利用計算流體力學模擬A類粒子於流體化床中之流力行為

Simulation of Geldart Group A Particles in Fluidized Beds by Computational Fluid Dynamics

許勝傑(Sheng-Jie Hsu)

指導教授 ： 呂理平

國立臺灣大學/工學院/化學工程學研究所/碩士(2009年)

https://doi.org/10.6342/NTU.2009.00921

摘要

本研究利用FLUENT軟體以計算流體力學之雙流體模型模擬A類粒子於流體化床內之流力行為。由於A類粒子粒徑較小，粒子間之內聚力（cohesive force）對於流體流動影響甚大，而雙流體模型將固體視為連續流體，不易推導出考慮固體粒子間內聚力之全區域流態之氣–固拖曳力模型。一些學者發展出基於一般氣–固拖曳力模型之修正模型，但僅適用於特定流態。本模擬依床體流態及氣體表觀速度的不同分為二部分。
第一部分利用二種一般型之氣–固拖曳力模型（Gidaspow drag model及Syamlal–O’Brien drag model）模擬較低氣速之流體化床。此區域之流態為固定床、均一流體化床與氣泡流體化床。模擬之流體皆為空氣，系統分為粒徑為50 μm、密度為2500 kg/m3之玻璃珠（System–1）；及粒徑為60 μm、密度為2510 kg/m3之玻璃珠（System–2）。利用模擬床體之平均空隙度對氣體表觀速度做圖，可求得粒子之最小流體化速度與最小氣泡速度。System–1之模擬結果與文獻上利用分離粒子模型（discrete element model）模擬之結果接近；System–2則與文獻上利用壓力擾動量測所求得之最小氣泡速度亦相當接近。由此可知利用此二氣–固拖曳力模型模擬床體之時間平均空隙度求得之最小氣泡速度為一有效的方法。
第二部分則是利用修正型之氣–固間拖曳力模型模擬較高氣速之流體化床，分別以modified Gibilaro drag model模擬氣泡流體化床，以及Yang drag model模擬快速流體化床。此二系統使用之粒子粒徑皆為78 μm、密度為1880 kg/m3之FCC粒子。利用此二種不同之修正型氣–固拖曳力模型模擬之結果皆顯示固體粒子由床中央區域向上流動，由管壁處向下流動。而模擬之結果與文獻上實驗量測所得之速度分布亦相當接近。另外利用Yang drag model模擬粒徑為58 μm、密度為1780 kg/m3之FCC粒子於快速流體化床中之軸向空隙度分佈，並在相同固體回流量下，比較不同氣體表觀速度間之空隙度分佈的差異。利用Yang drag model模擬之結果顯示其床體內之空隙度分布皆為S曲線，並未隨著氣速的改變而有明顯的改變，與文獻上之實驗結果不同。而Yang drag model是在 = 1.52 m/s、 = 14.3 kg/m2–s之條件下利用EMMS模型所推導出之氣–固拖曳力模型，因此若要模擬不同氣速及固體回流量的條件下，需以EMMS模型重新推導出不同氣速及固體回流量所適用之氣–固拖曳力模型，Yang drag model並不適用於模擬當速度及固體回流量不同於其推導中所使用之情況（ = 1.52 m/s、 = 14.3 kg/m2–s）。

關鍵字

氣固流體化床 ； 計算流體力學 ； 氣固拖曳力模型

並列摘要

Gas–solid flow behavior in fluidized beds with Geldart Group A particles was numerically simulated by using a computational fluid dynamics (CFD) package FLUENT which is based on the two–fluid model (TFM). Due to the fine nature of Geldart Group A particles, cohesive interparticle forces are significant in comparison to the particle weight and drag force. This effect is very difficult to model in TFM because the continuum basis of the model makes it difficult to consider cohesive forces between individual particles. In this study, five different particles and ambient air were used to simulate.
These particles are the glass beads with the diameter 50 μm and the density 2500 kg/m3 for System–1, the glass beads with the diameter 60 μm and the density 2510 kg/m3 for System–2, the FCC (fluid catalytic cracking) particles with the diameter 78 μm and the density 1880 kg/m3 for System–3 and System–4, and the FCC particles with the diameter 58 μm and the density 1780 kg/m3 for System–5. The gas–solid drag models used are Gidaspow drag model and Syamlal–O’Brien drag model for System–1 and System–2, modified Gibilaro drag model for System–3, and Yang drag model for System–4 and System–5.
The minimum fluidization velocity and the minimum bubbling velocity was obtained by using the relation between the computed time–averaged solid void fraction and the superficial gas velocity for System–1 and System–2. The simulation results were in agreement with the literature data. The computed time–averaged axial particle velocity in the radial profile at the different bed heights in the bubbling fluidization regime (System–3) and the fast fluidization regime (System–4) were in good agreement with the literature experimental results. The results showed that the particles flow upward in the center of the bed and downward near the wall. For System–5, the axial voidage distribution in the fast fluidization regime were calculated with different superficial gas velocities but at the same solid circulating rate. The simulation results showed that the solid phase void fraction distribution along the bed was the S-curve, and it does not change with the superficial gas velocity changed. It was different from the literature experiment data. Yang drag model was derived from the EMMS model at = 1.52 m/s and = 14.3 kg/m2–s. Therefore, if the simulation conditions were different from the above condition, the drag model would be derived again from the EMMS model at specific simulation conditions.

並列關鍵字

gas-solid fluidized bed ； CFD ； gas-solid drag model

參考文獻

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