Andre Bakker

Retired

The application of large eddy simulation (LES) to the prediction of large-scale chaotic structures in stirred tanks is investigated. Typical stirrer configurations representing different flow regimes are assessed: a single radial pumping impeller, single axial pumping impellers, multiple hydrofoil impellers, and a retreat curve impeller. The dynami...

High-viscosity mixing applications occur in most chemical process industries (CPI) plants. High-viscosity applications occur in the production of food, paint, drilling mud, and greases, to name a few. Mixing can occur in pipeline systems with motionless mixers, or in vessels using mechanical agitators, depending on the application and the process r...

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 Introductio...

This document contains the lectures that I gave for the Reacting Flows (ENGS 199) class at the Thayer School of Engineering at Dartmouth College in 2006. Reacting flows is a wide field: in this class the focus was on modeling of liquid reaction in stirred tanks as used in the process industries. These lectures are provided at no charge for educatio...

This document contains the lectures for the Computational Fluid Dynamics (ENGS 150) class that I taught at the Thayer School of Engineering at Dartmouth College from 2002-2006. These lectures are provided – at no charge - for educational and training purposes only. You are welcome to include parts of these lectures in your own lectures, courses, or...

The absence of a systematic scale-up approach for biological conversion of cellulosic biomass to commodity products is a significant bottleneck to realizing the potential benefits offered by such conversion. Motivated by this, we undertook to develop a scale-up approach for conversion of waste paper sludge to ethanol. Physical properties of the sys...

The model of South et al. [South et al. (1995) Enzyme Microb Technol 17(9): 797-803] for simultaneous saccharification of fermentation of cellulosic biomass is extended and modified to accommodate intermittent feeding of substrate and enzyme, cascade reactor configurations, and to be more computationally efficient. A dynamic enzyme adsorption model...

The aim of this work is to investigate the effect of the shaft eccentricity on the hydrodynamics of unbaffled stirred vessels. The difference between coaxial and eccentric agitation is studied using a combination of experiments carried out by particle image velocimetry, that provide an accurate representation of the time-averaged velocity, and comp...

Bakker A. (2006) The Future of CFD in Engineering Design. Presented at the Design Engineering Conference. The 16th National Manufacturing Week. Rosemont, IL, USA, March 20-23, 2006. This paper describes long-term trends in the field of fluid dynamics and how they will affect design engineering. Forecasts are made of what types of analyses designer...

So far, most of the computations aimed at providing detailed predictions of the flow field of mechanically stirred vessels have regarded baffled tanks, while less effort has been devoted to unconventional stirred vessels. Unbaffled vessel with eccentrical impeller can be adopted as an alternative to baffled vessels as this configuration is less pro...

This paper presents a way of modelling turbulence in multiphase flows within the context of the Reynolds-Stress Model. The model has been implemented in the general-purpose unstructured finite volume code Fluent V6. Two multiphase turbulence approaches were considered: mixture and dispersed models. The mixture model solves the Reynolds-stress trans...

THE THAYER SCHOOL OF ENGINEERING at Dartmouth College has offered a graduate class on CFD for several years now. Students learn about the theory behind ?uid flow and heat transfer, turbulence modeling,and numerical methods. The class ends with the students choosing a research topic of their own interest, and studying it using CED. A wide range of t...

Agrawal M., Bakker A., Prinkey M.T. (2004) Macroscopic Particle Model - Tracking Big Particles in CFD. AIChE 2004 Annual Meeting. Particle Technology Forum - Paper 268b. November 7-12, Austin, Texas, USA. Typical particle tracking models used in CFD software assume that particles are point masses that do not interact. Large particles immersed in t...

Montante G., Bakker A. (2004) Stirring Up the Phases in Tall Tanks, Fluent Newsletter, Fall 2004, page 8-9.

Bakker A., Wyman C.E., Lynd L.R., Mielenz J. (2004) Fermentation Modeling - Cellulosic Biomass Conversion. Presented by Bakker at ISMIP 5. The 5th International Symposium on Mixing in Industrial Processes. Seville, Spain, June 1-4, 2004. Today I will speak about the modeling of fermentations involving the conversion of cellulosic biomass to ethano...

An overview of computational fluid dynamics (CFD) is given, with an emphasis on the methods used to simulate mixing processes. Several techniques for modeling impellers are described in depth, including validation with experimental results. At the end of the chapter, a number of examples are presented that illustrate how CFD results can be used to...

Prediction of the mixing of multi-component fluids is important in many chemical process applications. Although laminar mixing is a complicated process per se (involving multi-component diffusion coefficients, for example), there is a far greater challenge in predicting mixing in turbulent flows because of their intrinsic, chaotic nature. In turbul...

Particle tracking models in most commercial CFD software, such as the discrete phase model (DPM) in FLUENT, assume that particles are point masses that do not interact. Large particles immersed in the fluid flow cannot be modeled using this type of approach. The modeling of large (macroscopic) particles requires special treatment to take into accou...

The performance of the concave blade BT-6 impeller was evaluated in a dual-impeller agitated tank (T= 0.48m, H/T=2). Power draw was measured and almost constant Pg/Pu ratio with aeration was found in agreement with what had been reported previously. Gas hold-up exhibited the same dependence on power per unit volume and superficial velocity as shown...

The photographs presented here illustrate the characteristics of turbulence.

Montante G., Rondini D., Bakker A., Magelli F. (2002) CFD Predictions of Solid Concentration Distributions in a Baffled Stirred Vessel Agitated With Multiple PBT Impellers. Presented by Montante at CHISA 2002, Prague, August 25-29, 2002. In the present work the predictive capabilities of CFD techniques as applied to solid-liquid stirred vessels ar...

Several recent approaches provide dramatic improvements in simulating flow in an agitated system. An investigation of the advantages and disadvantages of these new modelling options shows that the latest computational fluid dynamics (CFD) software provides excellent accuracy, well within the range of error of experimental methods.

Oshinowo L.M., Bakker A., (2001) CFD Modeling of solids Suspensions in Stirred Tanks. Presented by Oshinowo at the Symposium on Computational Modeling of Metals, Minerals and Materials, TMS Annual Meeting, February 17-21, 2002, Seattle, WA.

An understanding of the parameters that govern the just-suspended impeller speed, Njs, and the distribution of solids, is critical to the efficient operation of hydrometallurgical and other processes involving solid-liquid suspensions. In this paper, the distribution of solids in stirred tanks under a range of solids loadings (0.5 to 50 vol%) was p...

Researchers, equipment designers, and process engineers are increasingly using computational fluid dynamics (CFD) to analyze the flow and performance of process equipment. CFD allows for an in-depth analysis of the fluid mechanics and local effects in these types of equipment. This article discusses the technology behind CFD and illustrates today's...

In this study, 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 configuratio...

The application of computational fluid dynamics (CFD) for cyclone modelling on three-dimensional unstructured meshes is reviewed. Results of numerical simulations show agreement with laser Doppler anemometry measurements. The quality of turbulence modelling determines the fidelity of CFD predictions. An unstructured mesh was used to model the flow...

Gas dispersion impellers, such as the Rushton turbine and the more recent concave blade turbines, all feature blades that are symmetric with respect to the plane of the disc. This despite the fact that the gas usually enters from the bottom, and thus causes a flow pattern that is distinctly asymmetric. This article discusses the performance of a ga...

Design of mechanical agitators for solids suspension under gassed conditions can be achieved using a constant ungassed torque criterion if the solids suspension task is more difficult than the gas dispersion task. When gas dispersion is the more difficult task, design for complete gas dispersion will ensure solids suspension. Evidence supporting th...

In most work on agitated liquid–liquid dispersions, Rushton turbines have been used. Here, mean drop size and drop size distributions are reported for six different impellers covering 3 generic types over a range of mean specific energy dissipation rates. Both viscous and non-viscous dispersed phases have been used at concentrations by volume of 1...

The use of axial-flow impellers in the up-pumping mode has been increasing, but little performance and design information is available. Up-pumping pitched-blade and high-efficiency impellers have been studied in solids suspension applications, and their performance has been compared to and contrasted with that of the more conventional down-pumping...

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 comp...

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 tw o chemicals, exhibiting a competitive pair of reactions, are com...

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...

Mixing time measurements were carried out in turbulently stirred vessels with miscible liquids of differing viscosities and densities. Depending on the flow characteristics and liquid properties, three mixing regimes can be defined for buoyant additions: the stirrer controlled regime, the gravity controlled regime and the intermediate regime. While...

Digital particle image velocimetry (DPIV) has been used to examine the flow field in a vessel agitated by an axial-flow impeller in turbulent operation. Both a pitched blade turbine and a high-efficiency impeller were studied. Time series analysis indicates that the flow field is not steady; rather, it is subject to transients with frequencies well...

A technique is proposed to predict the gassed power draw of mixed impeller systems as a weighted average of the gassed power draws of the individual impellers. The success of this method requires determining the gassed power draws of the individual impellers in the proper gassed environment. In the case of the upper impellers, this environment is p...

The focus of this work is the experimental determination of cloud height (the height to which solids are suspendedrpar; in an agitated slurry as a function of the agitation intensity, solid physical properties, impeller type (pitched-blade or high efficiency), and system geometry. Cloud height is not strongly dependent on impeller type or solid phy...

Here's a guide on when to consider static mixers vs. agitators. Mixing mechanisms in static mixers are explored, as well as design formulations for the various types of operation.

Flow and reaction in a typical commercial scale autoclave LDPE reactor were modeled by a three-dimensional computational fluid dynamic (CFD) k-∈ model in order to shed light on the macrosegregation effects that can occur in these reactors. It is shown that the CFD model predicts significant differences from CSTR behavior. Results are discussed in t...

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 favourably with experimental data obtained by laser-Doppler velocimetry....

Myers K.J., Reeder M.F., Bakker A., Dickey D.S. (1997) A Parametric Study of the Performance of the Prochem Maxflo T Impeller. Recents Progres en Genie des Procedes, Vol. 11, No. 52, page 115-122.

Flow patterns in agitated vessels are influenced by geometry, particularly impeller diameter and impeller off-bottom clearance. Large impellers and/or high off-bottom clearances lead to reversed flow in which the flow at the base of the vessel is radially-inward as opposed to radially-outward as expected with axial-flow impellers. Reversed flow is...

Efficient fermenter operation depends on a variety of mixing related variables. Gas-liquid mass transfer, proper feed distribution, local oxygen concentration, shear rate distribution, and local mixing intensity all affect the productivity of the reactor. The viscous properties of the fermentation medium play an important role, especially for stron...

The flow field generated by the pitched-blade turbine in both laminar and turbulent operation has been investigated using laser Doppler velocimetry (LDV) and digital particle image velocimetry (DPIV) experimental techniques. This data has been used to critically evaluate the performance of computational fluid mixing (CFM) tools. It has been found t...

Computational Fluid Dynamics (CFD) methods for turbulent flows have reached a stage where it is feasible to apply them to real problems such as mixer design in chemical process industries. However, there are many potential difficulties for CFD to predict turbulent mixing in complex geometries and in flows involving large-scale, time-dependent pheno...

Mixing of a gas in a liquid is required in fermentation operations and a variety of oxygenation and hydrogenation processes. Agitation increases the mass transfer between the gas and the liquid phase. Gas-liquid reactors equipped with agitators are often operated at high power input and large gas holdup, making these units among the most difficult...

A traditional agitator impeller often functions as a rather inefficient pump because of the way it produces fluid motion and pressure head. However, one can improve the amount of flow or shear generated by an impeller at constant power consumption and torque by changing its design. For example, a high-efficiency, axial-flow impeller produces more f...

This paper focuses on the gas-dispersion characteristics of axial flow impellers. Various impellers are compared: an inclined blade impeller with flat blades, an A315 impeller and a Leeuwrik impeller. The goal is to gain insight in the gas-dispersion properties of such axial flow impellers. Therefore, the effects of impeller position and sparger ge...

In this paper a model is described for calculating the gas-liquid flow in stirred vessels. The general fluid flow code FLUENT is used for calculating the single-phase flow pattern. This flow pattern is used as input for an in-house code named GHOST! which calculates the distribution of the gas over the vessel on the basis of balance equations. A ma...

Single-phase flow patterns in stirred reactors were investigated both computational, with FLUENT, and experimental, by Laser Doppler Velocimetry (LDV). This was done for three different impeller types, viz. two axial flow impellers and a disc-turbine. The effect of geometrical modifications to the vessel geometry was studied as well. In particular,...

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 s...

By using flow-analysis and visualization techniques, engineers can identify potential trouble spots in mixing systems. Increasingly, designers of various types of mixing equipment are turning to modern tools for flow analysis, such as computational fluid dynamics, Laser Doppler velocimetry and particle image velocimetry. Such quantitative design me...

The mixing and flow characteristics of the Kenics Static Mixer have been assessed by both laboratory experimentation and, more recently by the use of Computational Fluid Dynamics modelling. Accurate visualization of the mixing process, prediction of mixture quality, pressure loss and heat transfer coefficients now enable the designs to be optimized...

Hicks M.T., Myers K.J., Corpstein R.R., Bakker A., Fasano J.B. (1993). Cloud Height, Fillet Volume, and the Effect of Multiple Impellers in Solids Suspension. Presented by Myers at the MIXING XIV Conference, June 20-25, 1993, Santa Barbara, California.

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 locat...

Bakker A., LaRoche R. (1993) Flow and Mixing with Kenics Static Mixers. Cray Channels, Volume 15, Number 3, page 25-28.

Bakker A. (1992) Hydrodynamics of Stirred Gas-Liquid Dispersions. Ph.D. Dissertation, February 1992, Delft University of Technology, Delft, The Netherlands.

The numerical computation of the flow field in stirred tank reactors has received attention since the beginning of the 1980’s, starting with Harvey (1980) and Harvey and Greaves (1982). Most times computations are limited to single-phase flow. Reviews of the work done on these single-phase computations have been given by Ranade and Joshi (1990) and...

The suspension of settling solids with gassed impellers has been studied taking into account the influence of cavity formation behind the impeller blades in gassed systems. It is shown that the increase of stirrer speed that is required to maintain complete suspension under gassed conditions correlates strongly with the power drop that results from...

In recent years a variety of profiled axial flow impellers has been developed. The aim of the present research work is to investigate the performance of these impellers and to contribute to the understanding of the gas dispersion mechanism. Three impeller types have been compared: a standard inclined blade impeller and the profiled A315 and Leeuwri...

The suspension of floating solids under gased conditions has been studied in a fully baffled 1.20 m diameter stirred vessel. The performance of various impellers is compared at two immersion depths. It is concluded that impeller hydrodynamics, especially the formation of large gas filled cavities behind the blades, has a significant influence on th...