葉輪攪拌槽中之流場計算

Abstract

摘 要 本研究主要是針對等間距葉片攪拌器(PBT)作流場之計算。早期的模擬是將流場簡化為二維流場，不作圓周方向的計算。而對於葉片區域內的流場，也只以實驗量測而不作計算。本研究所計算的範圍是三維紊流流場，包含葉片區內和葉片區以外的範圍。假設流場為擬似穩態如同一瞬時狀態之近似，並以旋轉座標系及靜止座標系分別計算葉片區內和葉片區以外之流場，紊流模式是使用高雷諾數的 模式，對近壁面流場的處理是使用壁函數。由於攪拌槽葉片的尺寸外型皆相同，且流場為週期性變動，因此只取一適當計算範圍作為週期性邊界，計算此一範圍之流場以減少計算量，計算網格是使用非結構性與非交錯式網格。 本研究主要是對Dong[2]與Ranade[4]的實驗模型作模擬，並分析垂直面與水平面流場結構，渦流發展情形，速度場與紊流動能分佈，且將模擬與實驗作比較，並研究葉片中心高度與葉片角度對流場之影響，以及計算其攪拌能力之無因次參數。 等間距葉片攪拌器(PBT)是屬於軸向紊流攪拌器，結果顯示當葉片角度垂直 時，且葉片中心的高度位於攪拌槽高度的一半，流場形成上下兩個對稱的循環，而射出方向也非一般軸向紊流攪拌器應有的軸向射出方向，反而類似徑向紊流攪拌器，如盤式直葉攪拌器(Rushton turbine)，的徑向射出方向。而降低葉片中心位置高度並沒有太大改變，只有將流場中心下移，並沒有改變徑向射出的方向及上下兩個循環的結構。不同葉片角度對流場結構的影響較大，角度愈大攪拌能力愈好。但結果也顯示葉片角度大到某一範圍，循環的程度會下降，且角度愈大所消秏的功率也同時變大。

ABSTRACT This thesis is mainly aimed the calculation of the flow in pitched blade turbine. The simulation of the flow was simplified to 2-D regardless of the changes in azimuth. The region swept by impeller was measured rather than calculated. I calculate both inside and outside turbulent flow of the impeller swept region in 3-D. Assuming that the flow is quasi-steady like a snapshot approach I calculate the inside and outside flows of the impeller region by means of rotative and stationary frame of reference individually. Turbulent model is High Reynolds model; wall function is used to calculate the flow near wall. Because the sizes and shapes of the blades in a stirred tank are the same and the flow cycles, I select a suitable calculation range as cyclic boundary to reduce the amount of the calculation. Unstructured and no staggered meshes are adopted in our calculation. Simulating the experimental models of Dong [2] and Ranade [4] mainly, I analyze the vertical and horizontal planes of the flow structure, the development of the vortices, the velocity of flow fields, turbulent kinetic energy, and the comparison between simulation and experimental data. Studying the influence of the impeller clearance and that of blade angle on flow structure, I calculate the dimensionless parameters of mixing capacity. Although pitched blade turbine belongs to axial turbulent impeller stirred tank, the flow forms two symmetrical circulation above and below when the angle of blade is and the impeller clearance is half tank height. The direction of discharge of this axial turbulent impeller stirred tank is like that of radial impeller stirred tank such as Rushton turbine rather than that of itself. The center of the flow will go down, provided that I reduce impeller clearance. In addition, reducing impeller clearance will not change two circulating flow structures and the discharge direction of radial turbulent impeller stirred tanks. The influence of different blade angles on flow structure is of much. The larger the blade angle is, the better the mixing capacity is. The degree of circulation will be down, providing that the angle of blade is big to some extent. The bigger the angle of blade becomes, the more the turbulent dissipation rate rises.