A critical review of the published literature regarding the computational fluid dynamics (CFD) modelling of single-phase turbulent flow in stirred tank reactors is presented. In this part of review, CFD simulations of radial flow impellers (mainly disc turbine (DT)) in a fully baffled vessel operating in a turbulent regime have been presented. Simulated results obtained with different impeller modelling approaches (impeller boundary condition, multiple reference frame, computational snap shot and the sliding mesh approaches) and different turbulence models (standard k − ε model, RNG k − ε model, the Reynolds stress model (RSM) and large eddy simulation) have been compared with the in-house laser Doppler anemometry (LDA) experimental data. In addition, recently proposed modifications to the standard k − ε models were also evaluated. The model predictions (of all the mean velocities, turbulent kinetic energy and its dissipation rate) have been compared with the experimental measurements at various locations in the tank. A discussion is presented to highlight strengths and weaknesses of currently used CFD models. A preliminary analysis of sensitivity of modelling assumptions in the k − ε models and RSM has been carried out using LES database. The quantitative comparison of exact and modelled turbulence production, transport and dissipation terms has highlighted the reasons behind the partial success of various modifications of standard k − ε model as well as RSM. The volume integral of predicted energy dissipation rate is compared with the energy input rate. Based on these results, suggestions have been made for the future work in this area.Nous présentons un examen critique de la littérature concernant la modélisation de la dynamique des fluides numérique (DFN) de l'écoulement turbulentà une phase dans les réacteursà cuve agitée. Dans cette partie de l'examen, nous présentons les simulations de DFN de turbinesà ecoulement radial (principalement des turbinesà disque (TD)) dans un réservoir entièrement cloisonné effectuées dans un régime turbulent. Les résultats des simulations obtenus grâceà différentes approches de modélisation des turbines (couche limite turbulente, méthode des référentiels multiples, snap-shot de modélisation numérique, maillage glissant) età différents modèles de turbulence (modèle standard k-e, modèle RNG k-e, modèle aux tensions de Reynolds et simulation des grandeséchelles) ontété comparés aux données expérimentales internes d'allocation de Dirichlet latente (ADL). De plus, les modifications des modèles standards k-e récemment proposées ontégalementétéévaluées. Les prédictions du modèle (de toutes les vitesses moyennes, de l'énergie cinétique turbulente et de son taux de dissipation) ontété comparées aux données expérimentales relevéesà différents endroits de la cuve. Une discussion présente les points forts et les points faibles des modèles de DFN actuellement utilisés. Une analyse préliminaire de la sensibilité des hypothèses de modélisation liées aux modèles k-e et RSM aété menée en utilis...
In the first part of the review, published literature regarding the CFD modelling of single-phase turbulent flow in stirred tank reactors with radial flow impellers was critically analysed. A brief overview of different turbulence models (standard k − ε model, RNG k − ε model, the Reynolds stress model and large eddy simulation) as well as impeller baffle interaction models has been presented in the previous part. This part is concerned with the review of literature regarding CFD simulation of axial flow impellers. Comprehensive simulations have been carried out using various turbulence models and the model predictions (of all the mean velocities, turbulent kinetic energy and its dissipation rate) have been compared with the experimental measurements at various locations in the tank. The strengths and weaknesses of various turbulence models for axial flow impellers is presented. The quantitative comparison of exact and modelled turbulence production, transport and dissipation terms has highlighted the reasons behind the partial success of various modifications of standard k − ε model as well as Reynolds stress model. Literature efforts on multiple impeller systems and multiphase systems have been discussed in a separate section. Based on these results, suggestions have been made for the future work in this area.Dans la première partie de l'étude, on a procédéà une analyse critique de la littérature concernant la modélisation de la dynamique des fluides numérique de l'écoulement turbulentà une phase dans les réacteursà cuve agitée dotés de turbinesàécoulement radial. Une vue d'ensemble rapide des différents modèles de turbulence (modèle standard k-e, modèle RNG k-e, modèle aux tensions de Reynolds et simulation des grandeś echelles), ainsi que des modèles d'interaction des déflecteurs de turbine, aété présentée dans la partie précédente. Cette partie se concentre sur l'analyse de la littérature concernant la simulation de DFN de turbinesàécoulement axial. Des simulations complètes ontété effectuées en utilisant plusieurs modèles de turbulence et les prédictions des modèles (de toutes les vitesses moyennes, de l'énergie cinétique turbulente et de son taux de dissipation) ontété comparées aux données expérimentales relevéesà différents endroits de la cuve. On a présenté les points forts et les points faibles de plusieurs modèles de turbulence concernant les turbinesàécoulement axial. La comparaison quantitative des données exactes et modélisées de la production, du transport et de la dissipation de la turbulence a mis enévidence les raisons qui expliquent la réussite partielle de plusieurs modifications apportées au modèle standard k-e ainsi qu'au modèle aux tensions de Reynolds. Une partie distincte est consacréeà la discussion des résultats indiqués dans la littérature concernant les systèmesà roues multiples et les systèmes multiphases. Sur la base de ces résultats, desétudesà venir dans ce domaine ontété suggérées.
Most chemical engineering equipment is operated in the turbulent regime. The flow patterns in this equipment are complex and are characterized by flow structures of wide range of length and time scales. The accurate quantification of these flow structures is very difficult and, hence, the present design practices are still empirical. Abundant literature is available on understanding of these flow structures, but in very few cases efforts have been made to improve the design procedures with this knowledge. There have been several approaches in the literature to identify and characterize the flow structures qualitatively as well as quantitatively. In the last few decades, several numerical as well as experimental methods have been developed that are complementary to each other with the onset of better computational and experimental facilities. In the present work, the methodologies and applications of various experimental fluid dynamics (EFD) techniques (namely, point measurement techniques such as hot film anemometry, laser Doppler velocimetry, and planar measurement techniques such as particle image velocimetry (PIV), high speed photography, Schlieren shadowgraphy, and the recent volume measurement techniques such as holographic PIV, stereo PIV, etc.), and the computational fluid dynamics (CFD) techniques (such as direct numerical simulation (DNS) and large eddy simulation (LES)) have been discussed. Their chronological developments, relative merits, and demerits have been presented to enable readers to make a judgment as to which experimental/numerical technique to adopt. Also, several notable mathematical quantifiers are reviewed (such as quadrant technique, variable integral time average technique, spectral analysis, proper orthogonal decomposition, discrete and continuous wavelet transform, eddy isolation methodology, hybrid POD−Wavelet technique, etc.). All three of these tools (computational, experimental, and mathematical) have evolved over the past 6−7 decades and have shed light on the physics behind the formation and dynamics of various flow structures. The work ends with addressing the present issues, the existing knowledge gaps, and the path forward in this field.
Solid−fluid heat-transfer coefficients have an important role in the design of chemical processing equipment. The major resistance to heat transfer lies in a region very close to the wall, where experimental measurements are very difficult. The validity and accuracy of the models developed for the estimation of the heat- and mass-transfer coefficient still do not have general applicability for the entire range of Reynolds and Prandtl numbers, because of the limited knowledge of near-wall turbulence. There have been two approaches for such model development: one is an analytical approach, which considers the momentum, mass, and heat transfer to be analogous in nature and the understanding of one of these processes can be used to predict the other two; the other approach is heuristic, based on the visualization of the behavior of the coherent structures in the near-wall region. The continuous movement of fluid elements to and away from the wall (coherent structures) affects the transport phenomena. The models for the quantification of this behavior have been developed for the estimation of heat- and mass-transfer rates in the literature. However, both these approaches contain parameters fitted empirically to obtain good agreement with the experimental heat- and mass-transfer data. These models must be tested for their formulation and empirical constants on the basis of accurate solutions of governing equations of heat, mass, and momentum transfer. This is possible using direct numerical simulation (DNS) and large eddy simulation (LES), which can accurately predict the near-wall flow pattern. An attempt has been made to exploit the ability of DNS and LES to develop insight into hitherto used models, based on analogies and/or heuristic arguments.
There have been several approaches in the literature to identify and characterize flow structures qualitatively as well as quantitatively. In the first part of this review, the methodologies and applications of various experimental fluid dynamics and computational fluid dynamics techniques, as well as mathematical techniques, have been discussed. Their chronological developments, and relative merits and demerits, have been presented to allow readers to make a judgment as to which techniques to adopt. In the present part of the review series, a stepwise procedure is suggested for the design of equipment using flow structure knowledge. An attempt has been made to relate the structure properties (such as age, penetration depth, size, shape, and energy content distribution) to the design parameters (such as mixing time, heat- and mass-transfer coefficient, drag coefficient, dissipation rate, etc.). This understanding of flow structures has brought improvements in the formulations of heuristic models of mass and heat transfer. This review makes an effort in developing insights into the views of earlier established analytic and heuristic theories of heat and mass transfer. The recently revealed dynamics of flow structures (as uncovered through the use of various techniques) has helped in furthering the efforts of developing new theories of heat, mass, and momentum transfer. Such an understanding between the structure dynamics and the transport phenomena has helped in the optimization of flow pattern (for instance, maximization of ratios of heat and mass transfer, as well as mixing, with respect to energy input). In this direction, some success stories have been described that have already been implemented in industry and have good potential for implementation.
A computational analysis using standard k-e model, RSM and LES has been carried out for jet loop reactors (JLR) to investigate the mean and turbulence quantities. These simulations have revealed that the flow in JLR was different from the self-similar round jets. RSM and LES showed better agreement with PIV measurements compared with standard k-e model. The modeled turbulence production and transport in k-e model overpredicted those estimated from LES data. To reduce the limitations, modified k-e models have been evaluated for JLR. Also, a hybrid k-e model has been suggested, which was found to perform better than other modified k-e models. This model was also found to hold for stirred tank reactors (STRs). Mixing time analysis has been carried out for JLR and STR at same power consumption. It has been shown that JLR can be inferior to STR if proper nozzle diameter is not selected. V V C 2009 American Institute of Chemical Engineers AIChE J,
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