Stirred Tank

This paper presents an experimental study on the mass transfer performance in a gas-liquid stirred vessel. Three impellerscircular with flat blades, rectangular with slightly curved blades, and trapezoidal with flat perforated bladeswere tested for two aspect ratios of the water column inside a tank. The study aimed at identifying the type of impeller and operating conditions that maximize the aeration efficiency of vortex aerators and at verifying whether the usage of impellers with perforated blades could decrease the energy consumption while maintaining a good aeration efficiency. The results revealed the existence of two critical impeller speeds. At the first critical speed, the operating regime changes from subcritical or non-aerated to supercritical or aerated. The vortex specific interfacial area increases almost linearly up to the second critical speed and then tends to stabilize. The impeller with perforated blades did not confirm the expectations, its efficiency being very poor. The rectangular impeller was the most efficient in the subcritical regime. The circular impeller performed better in the supercritical regime. These two impellers were found to operate roughly in the same range as other conventional impellers for an aspect ratio of 1. This finding, coupled with the geometrical simplicity, recommends the circular and rectangular impellers for usage in tank aerators and, possibly, in bioreactors.

Computational and experimental methods have been used to investigate the flow field, power and mixing time in a fully baffled stirred vessel with two six-blade Rushton turbines. Flow in a stirred tank involves interactions between flow around rotating impeller blades and stationary baffles. In computational fluid dynamics (CFD), the flow field was developed using the sliding mesh (SM) approach. The large eddy simulation (LES) was used to model the turbulence. For validation of simulation results two series of experiments were performed: (i) velocity measurements of the liquid phase using particle image velocimetry (PIV) and (ii) concentration measurements of the determining tracer in the liquid phase using the planar laser-induced fluorescence (PLIF) technique. In each series three different rotational speeds of impellers: 225, 300 and 400 rpm were employed. The stirring power input was also calculated based on the PIV results. A considerable reduction in mixing time was achieved and stirring power input was increased by increasing the impeller speed. The satisfactory comparisons indicate the potential usefulness of this CFD approach as a computational tool for designing stirred reactors.

Dispersion of two immiscible liquids is commonly used in chemical industry as wall as in metallurgical industry e. g. extraction process. The governing property is droplet size distribution. The droplet sizes are given by the physical properties of both liquids and flow properties inside a stirred tank. The first investigation stage is focused on in-situ droplet size measurement using image analysis and optimizing of the evaluation method to achieve maximal result reproducibility. The obtained experimental results are compared with multiphase flow simulation based on Euler-Euler approach combined with PBM (Population Balance Modelling). The population balance model was, in that specific case, simplified with assumption of pure breakage of droplets.

The optimum positions for the additive injection and probe location in a dual-Rushton stirred tank have been investigated using Response Surface Methodology. Therefore, by keeping these two parameters (probe location and additive injection point) fixed during consequent experiments, the single Rushton impeller has been compared with a dual Rushton impeller stirred tank. The results have shown that the mixing time for single impeller systems, is lower than that for double impeller systems, so to obtain a lower mixing time in multi impeller systems, it is necessary to obtain the relation between the height of the liquid and the number of impellers. In the next part, the obtained mixing time is compared against simulation results and published data based on experimental works for a Rushton impeller in different configurations of a vessel. Also in this work the effects of superficial gas velocity and impeller rotational speed for both systems have been investigated.

The local shear rate generated in a cylindrical tank equipped with a Rushton turbine was investigated using particle image velocimetry in a shear-thinning fluid (Carbopol). This non-Newtonian fluid was used in an attempt to mimic fermentation broths. Three Reynolds numbers corresponding to the transition regime were investigated. The hydrodynamics is analyzed, and the velocity field is decomposed by proper orthogonal decomposition into mean flow, organized motion, and turbulence. Then, the contributions of each flow structure to the total dissipation of kinetic energy are presented. The spatial heterogeneity of shear rate is discussed and a new expression is proposed for shear rate. This work shows that the local shear rate is highly heterogeneous in a tank. Future works will need to focus on other types of stirrer and investigate the effect of scaling up reactors on the shear rate heterogeneity. V

The discharge flow of a Rushton turbine is characterized by the formation of coherent vortex structures induced by the blade motion and called trailing vortices. The objective here is to assess the ability of computational fluid dynamics (CFD) to represent the trailing vortices and their relationship with turbulence properties. To this end, two simulations have been realized: an unsteady Reynolds-Averaged Navier-Stokes (URANS) simulation and a Large Eddy Simulation (LES) simulation. The trajectory of the trailing vortices predicted by the simulations has been compared with previous works. This comparison shows that the URANS simulation does not predict properly the trailing vortices while the LES results are very close to the experimental ones. As a consequence, the turbulence properties spatially correlated to the trailing vortices are well predicted by LES but not by URANS simulation.

Dissipation rate of turbulent kinetic energy is a key parameter in a number of chemical and biochemical processes. In stirred tank, its overall value is readily estimated; however its distribution is far from homogeneous. The experimental determination of dissipation rate is difficult and often limited to small regions of a reactor. Therefore, CFD appears to be a useful tool to estimate the distribution of the turbulence dissipation rate. In this work, two kinds of simulation have been performed: a simulation based on the URANS equations and a Large Eddy Simulation with the Smagorinski subgrid scale model. The aims of the present paper are (1) to compare URANS and LES simulations to PIV data in terms of velocity, kinetic energy and turbulence dissipation rate and (2) to study the influence of the constant in the subgrid scale model used in LES. In term of mean velocity, URANS simulation and especially LES are in good agreement with PIV experiments. In term of total kinetic energy, both simulations also give good results; however URANS simulation fails to predict the repartition of total kinetic energy between its periodic and turbulent components in the vicinity of the impeller. The mean velocity and the kinetic energy are shown to be independent of the value of the constant C S in the Smagorinski model. The overall dissipation rate is well predicted by URANS simulation but not its distribution in the impeller stream. For LES, the amplitude of local and overall dissipation rate is shown to be strongly dependent of the value of the constant C S. However, its distribution in the stream is similar regardless of the value of C S and best reproduced than by the URANS simulation.

Department of Chemical Engineering, Institute of Chemical Technology, Mumbai-40019, India<br> ASAL, Department of Chemical Engineering, Vidyasagar National Institute of Technology,<br> Nagpur-440 010, Maharashtra, India<br> E-mail: ab.pandit@ictmumbai.edu.in<br> Manuscript received online 07 April 2020, accepted 12 June 2020 Metal salts deposition on the surfaces of heat transfer equipment is one of the critical problems faced in chemical industries.<br> A slight change in operating parameters of the process fluid after coming in contact with a foreign surface or particle often<br> results in equilibrium shift of the dissolved metal ions leading to scale deposition. This causes reduced thermal efficiency,<br> high maintenance cost and increased pressure drop in the unit has been observed followed by plant shut down for maintenance.<br> The present work involves a systematic simulation and modelling of a Stirred Tank Reactor (STR) with ...

Local and temporal variations of the particle cloud formed in a cylindrical mixing vessel were investigated experimentally. Different particle sizes (0.5, 1, and 2 mm) and volumetric concentration up to 20 vol % were evaluated at different impeller speeds. The time-averaged cloud height was linear with impeller frequency and with volume concentration. Suspensions with larger particles had a lower average cloud height, while the standard deviation for the temporal cloud height variation was larger. Two strong periodic phenomena were identified to be dominating the particle cloud height variations. The frequencies were linear with impeller speed, resulting in dimensionless frequencies of S 1 50.02-0.03 and S 2 50.05-0.06. The frequencies were affected by neither the particle size nor the volumetric concentration. The amplitude showed no dependency on the particle size, but the S 2 amplitude significantly decreases and S 1 increases with increasing solid concentration. The results were compared to LES/discrete element method simulations and showed a fair agreement.

h i g h l i g h t s Direct Numerical Simulations were performed in baffled and unbaffled stirred tanks. Transition from creeping to early turbulent flow was studied in both systems. Bifurcation between baffled and unbaffled vessels was correctly predicted and explained. Bifurcation was found not to strictly coincide with the transition to turbulence. A travelling wave disturbance rotating with the impeller was found in the unbaffled tank.

We analyse the steady-state operation of a continuous flow bioreactor in which the biochemical reaction is governed by noncompetitive substrate inhibition (Andrews kinetics). A generalized reactor model is used in which the well-stirred bioreactor and the idealized membrane bioreactor are special cases. As generic properties of systems subject to substrate inhibition have been obtained by other authors, we discuss reaction engineering features specific to Andrews kinetics.

In process engineering as chemical and biotechnological industry, agitated vessels are commonly used for various applications; mechanical agitation and mixing are performed to enhance heat transfer and improve specific Physico-chemical characteristics inside a heated tank. The research subject of this work is a numerical investigation of the thermo-hydrodynamic behavior of viscoplastic fluid (Casson–Papanastasiou model) in a stirred tank, with introducing a new anchor impeller design by conducting some modifications to the standard anchor impeller shape. Four geometry cases have been presented for achieving the mixing process inside the stirred vessel, CAI; classical anchor impeller, AI1; anchor impeller with added horizontal arm blade, AI2 and AI3 anchor impeller with two and three added arm blades, respectively. The investigation is focused on the effect of inertia and plasticity on the thermo-hydrodynamic behavior (flow pattern, power consumption, and heat transfer) by varying th...

Several industrial fields require mixing and mechanical agitation processes. This operation is mainly used to enhance heat and mass transfer inside stirred tank systems and improve the degree of homogeneity to obtain a high-quality final product. The main goal of this research paper is to analyze the thermal and hydrodynamic behavior of non-Newtonian nanofluid (Bingham–Papanastasiou–Al2O3) inside a symmetrically stirred tank. A 3D numerical study has been conducted for a stationary laminar flow inside a symmetric cylindrical vessel under influencing parameters, including the inertia parameter (Re=1, 20, 100) and the volume fraction of nanoparticles (Ø=0.02, 0.06, 0.1) with different geometric configurations, has been introduced into the stirring system. According to the findings, with high inertia (Re=100), the heat transfer inside the stirred tank is enhanced. Furthermore, increasing the nanoparticle fraction volume had a significant impact on the acceleration of heat transfer alon...

Hydromechanics of a stirred tank generated by a classical anchor and an anchor blade are numerically simulated. The numerical results gives good prediction of the hydrodynamics such as the velocity field, the turbulent kinetic energy and its dissipation rate. These results are obtained by the computational fluid dynamics (CFD) code. The mechanical deformation of these anchors is calculated by the computational structural dynamics (CSD) code. The deformed shape of the anchor blade has been found in turbulent flow. However, the anchor blade has not been affected by this flow. Predictions of the numerical results have been compared with literature data and a satisfactory agreement has been found.

A large number of chemical operations, biochemical or petrochemical industry is very depending on the rheological fluids nature. In this work, we study the case of highly viscous of viscoplastic fluids in a classical system of agitation: a cylindrical tank with plate bottom without obstacles agitated by gate impeller agitator. We are interested to the laminar, incompressible and isothermal flows. We devote to a numerical approach carried out using an industrial code CFD Fluent 6.3.26 based on the method of finites volumes discretization of Navier-Stokes equations formulated in variables (U.V.P). The threshold of flow related to the viscoplastic behavior is modeled by a theoretical law of Bingham. The results obtained are used to compare between the five configurations suggested of power consumption. We study the influence of inertia by the variation of Reynolds number. This is an Open Access article distributed under the terms of the Creative Commons Attribution License 4.0, which permits distribution, and reproduction in any medium, provided the original work is properly cited.

Flotation in mechanically agitated cells has been the workhorse of the mining industry, but our quantitative understanding of the effect of microturbulence generated by agitation on flotation is still very limited. This paper aims to review the literature on quantifying the microturbulence effects on bubble-particle interactions in flotation. The particular focus is on the stochastic description of bubble-particle interactions in the turbulent flow which is a random field. We briefly review the stochastic description of microturbulence and motions of particles of micrometre sizes and bubbles of millimetre sizes in the isotropic turbulence of mechanical flotation cells. The key starting point is the generic equation of motion, which can be decomposed into the mean turbulent variables and fluctuating turbulent variables. The turbulent flow of the carrying liquid is characterised using isotropic turbulence theory. The next focus is on reviewing bubble-particle turbulent collision and detachment interactions. Bubble-particle turbulent collision is poorly quantified; no quantitative models of the bubble-particle turbulent collision efficiency relevant for flotation are available. Current theories on bubble-particle turbulent detachment face some deficiencies. In assessing the microturbulence effect on bubble-particle detachment, the majority of studies only consider the particle acceleration in the centrifugal direction but ignore the transverse acceleration of particles, which is due to turbulent shear flow. Critically, contact angle required in quantifying the detachment is not constant, single-valued as considered in the theories, but can vary from receding to advancing value during the relaxation of the triple contact line on the particle surface. The latest experiments show that multiple-valued contact angle can significant affect stability and detachment of floating particles. Finally, quantifying the microturbulence effect on flotation requires further research.

Computational and experimental methods have been used to investigate the flow field, power and mixing time in a fully baffled stirred vessel with two six-blade Rushton turbines. Flow in a stirred tank involves interactions between flow around rotating impeller blades and stationary baffles. In computational fluid dynamics (CFD), the flow field was developed using the sliding mesh (SM) approach. The large eddy simulation (LES) was used to model the turbulence. For validation of simulation results two series of experiments were performed: (i) velocity measurements of the liquid phase using particle image velocimetry (PIV) and (ii) concentration measurements of the determining tracer in the liquid phase using the planar laser-induced fluorescence (PLIF) technique. In each series three different rotational speeds of impellers: 225, 300 and 400 rpm were employed. The stirring power input was also calculated based on the PIV results. A considerable reduction in mixing time was achieved and stirring power input was increased by increasing the impeller speed. The satisfactory comparisons indicate the potential usefulness of this CFD approach as a computational tool for designing stirred reactors.

Computational and experimental methods have been used to investigate the flow field, power and mixing time in a fully baffled stirred vessel with two six-blade Rushton turbines. Flow in a stirred tank involves interactions between flow around rotating impeller blades and stationary baffles. In computational fluid dynamics (CFD), the flow field was developed using the sliding mesh (SM) approach. The large eddy simulation (LES) was used to model the turbulence. For validation of simulation results two series of experiments were performed: (i) velocity measurements of the liquid phase using particle image velocimetry (PIV) and (ii) concentration measurements of the determining tracer in the liquid phase using the planar laser-induced fluorescence (PLIF) technique. In each series three different rotational speeds of impellers: 225, 300 and 400 rpm were employed. The stirring power input was also calculated based on the PIV results. A considerable reduction in mixing time was achieved and stirring power input was increased by increasing the impeller speed. The satisfactory comparisons indicate the potential usefulness of this CFD approach as a computational tool for designing stirred reactors.

HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

T he mixing of two water feedstreams is investigated in a 0.02 m 3 continuous stirred tank, from concentration measurements. These measurements are carried out with a laser sheet in a very thin vertical plane through the vessel tangent to the impeller. The proportionality between the grey level obtained by planar laser induced èuorescence and the local concentration has been previously determined. The éelds of instantaneous concentration, temporal variance and mixing index are processed from the grey level images.

Understanding and modeling the complex interactions between biological reaction and hydrodynamics are a key problem when dealing with bioprocesses. It is fundamental to be able to accurately predict the hydrodynamics behavior of bioreactors of different size and its interaction with the biological reaction. CFD can provide detailed modeling about hydrodynamics and mixing. However, it is computationally intensive, especially when reactions are taken into account. Another way to predict hydrodynamics is the use of "Compartment" or "Multi-zone" models which are much less demanding in computation time than CFD. However, compartments and fluxes between them are often defined by considering global quantities not representative of the flow. To overcome the limitations of these two methods, a solution is to combine compartment modeling and CFD simulations. Therefore, the aim of this study is to develop a methodology in order to propose a compartment model based on CFD simulations of a bioreactor. The flow rate between two compartments can be easily computed from the velocity fields obtained by CFD. The difficulty lies in the definition of the zones in such a way they can be considered as perfectly mixed. The creation of the model compartments from CFD cells can be achieved manually or automatically. The manual zoning consists in aggregating CFD cells according to the user's wish. The automatic zoning defines compartments as regions within which the value of one or several properties are uniform with respect to a given tolerance. Both manual and automatic zoning methods have been developed and compared by simulating the mixing of an inert scalar. For the automatic zoning, several algorithms and different flow properties have been tested as criteria for the compartment creation.

Hydromechanics of a stirred tank generated by a classical anchor and an anchor blade are numerically simulated. The numerical results gives good prediction of the hydrodynamics such as the velocity field, the turbulent kinetic energy and its dissipation rate. These results are obtained by the computational fluid dynamics (CFD) code. The mechanical deformation of these anchors is calculated by the computational structural dynamics (CSD) code. The deformed shape of the anchor blade has been found in turbulent flow. However, the anchor blade has not been affected by this flow. Predictions of the numerical results have been compared with literature data and a satisfactory agreement has been found.

-Dans cet article, uneétude comparative entre les caractéristiques hydrodynamiques de l'écoulement laminaire généré en cuves agitées par différents mobiles de proximité de type mono et double vis avec profils simple et modifié aété menée en utilisant la simulation numérique. Cetteétude aété abordéeà l'aide d'un code spécifique de dynamique des fluides numérique (CFD), basé sur la résolution deséquations de Navier-Stokes par la méthode des volumes finis. Les résultats numériques obtenus montrent bien l'influence de la forme du mobile d'agitation sur le comportement de l'écoulement au sein de la cuve. En effet, on remarque que l'utilisation d'une visà profil modifié favorise un champ de vitesse plus actif que dans le cas des configurationsà profil simple. Par ailleurs, et dans le cas d'une double vis, on note que l'effet de la dissipation visqueuse et du pompage est nettement plus prépondérant, présentant ainsi une meilleure efficacité de pompage et de consommationénergétique. La comparaison de nos résultats numériques avec les résultats expérimentaux tirés de la littérature montre une bonne adéquation, ce qui prouve la validité de la méthode d'analyse adoptée.

A new experimental approach has been set up for the identification of reasonable break-up mechanisms in stirred dispersions and of a physically-based model for the daughter drop size distribution. In the experiments, breakage of a single organic droplet and the subsequent fragments formation are analyzed by image processing techniques. The experimental data are then fitted by means of a daughter drop size distribution function written in terms of a probability density function (pdf). The mathematical approach that matches the theoretical requirements and that provides the best fit of the experimental data is the pdf proposed by Diemer and Olson (2002). This is a purely statistical model that does not contain any dependence on physical parameters, such as turbulence intensity and mother drop size. Based on the experimental results, an extension of the Diemer and Olson model is derived, in which dependence on the Weber number as well as on the mother drop diameter can be inserted. Other models appeared in the literature (Coulaloglou and Tavlarides, 1977; Konno et al., 1983; Martínez-Bazán et al., 1999) are also discussed and compared with experimental data. Out of them, the only model that approaches experimental data in a satisfactory way is the model by Martínez-Bazán et al. (1999). Eventually the presence of the maximum break-up probability for symmetric breakage, in contrast with some theoretical models published in the literature, is explained and justified by means of energy considerations.

The performance of three sparger diameters (D S = 0.6D, D S = D, D S = 1.6D) in combination with three positions (below, above or level with the impeller) for gas-liquid dispersion and mass transfer were evaluated in the case of the Rushton turbine and the A315 propeller in up-or down-pumping mode. The results show that the best results in terms of gas handling and mass transfer capacities are obtained for all impellers with the sparger placed below it and with a diameter at least equal to the impeller diameter. For the sparger position below the agitator, the k L a values of the Rushton turbine are greater than those of the A315 propeller, whatever the pumping mode. The A315 propeller in up-pumping mode is, however, more economically efficient in terms of mass transfer. In all cases, the up-pumping mode gives better results than the down-pumping one.

Impellers play a key role in flotation cells, as the turbulence generated through agitation aids particle suspension, air dispersion and particle-bubble collision. Therefore, it is important to understand the effect that different impeller designs have on flotation hydrodynamics, as small variations could enhance flotation performance. The study of flotation hydrodynamics is, however, a complex task due to the nature of flotation systems, which are opaque, multiphase, and polydisperse. In this paper, the impact of impeller design modifications on the hydrodynamics of a flotation cell was quantified for the first time in a three-phase system. Two different impeller designs, with and without a stator, were assessed using positron emission particle tracking (PEPT), a technique that allows the position and velocity of radioactive particle tracers within an opaque vessel to be determined. A novel PEPT data analysis strategy, as well as a statistical analysis on the basis of the Jensen-Shannon distance, were used. This statistical analysis, applied for the first time to PEPT data, facilitated the comparison of the different designs, by generating a robust quantification of their hydrodynamic differences. The experimental results showed that the stator significantly modified the hydrodynamics within the flotation cell, distorting the lower mixing loop that is characteristic of radial impellers. The use of a stator also resulted in the reduction of particle velocity and swirling outside of the impeller-stator region, both at the level of the impeller and, notably, at the pulp-froth interface. These findings have important implications for impeller-stator design, evidencing that the impeller has a direct effect on the hydrodynamics of the pulp and froth.

We analyse the steady-state operation of a continuous flow bioreactor in which the biochemical reaction is governed by noncompetitive substrate inhibition (Andrews kinetics). A generalized reactor model is used in which the well-stirred bioreactor and the idealized membrane bioreactor are special cases. As generic properties of systems subject to substrate inhibition have been obtained by other authors, we discuss reaction engineering features specific to Andrews kinetics.

Umbilical cord blood transplantation is clinically limited by its low progenitor cell content. Ex vivo expansion has become an alternative to increase the cell dose available for transplants. Expansion has been evaluated in several ways such as static cultures combining growth factors or mimicking the natural microenvironment using co-culture systems. However, static cultures have a small volume capacity and therefore large-scale expansion has been addressed using bioreactors. These and other biotechnological approaches for the expansion of hematopoietic progenitors and their utility to study several aspects of hematopoietic stem cell biology are discussed here. Keywords 2D-culture Á 3D-culture Á Human cells Á Leukemia Á Rotating wall vessel Á Stirred tank Á Transplant

In this work the characterization of hydrodynamic fields of incompressible yield stress fluid with regularization model of Bercovier and Engelman in a cylindrical vessel not chicaned equipped with circular anchor stirrer was undertaken by means of numerical simulation using computational fluid dynamics. Simulations flow of a Bingham fluid agitated by straights blades anchor was used to validate the rheological model implemented of the fluid treated. The flow structures, and especially the effect of inertia, the plasticity and the yield stress, are discussed. We have analyzed also the influence of rheological parameters on the hydrodynamic flow behaviours, such as the velocity components and the global characteristic like power consumption.

Sir: The authors are very much aware of Professor Fort's pioneering work and have referred to it in our comprehensive review 1 and in the present paper. 2 The previous works by Medek and Fort 3 and Fort and Medek 4 have calculated the pumping capacity of the impeller and total volumetric flow rate of the agitated liquid by means of mean circulation times of the flow-follower (the indicating particle). 5 However, the nonintrusive optical measurement techniques such as LDA (laser Doppler anemometer) can throw more precision in the measured flow numbers. The comprehensive study published by Kumaresan et al. 2 studied the influence of internals on the velocity distribution plus the importance of the secondary flow number on homogenization quantitatively. In the previous work of Patwardhan and Joshi 6 and in the reply to the comments by Professor Fort, 7 the authors emphasized the importance of the secondary flow number and its importance for the purpose of homogenization. The presence of baffle clearance with the increase/decrease in the number of baffles has a significant effect both on the primary and secondary flow number (Table 1). It is important to note that the value E decreased in the presence of baffle clearance by 37, 58, and 74% for 2-, 4-, and 6-baffle configurations, respectively. Similarly, the value E T decreased in the presence of baffle clearance by 55, 62, and 77% for 2-, 4-, and 6-baffle configurations, respectively. So the variation in the flow efficiencies has to affect the operation (crystallizers and bioreactors) significantly. The flow pattern and/or mixing time corresponding to the conical draft tube 8 will be entirely different from the results of the narrow draft tube. 2 The study by Kumaresan et al. 2 not only presents the flow pattern for the narrow draft tube but also illustrates the effect of the length of the draft tube on the flow pattern and, hence, on the mixing performance. The present work, in particular, notes that the draft tube configuration with L dr = 0.167 results in an increase in secondary flow number 4850

h i g h l i g h t s Direct Numerical Simulations were performed in baffled and unbaffled stirred tanks. Transition from creeping to early turbulent flow was studied in both systems. Bifurcation between baffled and unbaffled vessels was correctly predicted and explained. Bifurcation was found not to strictly coincide with the transition to turbulence. A travelling wave disturbance rotating with the impeller was found in the unbaffled tank.

h i g h l i g h t s Direct Numerical Simulations were performed in baffled and unbaffled stirred tanks. Transition from creeping to early turbulent flow was studied in both systems. Bifurcation between baffled and unbaffled vessels was correctly predicted and explained. Bifurcation was found not to strictly coincide with the transition to turbulence. A travelling wave disturbance rotating with the impeller was found in the unbaffled tank.

Large-eddy simulation (LES) of mixing process in a baffled tank was presented. The impeller rotation was modeled using the sliding mesh technique. In this study the CFD code was used for simulation of a standard vessel agitated by a 6-blade Rushton turbine and results were evaluated in terms of the predicted flow field, power number, mean velocity components, mixing time, turbulent kinetic energy and turbulent dissipation rate using published experimental data. Subsequently, the effects of varying injection position of the passive scalar have been investigated. The results show that LES is a reliable tool to investigate the unsteady behavior of the turbulent flow in stirred tank.

Experimental data for the flow induced by an impeller in a cylindrical tank were presented and analyzed in Part I (Dorm et aZ.. 1994, Chem. finnnu Sci. 49,549-560). In Part II, a Coriolis computer model is de&loped and employed to predict flow characteristics in a 3D calculation in conjunciion with the standard k-e turbulence model. The computer model involves a coordinate transformation in which the computational coordinates rotate with the impeller, allowing the impeller to be treated as a stationary blade in a rotating tank. Mean velocities and turbulent energy in the bulk liquid predicted by the computer model are observed to be in satisfactorv agreement with the measurements. The main discrepancies between the prediction and the experimentaidaia are analyzed.

In this work, the fluid dynamics behaviour of a model bioreactor specifically designed for a fermentative hydrogen production process is investigated by Computational Fluid Dynamics (CFD) simulations. The geometrical features of the bioreactor, which is a dual impellers baffled stirred tank provided with a draft tube and the gas-liquid characteristics of the vortex ingesting operating mode, make the modelling and the numerical solution tasks particularly challenging. The computational strategy is based on the two-phase formulation of the Reynolds Averaged Navier-Stokes equations in an Eulerian framework for both the continuous and the dispersed phase. The results of the simulations are compared with available experimental data under the same operating conditions and an identical bioreactor geometry. The reliability of the predicted overall hydrodynamics behaviour and the accuracy of the turbulent two-phase mean velocity field are evaluated and critically discussed.

Hydromechanics of a stirred tank generated by a classical anchor and an anchor blade are numerically simulated. The numerical results gives good prediction of the hydrodynamics such as the velocity field, the turbulent kinetic energy and its dissipation rate. These results are obtained by the computational fluid dynamics (CFD) code. The mechanical deformation of these anchors is calculated by the computational structural dynamics (CSD) code. The deformed shape of the anchor blade has been found in turbulent flow. However, the anchor blade has not been affected by this flow. Predictions of the numerical results have been compared with literature data and a satisfactory agreement has been found.

In this work study the location of interface in a stirred vessel with a Concave impeller by computational fluid dynamic was presented. To modeling rotating the impeller, sliding mesh (SM) technique was used and standard k-ε model was selected for turbulence closure. Mean tangential, radial and axial velocities and also turbulent kinetic energy (k) and turbulent dissipation rate (ε) in various points of tank was investigated. Results show sensitivity of system to location of interface and radius of 7 to 10cm for interface in the vessel with existence characteristics cause to increase the accuracy of simulation.

The fluid flows and power consumption in a vessel stirred by anchor impellers are investigated in this paper. The case of rheologically complex fluids modeled by the Bingham-Papanastasiou model is considered. New modifications in the design of the classical anchor impeller are introduced. A horizontal blade is added to the standard geometry of the anchor, and the effect of its inclination angle (α) is explored. Four geometrical configurations are realized, namely: α = 0°, 20°, 40°, and 60°. The effects of the number of added horizontal blades, Reynolds number, and Bingham number are also examined. The obtained findings reveal that the most efficient impeller design is that with (case 4) arm blades inclined by 60°.This case allowed the most expansive cavern size with enhanced shearing in the whole vessel volume. The effect of adding second horizontal arm blades (with 60°) gave better hydrodynamic performance only with a slight increase in power consumption. A significant impact of Bi...

This work is aimed at investigating the mixing process of highly viscous paints, used to colour leathers in the tanning industry, through Computational Fluid Dynamics (CFD). In particular, a mixing tank is fed with a master liquid and different liquid pigments and then stirred by Cowles impellers in order to obtain a paint of a uniform colour. The typical dynamic viscosity of the liquids in this process is μ ~ O(0.1-10) Pa·s, while the Cowles rotational speed is usually very high, i.e. 3000-5000 rpm. The numerical model is based on the solution of the unsteady Reynolds-Averaged Navier–Stokes (RANS) equations for continuity, momentum and species mass fractions, the latter being used to describe the different components. The impeller motion is modelled through the Sliding Deforming Mesh (SDM) approach, using rotating (unstructured) meshes in the impeller region and a static (structured) mesh in the remainder of the tank. The master liquid and coloured pigments are assumed to stratify ...

Understanding the flow in stirred vessels can be useful for a wide number of industrial applications. There is a wealth of numerical simulations of stirring vessels with standard impellers such as the Rushton turbine and pitch blade turbine. Here, a CFD study has been performed to observe the spatial vari-ations (angular, axial and radial) of hydrodynamics (velocity and turbulence field) in unbaffled stirred tank with concave-bladed disc turbine (CD-6) impel-ler. Three speeds (N =2 96, 638 and 844.6 rpm) have been considered for this study. The angular variations of hydrodynamics of stirred tank were found to be lower as compared to axial and radial variations.

In this study computational fluid dynamics (CFD) approach was used to study mixing in an Industrial gold leaching tank. The objective was to analyze the extent of mixing in the tank by producing visual images of the various mixing zones in the tank domain. Eddy viscosity plots that characterise the extent of mixing in the tank were generated in the flow field obtained by an Eulerian-Eulerian approach. The extent of mixing was found to be greatest in the circulation loops of the impeller discharge region and least at the top and bottom portions of the tank. Trailing vortices that contribute to some level of mixing were identified in between the impeller blades. This approach could be used to enhance optimum design of mixing vessels and to eliminate the need for pilot plants.

Sir: The authors are very much aware of Professor Fort's pioneering work and have referred to it in our comprehensive review 1 and in the present paper. 2 The previous works by Medek and Fort 3 and Fort and Medek 4 have calculated the pumping capacity of the impeller and total volumetric flow rate of the agitated liquid by means of mean circulation times of the flow-follower (the indicating particle). 5 However, the nonintrusive optical measurement techniques such as LDA (laser Doppler anemometer) can throw more precision in the measured flow numbers. The comprehensive study published by Kumaresan et al. 2 studied the influence of internals on the velocity distribution plus the importance of the secondary flow number on homogenization quantitatively. In the previous work of Patwardhan and Joshi 6 and in the reply to the comments by Professor Fort, 7 the authors emphasized the importance of the secondary flow number and its importance for the purpose of homogenization. The presence of baffle clearance with the increase/decrease in the number of baffles has a significant effect both on the primary and secondary flow number (Table 1). It is important to note that the value E decreased in the presence of baffle clearance by 37, 58, and 74% for 2-, 4-, and 6-baffle configurations, respectively. Similarly, the value E T decreased in the presence of baffle clearance by 55, 62, and 77% for 2-, 4-, and 6-baffle configurations, respectively. So the variation in the flow efficiencies has to affect the operation (crystallizers and bioreactors) significantly. The flow pattern and/or mixing time corresponding to the conical draft tube 8 will be entirely different from the results of the narrow draft tube. 2 The study by Kumaresan et al. 2 not only presents the flow pattern for the narrow draft tube but also illustrates the effect of the length of the draft tube on the flow pattern and, hence, on the mixing performance. The present work, in particular, notes that the draft tube configuration with L dr = 0.167 results in an increase in secondary flow number 4850

Processing in the minerals and energy resources industries almost always involves complex multiphase flow systems designed to achieve mixing, reaction or separation. Computational fluid dynamics (CFD) modelling can be a powerful tool to assist in improving these processes or designing new processes and equipment. However proper account needs to be taken for multi-phase interactions, multi-scale effects, and additional non-flow physics and chemistry that can be critical to the process.

Heat transfer during agitation of Bingham viscoplastic fluid is studied in this paper. The fluid is agitated with an anchor impeller and the heating is made by a jacketed wall of the stirred vessel. Transfers in the agitated vessel, translating hydrodynamic and thermal phenomena, are numerically predicted by means of Computational Fluid Dynamics (CFD) in transient regime. The purpose of this numerical study is to identify the rigid zones and to optimize mixing and heating performances. The Navier-Stokes and energy equations are discretized using finite volume method, and a two-dimensional analysis of the hydrodynamic and transient thermal behaviours generated in the agitated vessel are performed. Fluid rheology is modeled by the Bingham approximation and Papanastasiou's regularization model. Results show the presence of recirculation zones and permit to explain the unpredicted Nusselt number increasing when Oldroyd number increases. This study shows also the importance of the anchor position on the size and the shape of the rigid zones and on the heating performances.

This work shows improvements made in mixing operations at water treatment plants, as a result of the hydrodynamic analysis of the mixing processes carried out by the use of a Finite Element Model. The code, developed in the Civil Engineering Department of the University of La Coruña, Spain, solves the Navier-Stokes equations that rule viscous incompressible flow by using a Streamline Upwind/Petrov-Galerkin (SUPG) stabilization technique. The incorporation of the SUPG formulation leads to obtaining stable solutions for Reynolds numbers of a moderate order in connection with meshes that are not very refined. Some water treatment units present significant deficiencies in their design. The numerical evaluation of the flow avoids the high expenses of the trial-and-error processes involved in installing and removing the mixing mechanisms and those derived from the need to halt the water treatment processes. As a result, an optimum design of the treatment plant is obtained at a low cost. Water Environ. Res., 79 625 (2007).