無擋板攪拌槽自由液面與漩渦形狀之數值模擬

本研究針對圓盤渦輪葉片及螺旋槳式葉片之無擋板攪拌槽進行自由液面與漩渦形狀之三維電腦數值模擬分析，採用有限體積法之計算流體力學軟體Fluent 6.3來模擬攪拌槽的自由液面與漩渦形狀。假設流場為擬似穩態，應用多重參考座標系統，以旋轉參考座標系統計算葉片旋轉區域及其附近之流場，以靜止坐標系統計算其餘靠近壁面及柵欄之流場；採用雷諾應力紊流模式描述流體的紊流現象，對靠近壁面的流場使用壁函數來處理，使用尤拉多相流模式來確立自由液面與漩渦形狀。為了評估自由液面預測之準確度，以文獻裡的實驗與模擬數據進行驗證比對，並改變五種參數來觀察漩渦形狀與深度，分別為渦輪葉片傾斜角度、渦輪葉片至攪拌槽底部間距、改變軸心位置之偏心攪拌方式、增加柵欄及增加柵欄後改變其攪拌槽直徑的方式。經研究發現，本研究之標準案例模擬結果在自由液面與漩渦形狀的預測與實驗的趨勢相當吻合，並且其CVRMSE值僅在2.3%以下。而在增加柵欄後能確實增加漩渦深度及紊流動能，並且即使將此增加柵欄後的圓盤渦輪葉片應用在數倍大直徑的攪拌槽裡，只要增加其轉速也能夠維持其漩渦形狀而不失其混合效率。

關鍵字

攪拌槽；圓盤渦輪葉片；自由液面；漩渦；計算流體力學；柵欄；混合；偏心攪拌

並列摘要

In this study, numerical simulations of turbulent flows with free-surface vortex in unbaffled agitated vessels, by Rushton turbine, and propeller impeller were presented. The predictions were obtained using the Fluent 6.3 computational fluid dynamics by a finite-volume method. The flow was assumed as a quasi-steady state, and this study applied a Multiple Reference Frame to calculate flow field. A rotational frame was used to calculate the flow of the impeller swept region, and the surrounding flow. A stationary frame was use to calculate the flow of the remaining region near the fence or the wall. The turbulent model used a Reynolds Stress Model (RSM); and wall function was used to calculate the flow near the wall. A Eulerian multiphase model was used to determine the free-surface and shape of the vortex. In order to assess the accuracy of the vortex shape, as identified above, this study used experimental data for analysis of the free-surface. Five parameters were changed to observe the transform of the vortex shape and depth in the stirred tanks, including angle of the impeller blades, clearance between the bottom of the vessel and the midsection of the impeller disk, eccentric distance between the stirring shaft and the Z-axis, and increasing the fence and diameter of the tank. It was shown that in the standard case, the predicted general vortex shape of the liquid free-surface is in good agreement with measurements, and the predicted vortex depth is good. The CVRMSE is less than 2.3%. In addition, the fence settings can increase the vortex depth and the turbulence kinetic energy. It can be used in large-diameter agitated vessels when the increased rotational velocity is able to maintain the vortex shape without losing its mixing efficiency.

並列關鍵字

Unbaffled agitated vessels ； Rushton turbine ； Free-surface ； Vortex ； CFD ； Fence ； Mixing ； Eccentric agitation

參考文獻

[3] G.B. Tatterson, Fluid Mixing and Gas Dispersion in Agitated Tanks, New York: McGraw-Hill, 1991.

[7] H.A. Jakobsen, Chemical Reactor Modeling, Berlin Heidelberg: Springer Verlag, 2008.

[9] J. Karcz, M. Major, "An effect of a baffle length on the power consumption in an agitated vessel," Chemical Engineering and Processing, vol. 37, 1998, pp. 249–256.

[10] B. Freudig , S. Hogekamp, H. Schubert," Dispersion of powders in liquids in a stirred vessel," Chemical Engineering and Processing, vol. 38, 1999, pp. 525-532.

[11] K. Suzukawa, S. Mochizuki, H. Osaka, "Effect of the attack angle on the roll and trailing vortex structures in an agitated vessel with a paddle impeller," Chemical Engineering Science, vol.61, 2006, pp. 2791-2798.

國際替代計量

無擋板攪拌槽自由液面與漩渦形狀之數值模擬

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