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|Title: ||噴霧乾燥製程之熱流模擬;THERMAL FLOW SIMULATION OF A SPRAY DRYING CHAMBER|
|Authors: ||兒德娜;Tsedendolgor, Bat-Erdene|
|Keywords: ||含水量;牛奶乾燥;熱流模擬;噴霧乾燥;相對溼度;計算流體力學;moisture content;milk drying;airflow simulation;spray drying process;relative humidity;CFD|
|Issue Date: ||2017-10-27 16:18:15 (UTC+8)|
工業製程特別是食物加工業，普遍採用噴霧乾燥方式來保存牛奶以及蔬菜水果。噴霧乾燥以壓力噴嘴將待乾燥物噴入乾燥機，以高溫順流(co-current)空氣將待乾燥物水分蒸發而達到乾燥效果。預測空氣流速、溫度以及濕度分布對於設計噴霧乾燥機有重大的助益。 本文採用計算流體力學(Computational Fluid Dynamics, CFD)商用軟體Fluent 14.5為噴霧乾燥製程建立熱流模擬模型。模型將空氣流視為連續體(Continuum) 以歐拉法 (Eulerian approach) 模擬其熱流行為，液滴群則視為離散的顆粒以拉格朗日法 (Lagrangian approach) 來模擬個別顆粒的移動軌跡與水分蒸發行為，二種方法耦合一併求解。我們先建立二維模型測試上述二相流模型的可行性，再延伸至實際的三維計算，並嘗試以水滴蒸發來近似牛奶的乾燥模式。奶粉製程要求乾燥機出口的空氣相對濕度需介於4-6%，而入口空氣溫度上限則為200℃，本文以此標準設定模擬條件，探討入口空氣溫度與液滴流量對於出口濕度的影響。三維模擬結果顯示本文模型可有效模擬空氣流速、濕度及溫度，亦可模擬液滴的飛行軌跡與顆粒尺寸因蒸發產生的變化與粒徑分布，結果將可以提供奶粉噴霧乾燥製程作為參考。
Spray drying is one of the significant processes of various manufacturing sectors, especially for food processing industries which commonly use the process for drying milk, fruit juice, and vegetable juice. Prediction of the air flow, temperature, and humidity patterns in the co-current spray drying chamber with a pressure nozzle fitted at top of the drying chamber can help the design of the spray drying process.
This thesis presents thermal fluid simulations of a spray drying chamber. The demonstrations were performed with two-dimensional and three-dimensional models using a Computational Fluid Dynamics package (CFD) ‘Fluent 14.5’. The gas phase was exhibited as a continuum using the Euler approach and the droplet phase was exhibited by the Lagrange approaches. We investigated the drying process considering the effects of air flow interacted with atomizing water droplets. The two-dimensional simulation generally predicted the fast downward flowing core and slow recirculation zones around it which successfully verify the simulation model. After that we performed the three-dimensional simulations, considering the application to the milk drying process.
Most of the previous investigations have shown that the moisture content of the milk-air mixture is better to be around 4-6% (entering hot air up to 200℃) when the milk powders leave the chamber. This criterion was adopted to test our simulation parameters. The three-dimensional simulations correctly predicted the patterns of air flow, humidity, and temperature as well as particle tracks and distributions. The CFD simulation predicted the relationships between the inlet and outlet properties of the drying chamber. Variations of the inlet air temperature and water flow rate affect the outlet humidity of the exhaust air as well as the droplet sizes.
|Appears in Collections:||[機械工程研究所] 博碩士論文|
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