鹽水液滴在常壓環境下乾燥過程的建模與模擬

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题名:	鹽水液滴在常壓環境下乾燥過程的建模與模擬
作者:	吳帛軒;WU, PO-HSUAN
贡献者:	機械工程學系
关键词:	噴霧乾燥;蒸發;固化析出;流體體積法;固體顆粒結構;Spray drying;evaporation;solidification;Volume of fluid method;solid particle structure
日期:	2017-11-23
上传时间:	2018-01-16 11:07:38 (UTC+8)
出版者:	國立中央大學
摘要:	噴霧乾燥技術，因為具有成本低廉、工藝簡單、適合於大規模工業化生產等優點，且容易控制粉末成型的結構，因此成了工業製程中最常用來製作功能性粉末的一種方法。由於功能性粉末的結構日漸複雜，故解析在功能性粉末製程中的噴霧乾燥技術，成了近年來工業發展的重要課題。噴霧乾燥技術理論模型的基礎，是建立在單顆液滴的乾燥及固化析出的模型。已知液滴蒸發過程的關鍵有二：蒸發過程與固化析出過程。本研究以數值模擬的方式，建立一個同時包含此兩種機制的數學模型，並使用ANSYS Fluent中的流體體積法(Volume of fluid ,VOF)來擷取介面，模擬一顆含有氯化鈉的水溶液液滴，在一個靜止的開放空間下進行的乾燥過程，並使用環境溫度為40℃、60℃ 及 80℃的條件，來探討不同環境溫度對最終形成固體顆粒結構的影響。由模擬計算結果可得知，當環境溫度改變，由於乾燥過程的差異，使得最終形成的鹽粒結構有明顯不同。當環境溫度為40℃時，鹽粒呈均勻的實心結構；環境溫度為60℃及80℃時，鹽粒呈空心結構，且環境溫度愈高，形成外鹽殼層的孔隙率愈小、鹽殼層內外的孔隙率差距愈大，而整體殼層的厚度也會愈小。故可推測，介於40℃與60℃之間存在著一個臨界的環境溫度，使鹽粒的結構由實心固體狀，轉變為空心殼狀。且若持續提高環境溫度，則會形成一薄殼球狀的鹽粒。 ;Spray drying is one of the most commonly used methods of producing functional powders in industrial processes because of its advantages for low cost, simple process, large-scale industrial production and easy control of powder-molded structures. Due to the increasingly complex structure of functional powders, the analysis of spray-drying technology in the manufacture of functional powders has become an important issue for the development of the industry in recent years. The basis of the theoretical model of spray drying technology is based on a single droplet which undergoes the drying and precipitation processes. Droplet evaporation has two key processes: evaporation process and solidification precipitation process. This study proposes a mathematical model that considers both two mechanisms. The volume of fluid method (VOF) in ANSYS Fluent was used to capture the interface and simulate a drying process of a saline droplet in a static open space. To investigate the influence of ambient temperatures on the final structure of solid particles, simulations were performed with ambient temperature of 40℃、60℃ and 80℃, respectively. Simulation results show that when the ambient temperature changes, due to the difference in the evaporation rate, the structure of the final salt particles is obviously different. When the ambient temperature is 40℃, the salt particles is a uniform solid structure. When the ambient temperature is 60℃ and 80℃, the salt particles are hollow structure. The higher the ambient temperature, the smaller the porosity of the outer salt shell formed, the bigger the porosity difference between the inner and outer salt shell, and the thinner of the shell thickness. Therefore, it can be speculated that there is a critical ambient temperature between 40℃ and 60℃ at which the structure of the salt particles will change from a solid particle to a hollow shell particle. Increasing the ambient temperature will result in a thin shell spherical salt particle.
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