以計算流體力學模擬紊流噴射流場和A類粒子在氣-固流體化床中的流力行為

本研究以計算流體力學來求解流體力學問題，以常用的商業軟體FLUENT來模擬流體流動流場，並計算出其相關的流體力學變數如速度、壓力等，最後將模擬的結果與文獻上的實驗數據作比較，來討論其正確性和可行性。第一部分為自由紊流噴流流場的模擬，以空氣為操作流體，從口徑為1 mm的圓孔高速噴發向一近半無限大的靜止空氣流場，在軸附近形成噴流的主要流動區域，區域內的軸向速度分佈會隨著與噴口距離增加而有相對應的變化，此為噴流的自相似性，從理論分析上的解析解方法可以證明出有此性質，同樣地，亦可利用數值解的方法來驗證。第二部份利用將氣、固兩相皆視為流體的雙流體模型來模擬氣-固流體化床，床中的粒子為A類粒子，因A類粒子粒徑小，當操作在較高氣體流速下的循環式流體化床時，粒子群傾向於聚集在一起形成絮狀物，在絮狀物影響的效應下，以原有的氣固拖曳力模型無法模擬出上稀下濃的軸向空隙度分佈，必須導入修正因子來修正原有拖曳力模型，利用修正型拖曵力模型來模擬粒徑為58 μm、密度為1780 kg/m3的 FCC粒子在固定粒子循環量下，不同的氣體流速其軸向空隙度分佈曲線，從模擬結果顯示，濃相區域的粒子濃度較實驗結果大，而稀相區域較符合實驗的結果。在決定A類粒子的最小氣泡化速度時，文獻中有人利用壓力擾動實驗數據的相關性來得到。在此分別以絕對壓力擾動法和相對壓力擾動法兩種方法來模擬粒徑為60 μm、密度為2510 kg/m3的玻璃珠在氣泡流體化時的流態，利用模擬的方式來驗證文獻中求最小氣泡化速度之方法。

關鍵字

計算流體力學；噴流；流體化床；軸向空隙度分佈；壓力擾動

並列摘要

Computational fluid dynamics ( CFD ) was used to solve the fluid mechanics problems by using the commercial software FLUENT in this study. The software was used to simulate flow field and calculate the relevant variables related to fluid mechanics such as velocity, pressure, etc. Finally, the simulation results were compared with the experimental data in the literatures to discuss its correctness and feasibility. The first part of this study was free turbulent jet flow field simulation. The fluid was air, which was at high speed and emerged from a 1 mm diameter circular hole into a semi-infinite stationary air flow. The main flow region was formed near the central axis, and in that region the axial velocity distribution profile changed similarly with increasing the distance from the jet exit. This is called self-similarity of jet flow. The problem was solved analytically in early days; now it also was solved numerically for comparison. The second part was gas-solid fluidized bed simulation. The two-fluid model ( TFM ) which considered the behavior of both gas phase and solid phase as the fluid was applied to simulate Group A particles flow pattern in the fluidized beds. Due to Geldart Group A particles, the particles which formed clusters when operating in the high gas flow circulating fluidized beds. Due to the effect of the clusters, the classical gas-solid drag force model can not simulate the dilute up and dense bottom axial voidage distribution successfully. With the help of the modified drag force model, the axial voidage distribution profile of FCC particles ( the particle size 58 μm and the density 1780 kg/m3 ) were simulated under fixed solid circulation rate but different superficial gas velocities. The simulation results showed that the particle concentration in the dense region was larger than the expected value, but in accordance with the experimental data in the dilute region. Leu and Tsai (2009) obtained the minimum bubbling velocity of Geldart group A particles by correlation of pressure fluctuations experimental data. The glass beads ( the particle size 60 μm and the density 2510 kg/m3 ) in bubbling fluidized beds were simulated by using both absolute pressure fluctuations method and differential pressure fluctuations method respectively. The method of determining minimum bubbling velocity in the literatures could be verified by the simulations.

並列關鍵字

Comptuational fluid dynamics ； jet flow ； fluidized beds ； axial voidage distribution ； pressure fluctuations

參考文獻

Abrahamsen, A. R. and D. Geldart, “Behavior of Gas–Fluidized Beds of Fine Powders Part I. Homogeneous Expansion”, Powder Technol., 26, 35–46 (1980).

Agrawal, A. and G. Verma, “Similarity Analysis of Planar and Axisymmetric Turbulent Synthetic Jets”, Int. J. Heat Mass Transfer, 51, 6194-6198 (2008).

Ali, A., N. Kechiche and H. B. Aissia, “Prandtl-Number Effects on Vertical Buoyant Jets in Forced and Mixed Convection Regimes”, Energy Convers. Manage., 48, 1435-1449 (2007).

Almuttahar, A. and F. Taghipour, “Computational Fluid Dynamics of High Density Circulating Fluidized Bed Riser: Study of Modeling Parameters”, Powder Technol., 185, 11-23 (2008).

Amano, R. S., “Turbulence Effect on the Impinging Jet on a Flat Plate”, Bulletin of the JSME, 26, 1891-1899 (1983).

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以計算流體力學模擬紊流噴射流場和A類粒子在氣-固流體化床中的流力行為

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