具單個或雙個斜葉圓盤渦輪攪拌槽流場分析

Abstract

本研究為對具單個或雙個圓盤渦輪的攪拌槽做流場模擬分析，其中在單個渦輪的部份，螺槳葉片將在具有不同角度的情況下計算。假設流場為擬似穩態，應用多重參考座標系，以旋轉座標系計算葉片旋轉區域及其附近之流場，以靜止座標系計算其餘靠近壁面及擋板之流場；紊流模式使用高雷諾數之 模式，對靠近壁面的流場使用壁函數來處理，由於攪拌槽具備軸對稱的特性，故本模擬只取一半的攪拌槽作為計算範圍以減少計算量。 在單渦輪攪拌槽的情況下，改變四種參數以觀察流場的改變，分別是：渦輪葉片的傾斜角度、渦輪距離攪拌槽底的距離、渦輪的直徑以及渦輪圓盤的直徑。當渦輪葉片的角度變逐漸變大(越接近垂直時)，到達某一特定角度時，流場會從軸向流場變成徑向流場，我們稱此特定角度為臨界角。當渦輪與攪拌槽底的間隙越小時臨界角越大，而渦輪碟盤對於流場的影響也越明顯，當渦輪尺寸與渦輪碟盤尺寸越大時，臨界角越小。本研究所定義的多重參考座標系與沈[19]所定義的不同，而本研究與實驗的比對較沈[19]來的好。 多重參考座標系亦應用在雙渦輪圓盤攪拌槽中，模擬雙渦輪攪拌槽中會出現的三個穩定流場，分別是平行流，結合流及分離流；將其與實驗結果、內外交替疊代法(數值模擬)、滑動網格法(數值模擬)比較，發現多重參考座標系能準確的模擬平行流，但是在結合流及分離流方面，雖然流場與實驗結果相似，但是功率數與實驗結果比較卻不甚相同。

This study aimed at the prediction of flow field agitated by single or dual pitched blade disc turbine by Multiple Reference Frame. Pitched blade disc turbine was only used in stirred tank with single impeller. The flow was assumed as quasi-steady state and we applied the Multiple Reference Frame to calculate flow field. Rotational frame was use to calculate flow of impeller swept region, and flow surrounding it; stationary frame was use to calculate flow of the rest region that neat the baffle or the wall. The turbulent model used was High Reynolds k-ε model; wall function is used to calculate the flow near the wall. Because of the axial symmetric of the stirred tank, the computational domain was only half of the tank and periodic boundary was used. Four parameters were changed to observe the transform of flow field in stirred tanks with single disc turbine. They are oblique angle of the impeller blades, clearance between impeller and vessel bottom, diameter of impeller disc, and diameter of impeller. When the blade angle increased, reaching a specific angle, the flow field changed from axial flow to radial flow rapidly. The significant oblique angle of impeller blade that made the flow field changed was defined as the critical angle. Decrease of the clearance made the critical angle become bigger and the effect of disc also increase. Larger impeller disc and larger impeller made the critical angle smaller. Different definition of MRF from Shen[19] was applied and its results shown good agreement with the experimental data. MRF technique was also used to predict the flow field of stirred tank agitated by dual impellers. Three stable flow field of stirred tank with dual turbines were predicted, they are parallel flow, merging flow and diverging flow. The prediction results were compared with experimental data and prediction of IO and SG technique. The results show that the MRF technique can predict the flow field agitated by dual impeller, but the power numbers were not good except the parallel flow.