Recently Liz Marshall and I published a booklet titled Computational Fluid Mixing. This 154 page booklet covers many topics in the field of simulating mixing processes using CFD. A printed version of this booklet was published by Fluent Inc. The booklet starts with a great Foreword by Prof. J.M. Smith that discusses the progress of understanding of mixing throughout the ages. It continues with an easily understandable Introduction to CFD; an Introduction to Numerical Methods; a chapter on Stirred Tank Modeling Using Experimental Data; a section on Stirred Tank Modeling Using the Actual Impeller Geometry; an overview of Evaluating Mixing from Flow Field Results; a very nice suite of Application Examples; and Closing Remarks giving you useful modeling advice.
The application of large eddy simulation (LES) to the prediction of large-scale chaotic structures in stirred tanks is investigated. Flow regimes representing typical stirrer configurations were assessed: a single radial pumping impeller and a single axial pumping pitched blade turbine. The turbulent flow field in each configuration was calculated using LES turbulence models. The impellers were modeled using the sliding mesh model. The predicted flow patterns compared well with digital particle image velocimetry data reported in the literature, and exhibited the long time scale instabilities seen in the experiments. The results of these studies open the way to a renewed interpretation of many previously unexplained hydrodynamic phenomena that are observed in stirred vessels.
(PDF File; 0.3 MB; December 22, 2001)
In addition to their work in the chemical and oil industries, chemical engineers work in a wide variety of other industries, such as those dedicated to the production of plastics and synthetic resins, man-made fibers, polymers, paints and varnishes, drugs and pharmaceuticals, agricultural chemicals, fats and oils, foods and beverages, and many others. They are responsible for the design, operation, optimization, and troubleshooting of manufacturing operations, and can be involved in applications that range from combustion to biological reactions. All of these industrial applications have one thing in common: raw materials are converted into final products by means of a chemical reaction in an environment that involves fluid flow. Depending on the physical state of the materials being converted and the specific operating conditions under which the selected reactions occur, different types and scales of reactors are used. These include, but are not limited to, batch reactors; continuous stirred tank reactors; plug flow reactors; fluidized, fixed, or moving bed reactors; bubble columns or airlift reactors; and film reactors. In this article, some of today’s most popular methods for simulating reacting flow are reviewed. Examples are used to emphasize the variety of applications and methods for tackling this complex behavior.
(PDF File; 0.4 MB; April 6, 2001)
The design, scale-up and operation of unit operations in the chemical industry rely heavily on empiricism and correlations of overall parameters for non-ideal or non-equilibrium conditions. Researchers, equipment designers and process engineers are increasingly using computational fluid dynamics (CFD) to analyze the flow and performance of process equipment such as chemical reactors, stirred tanks, fluidized beds, cyclones, combustion systems, spray dryers, pipeline systems, heat exchangers, and other equipment. CFD allows for an in-depth analysis of the fluid mechanics and local effects in these types of equipment. In many cases this results in improved performance, better reliability, more confident scale-up, improved product consistency, and increased plant productivity. In this chapter we first discuss the technology behind CFD, and then illustrate today's possibilities with a number of practical design applications. We will conclude with a discussion of expected future developments.
All of the disk-style gas dispersion impellers studied in the literature so far have blades that are symmetric with respect to the plane of the disk. This is not necessarily optimal, as the gas usually enters from the bottom, causing a distinctly asymmetric flow pattern. This paper discusses the performance of a new gas dispersion impeller with vertically asymmetric blades. The new impeller is designed to accommodate the different flow conditions above and below the impeller disk. The blade shape was optimized in a comparative study of more than twenty different geometries. This paper discusses the performance of a gas dispersion impeller with blades that are vertically asymmetric; i.e. the blade shape above the disk is different from the shape below the disk. It is shown that this impeller has a gassed power curve that is flatter than that of other impellers. Furthermore, it can disperse more gas before flooding than the impellers with symmetric blades. Both experimental data and the results of advanced CFD simulations are being discussed.
Blend time and chemical product distribution in turbulent agitated vessels can be predicted with the aid of Computational Fluid Mixing (CFM) models. The blend time predictions show good agreement with an experimental correlation. Calculations for turbulent, time dependent mixing of two chemicals, exhibiting a competitive pair of reactions, are compared with experimental results. The effects of the position of the inlet feed stream in the turbulent flow field are studied. It is concluded that process problems with turbulent chemical reactors can be avoided by incorporating the results of CFM simulations in the design stage.
The relation between the flow pattern and the spatial distribution of the solids in a stirred tank has been investigated. Both single impeller systems and multiple impeller systems were studied in tanks with a liquid level of up to 1.75 times the tank diameter, using pitched blade turbines and high efficiency impellers. The solids distribution is strongly affected by certain flow transitions. When the impeller diameter and/or impeller-bottom clearance are too large, the flow direction at the bottom reverses, seriously hampering solids suspension. Adding a second impeller does not decrease the just-suspended speed. A second impeller does increase the homogeneity of the suspension, provided that the spacing between the impellers is not too large.
A mathematical model for the combined laminar and turbulent flow of paper pulp has been developed. The results of the model predictions agree well with experimental data. The model is used to analyze the flow pattern in an industrial paper pulp chest equipped with a side entering impeller. Turbulent, laminar and stagnant regions can easily be located. The model is an excellent tool for the optimization of agitators for large industrial paper pulp chests.
The turbulent flow pattern and mixing characteristics of the High Efficiency Vortex (HEV) static mixer have been investigated by means of computational fluid dynamics simulations. Experiments showed that the mixer generates a complex vortex system, consisting of a steady longitudinal vortex and transient hairpin vortices. The steady state computer model correctly predicted the longitudinal vortex and a high turbulence intensity in the hairpin vortex region. The vortex system provides for efficient blending of gases or miscible fluids.
The flow pattern, pressure drop and the mixing characteristics of Kenics static mixers are investigated by means of computer simulations. The static mixer consists of a series of alternating left and right hand helical elements. The simulations gave new insights in the flow pattern in the helical mixing elements. The pressure drop predictions compare favorably with literature data. Mixing in the elements occurs through a combination of flow splitting and shearing at the junctions of successive elements and a stretching and folding mechanism within the elements. This makes the Kenics element an excellent radial mixing device.
The flow pattern created by a pitched blade turbine was calculated using a sliding mesh method for various Reynolds numbers, mostly in the laminar regime. This method allows flow pattern calculations without the use of any experimental boundary conditions. The results compared favorably with experimental data obtained by laser-Doppler velocimetry. At low Reynolds number the impeller creates a radial flow pattern, rather than axial. The pumping number decreases with decreasing Reynolds number. It is concluded that the sliding mesh method is suitable for the prediction of flow patterns in stirred tanks.
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