy equation and the radiation transfer equation for the distributions of moisture content and temperature at various drying conditions. We then solve for the drying rate. In terms of the numerical analysis schemes, we utilize the A.D.I method to solve the non-linear transient heat and mass transfer problems and use an interative procedure similar to the method of successive overrelaxation to accelarate the convergence. It is found in the numerical results that we increase radiant heat flux, Bi, Bim, and the heating area and decrease conduction to radiation parameter to raise the drying rate and decrease the drying time.