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Unformatted text preview: 1 TECHNICAL NOTES TN144 Computational Fluid Mixing Elizabeth M. Marshall and Andr Bakker Fluent Inc. 10 Cavendish Court, Centerra Resource Park Lebanon, NH 03766 Software: Fluent, Polyflow Submitted to the North American Mixing Forum (NAMF) Handbook on Mixing January, 2001 Fluent Incorporated, Jan. 12, 2001 2 Introduction Mixing processes can be based on a number of mechanisms, from agitation to sparging to static flow manipulation. Agitation in a stirred tank is one of the most common operations, yet presents one of the greatest challenges in the area of computer simulation. Stirred tanks typically contain an impeller mounted on a shaft, and optionally can contain baffles and other internals such as spargers, coils, and draft tubes. Modeling a stirred tank using computational fluid dynamics (CFD) requires consideration of many aspects of the process. First, any computational model requires that the domain of interest, in this case the volume occupied by the fluid inside the vessel, be described by a computational grid, a collection of small sub-domains or cells. It is in these cells that problem specific variables are computed and stored. The computational grid must fit the contours of the vessel and its internals, even if the components are geometrically complex. Second, the motion of the impeller in the tank must be treated in a special way, especially if the tank contains baffles or other internals. The special treatment employed impacts both the construction of the computational grid as well as the solution method used to numerically obtain the flow field. In this chapter, the process of modeling the flow inside a stirred tank is examined, and these special considerations are discussed at length. The material presented in subsequent sections is summarized below. Section 2: Introduction to Computational Fluid Dynamics Section 3: Introduction to Numerical Methods Section 4: Modeling a Stirred Tank with Experimental Data Section 5: Modeling a Stirred Tank using Actual Impeller Geometry Section 6: Interpretation of CFD Results for Mixing Section 7: Application Examples Section 8: Closing Remarks In Section 2, an introduction to the field of computational fluid dynamics is given, with an emphasis on the fundamental equations that are used to describe processes that are common in mixing applications. An overview of the numerical methods used to solve these equations is presented in Section 3. Numerical simulations of stirred tanks are normally done in either two or three dimensions. In 2D simulations, the geometry and flow field are assumed to be axisymmetric, or independent of the angular dimension. The solution domain extends from the axis of the vessel out to the vessel wall. Approximations are required for elements that do have angular dependence, such as the impellers and baffles. These approximate methods are discussed in Section 4. In 3D simulations, the impellers, baffles, and other internals can be modeled using their exact geometry. geometry....
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