LDA Measurements on a Turbulent Gas/Liquid/Fibre Suspension

Detailed continuous phase flow measurements (mean and RMS velocities) by Laser Doppler Anemometry (LDA) of a turbulent gas/fibre/liquid suspension in a rotary shear tester have been performed with simultaneous torque measurements. The model system comprises a refractive‐index matched suspension in the 4‐12% wt. range with gas contents up to 15% vol. The two transition points, found by Andersson and Rasmuson (2000), were detected also with gas present and appear at higher impeller speeds with increasing gas and fibre concentrations. Plotting RMS and mean velocities versus impeller speed and power input, it is found that both decrease with increasing gas and fibre contents.     

Demarcation of a new circulating turbulent fluidization regime

Transient flow behaviors in a novel circulating‐turbulent fluidized bed (C‐TFB) were investigated by a multifunctional optical fiber probe, that is capable of simultaneously measuring instantaneous local solids‐volume concentration, velocity and flux in gas‐solid two‐phase suspensions. Microflow behavior distinctions between the gas‐solid suspensions in a turbulent fluidized bed (TFB), conventional circulating fluidized bed (CFB), the bottom region of high‐density circulating fluidized bed (HDCFB), and the newly designed C‐TFB were also intensively studied. The experimental results show that particle‐particle interactions (collisions) dominate the motion of particles in the C‐TFB and TFB, totally different from the interaction mechanism between the gas and solid phases in the conventional CFB and the HDCFB, where the movements of particles are mainly controlled by the gas‐particle interactions (drag forces). In addition, turbulence intensity and frequency in the C‐TFB are significantly greater than those in the TFB at the same superficial gas velocity. As a result, the circulating‐turbulent fluidization is identified as a new flow regime, independent of turbulent fluidization, fast fluidization and dense suspension upflow. The gas‐solid flow in the C‐TFB has its inherent hydrodynamic characteristics, different from those in TFB, CFB and HDCFB reactors. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Friction reduction by chemical additives in the turbulent flow of fibre suspensions

The effect of friction reducing additives on the turbulent flow characteristics of fibre suspensions have been studied experimentally by using a small diameter straight tube and a free rotating disk. The fibre concentration varied between 0.1 and 5.0%. The friction reducing additives were polyethylene oxide and guar gum. The optimum conditions for friction reduction and effects of temperature, aging and mechanical degradation have been determined; the viscoelastic behaviour of fibre suspensions with and without additives have also been studied. It is shown that the use of friction reducing additives in the turbulent flow of paper making fibre suspensions results in a friction reduction comparable to that in water and the quality of fibre suspensions can be improved.     

Gas hold-up and liquid mixing at low and intermediate gas velocities I. Air-water system

Gas hold-up and liquid phase dispersion experiments have been carried out in a 0.06 m bubble column at varying liquid and gas velocities. The results obtained show that the coefficient of liquid mixing varies with the flow regime. The isotropic turbulence theory of Baird and Rice (Chem. Eng. J., 9 (1975) 171) was used to provide dimensionally consistent correlations for the chain bubbling, bubbly and churn turbulent flow regimes. The gas hold-up was determined to increase with gas velocity in the chain bubbling and bubbly flow regimes. The results obtained from this study also show that the Froude number represents a useful criterion for mapping flow regimes in vertical bubble columns.     

Bubble shape,gas flow and gas–liquid mass transfer in pulp fibre suspensions

Gas–liquid mass transfer in pulp fibre suspensions in a batch‐operated bubble column is explained by observations of bubble size and shape made in a 2D column. Two pulp fibre suspensions (hardwood and softwood kraft) were studied over a range of suspension mass concentrations and gas flow rates. For a given gas flow rate, bubble size was found to increase as suspension concentration increased, moving from smaller spherical/elliptical bubbles to larger spherical‐capped/dimpled‐elliptical bubbles. At relatively low mass concentrations (C_m = 2–3% for the softwood and C_m ? 7% for the hardwood pulp) distinct bubbles were no longer observed in the suspension. Instead, a network of channels formed through which gas flowed. In the bubble column, the volumetric gas–liquid mass transfer rate, k_La, decreased with increasing suspension concentration. From the 2D studies, this occurred as bubble size and rise velocity increased, which would decrease overall bubble surface area and gas holdup in the column. A minimum in k_La occurred between C_m = 2% and 4% which depended on pulp type and was reached near the mass concentration where the flow channels first formed.     

Fibre orientation distribution in turbulent fibre suspensions flowing through an axisymmetric contraction

The Reynolds averaged Navier–Stokes equation was solved numerically with the Reynolds stress model to get the mean fluid velocity and the turbulent kinetic energy in turbulent fibre suspensions flowing through an axisymmetric contraction. The fluctuating fluid velocity was represented as a Fourier series with random coefficients. Then the slender‐body theory was used to predict the fibre orientation distribution and orientation tensor. Some numerical results are compared with the experimental ones in the turbulent fibre suspensions flowing through a contraction with a rectangular cross‐section. The results show that the fibres with high aspect ratio tend to align its principal axis with the flow direction much easier. High contraction ratio makes the fibre alignment with the flow direction much easier. The contraction ratio has a strong effect on the fibre orientation distribution. Only a small part of the fibre is aligned with the flow direction in the inlet region, while most fibres are aligned with the flow direction when they approach to exit. The fibres are aligned with the flow direction rapidly in the inlet region, after that the fibre orientations change little in the most of the downstream region. The fibres with high aspect ratio are aligned with the flow direction faster when they enter the contraction. The randomising effect of the turbulence becomes significant in the downstream region because of the high turbulent intensity.     

Quantifying the Fibre Effect on Gas Holdup in a Co‐Current Air‐Water‐Fibre Bubble Column

Fibre type and mass fraction have significant effects on gas holdup in gas‐liquid‐fibre bubble columns. An experimental study is introduced to identify a parameter that simultaneously characterizes the fibre type and mass fraction effects on gas holdup in gas‐liquid‐fibre bubble columns. This parameter satisfies the following condition: when this parameter is constant, the gas holdup trend in different fibre suspensions is generally similar at most operating conditions. A method is proposed to identify a characterization parameter by combining the crowding factor and fibre number density. The identified parameter is I_c=1n(N_c^0.8N_f^0.2). This parameter can be used to model gas holdup in gas‐liquid‐fibre bubble columns and quantitatively compare the fibre effects in different fibre suspensions.     

Modeling of Gas‐Particle Turbulent Flow in Spout‐Fluid Bed by Computational Fluid Dynamics with Discrete Element Method

Gas and solid turbulent flow in a cylindrical spout‐fluid bed with conical base were investigated by incorporating various gas‐particle interaction models for two‐way coupling simulation of discrete particle dynamics. The gas flow field was computed by a k‐ϵ two‐equation turbulent model, the motion of solid particles was modeled by the discrete element method. Drag force, contact force, Saffman lift force, Magnus lift force and gravitational force acting on individual particles were considered in the mathematical models. Calculations on the cylindrical spout‐fluid bed with an inside diameter of 152 mm, a height of 700 mm, a conical base of 60° and the ratio of void area of 3.2 % were carried out. Based on the simulation, the gas‐solid flow patterns at various spouting gas velocities are presented. Besides, the changes in particle velocity, particle concentration, collision energy, particle and gas turbulent intensities at different proportions of fluidizing gas to total gas flow are discussed.     

Hydrodynamics and mass transfer in a bubble column with an organic liquid

Hydrodynamic and mass transfer data in Soltrol-130 (a mixture of C₉+ iso-paraffins) were measured in a 0.305 m diameter bubble column. The gas holdup structure (i.e., the contributions to holdup from the small and large bubble fractions of the dispersion) for this hydrocarbon liquid in the churn turbulent flow regime was analyzed using the dynamic gas disengagement technique. The validity of the assumption of axially uniform gas holdup structure was checked. Literature correlations were found inadequate to explain the observed gas holdup and the volumetric mass transfer coefficients for Soltrol-130. The volumetric mass transfer coefficient per unit volume of large bubbles is shown to be independent of superficial gas velocity for the fully developed churn turbulent regime. The present hydrodynamic and mass transfer data in the churn turbulent regime should be useful in the design and scale-up of bubble columns used in organic process industries.     

Gas holdup in a two‐phase reversed flow jet loop reactor

Experimental investigations have been carried out in Reversed Flow Jet Loop Reactor (RFJLR) to study the influence of liquid flow rate, gas flow rate, immersion height of two‐fluid nozzle in reactor and nozzle diameter on gas holdup without circulation, that is, gas–liquid mixture in draft tube only (E_gd) and gas holdup with circulation loop (E_g). Also critical liquid flow rate required for transition from draft tube to circulation loop has been determined. Gas holdup was measured by isolation valve technique. Gas holdup in draft tube and circulation loop increased with increase in liquid flow rate and gas flow rate. It is observed that the increased flow rate is required for achieving a particular value of gas holdup with larger nozzle diameter. Nozzle at the top edge of draft tube have higher gas holdup as compared to other positions. It has been noted that, no significant recirculation of gas bubbles into the top of draft tube from annulus section has been observed till a particular liquid flow rate is reached. A plot of gas holdup with no circulation and with circulation mode determines minimum liquid flow rate required to achieve complete circulation loop. Critical liquid flow rate required to achieve complete circulation loop increases with increase in gas flow rate and is minimum at lowest immersion height of two‐fluid nozzle.     

Numerical Modelling of the Flow of Fibre Suspensions through a Planar Contraction

This study presents results of numerical simulations of the flow of fibre suspensions in a Newtonian fluid through a 4:1 planar contraction. Two approaches are adopted to determine the fibre orientation. The first one uses orientation tensors defined as dyadic products of the orientation vector, while the second one is based on the fibre aligned assumption. An implicit time discretization scheme and a mixed finite element method based on the introduction of the rate of deformation tensor as an additional unknown are used to obtain the steady‐state flow. The numerical technique we use allows us to examine the flow of fibre suspensions in both dilute and semi‐dilute regimes at high values of the parameters controlling inertial and fibre effects. The predicted flow patterns and fibre orientation are discussed, and a systematic comparison between the predictions of the two approaches is presented.     

Heat transfer in a reciprocating plate column

A pilot‐scale (5.08 cm internal diameter) reciprocating plate column has been modified by the insertion of a brass test section for heat transfer measurements. Heat is supplied to liquid (water or a glucose solution) in the column from an electrical heating tape wound round the brass section, the walls of which contain thermocouples. Reciprocation of the plates in the column results in up to a seven‐fold improvement In heat transfer coefficient, to single phase liquids. Conditions are turbulent with oscillatory Reynolds numbers up to 20000. The effect of plate reciprocation is much less pronounced when the liquids are agitated by a stream of gas bubbles. The single‐phase heat transfer coefficients have been correlated for 5 different types of plates using approaches already available in the literature for turbulent systems in steady flow. The best‐fit oscillatory flow correlation differs slightly from the existing correlations for steady flow.     

Local gas holdup in a draft tube airlift bioreactor

Airlift column bioreactors are gas–liquid contact devices characterized by a rising channel and a down flow channel due to gas holdup differences in these two channels. Local gas holdup distribution strongly affects the overall gas–liquid flow dynamics in airlift columns. In this work, local gas holdup distributions in a draft tube airlift column covering both bubbly flow and churn–turbulent flow regimes have been studied using computed tomography (CT) technique as well as conventional techniques. The radial and axial evolutions of the gas holdup distribution will be discussed, together with the effects of superficial gas velocity and geometry parameters. The obtained gas holdup results will also be used to verify various empirical and semi-empirical correlations in the literature. Moreover, the obtained gas holdup information, combined with liquid flow dynamic information reported in Luo and Al-Dahhan, 2008a, Luo and Al-Dahhan, 2008b, forms a benchmark database for the design and scale-up of airlift column bioreactors and for computational fluid dynamic (CFD) modeling validations.     

A discrete model for non-wetting liquid flow from a point source in a packed bed under the influence of gas flow

A study of non-wetting flow in a packed bed under the influence of gas flow has been carried out. Departing from the usual continuum models, a discrete and deterministic model for liquid flow has been presented to model the liquid flow from single and multiple point sources. Liquid flow is modelled based on force balance approach considering gas drag, bed resistance and gravity forces. Gas flow is modelled using k-ε model for turbulent flow. An X-ray flow visualization technique, developed by our group, is used to study the liquid flow paths in the packed bed. Liquid flow path and velocity has been obtained for various liquid and gas flow rate. Flow paths obtained from the simulation results are in good agreement to those obtained from flow visualization procedure under various conditions. Also, liquid distribution at the bed bottom is reported and compared with the experimental results.     

Near Wall Studies of Pulp Suspension Flow Using LDA

A detailed study of the flow behaviour in the near wall region of pulp suspensions up to 4.7% have been performed using Laser Doppler Anemometry (LDA) in pipe flow. Axial mean velocity profiles show a distinct plug flow and an increase of the plug region as the flow rate decreases and fibre concentration increases. An attempt is made to relate the LDA data‐rate to fibre concentration, which indicates a dilution region at 1‐2 mm from the wall that is larger than the annulus region. The dilution region increases with increasing flow rate, decreasing concentration and when using longer pulp fibres.     

In‐line jet mixing of liquid‐pulp‐fiber suspensions: Effect of fiber properties,flow regime,and jet penetration

Mixing effectiveness was determined experimentally for side jet injection into pipe flow for water and pulp suspensions for a range of fiber mass concentrations (0–3.0%), mainstream velocities (0.5–5.0 m/s), and side‐stream velocities (1.0–12.7 m/s). The mixing quality was measured in cross‐sectional planes along the pipe using electrical resistance tomography and quantified by a modified mixing index, derived from the coefficient of variation of conductivity. Mixing depended strongly on the flow regime and jet penetration. For turbulent flow, the criteria for in‐line jet mixing in water are applicable to the mixing in suspensions, with small differences likely due to differences in fiber network strength and influences of fiber‐turbulence interactions in modifying turbulent structures in the bulk. When a suspension flows as a plug, however, the mixing differs greatly from that in water, depending on the fiber network strength in the core of the pipe. © 2012 American Institute of Chemical Engineers AIChE J, 59: 1420–1430, 2013     

基于气泡群相间作用力模型的加压鼓泡塔流体力学模拟

目前,多数文献报道了冷态加压湍动鼓泡塔内流动特征,并且通过实验数据回归相关经验关联式。然而,此类关联式适用范围有限,难以直接外推到工业鼓泡塔反应器条件。因此,在FLUENT平台上建立了基于气泡群相间作用力的、动态二维加压鼓泡塔计算流体力学模型。通过数值模拟考察了操作压力为0.5~2.0 MPa,表观气速为0.20~0.31 m·s^-1,内径0.3 m鼓泡塔内流场特性参数分布,并且与冷态实验数据进行比较。结果表明,采用修正后的气泡群曳力模型、径向力平衡模型以及壁面润滑力模型描述气泡群相间作用力,能够较为准确地反映平均气含率和气含率径向分布随操作压力和表观气速变化的规律。     

Possibilities and Limits of Application of Electrical Resistance Tomography in Hydrodynamics of Bubble Columns

Knowing the hydrodynamic regime in which is working a bubble column is of great importance because the regime affects strongly the mass transfer between the phases. To this end, we examine the potentialities of an Electrical Resistance Tomography (ERT) device. We analyse cross correlation of electrode pair measurements, of neighbouring pixels and power spectra of averaged pixels within a single plane as well as cross correlations of averaged pixels between two planes, without finding a clear signature of the churn turbulent flow. Variable gas flow rate inputs are used to determine the time resolution of the ERT.     

Numerical simulation of churn flow in a vertical pipe

A numerical simulation of the churn flow regime of air-water and R134a vapour-liquid mixtures by means of the volume of fluid (VOF) method is presented. The focus of the paper is on the inlet region of a vertical pipe. An axisymmetrical domain is used, reproducing the region next to the porous wall liquid injector of a typical test section for the investigation of vertical gas-liquid flows.A simplified model of the levitation process of the ring-type waves typical in churn flow is proposed. The influence of the gas Froude number on the waves amplitude is shown by means of the simplified model and used to explain the numerical results.A comparison of the numerical results with experimental wave frequency data and visualizations available in the literature is performed. The velocity field in the forming wave region and the pressure and shear stress variations along the interface are shown.Simulations have been performed at different liquid and gas superficial velocities and pipe diameters and the influence of these parameters on the gas-liquid interface is discussed.     

Flow regime identification of two-phase liquid-liquid upflow through vertical pipe

The present work has attempted to identify the flow patterns during liquid-liquid two phase flow through a vertical pipe. Dyed kerosene and water have been selected as the test fluids. The measurements have been made for phase velocities varying from 0.05 to 1.5 m/s for both the liquids. The conductivity probe technique has been adopted and three different probe designs have been used to identify the patterns under different flow conditions. A parallel wire type probe traversing the entire cross-section along a diametral plane has indicated the existence of bubbly flow at low phase flow rates and dispersed bubbly flow at high velocities of water. Apart from the visual appearance of the signals, different statistical analysis namely the probability density function and wavelet analysis have been performed for a better appraisal of the flow situation. The information in the PDFs have been quantified by means of the statistical moments. The existence of the core-annular flow at high kerosene and low water velocities has been confirmed from measurements using a different probe design. The intermediate region between the bubbly and annular flow patterns is characterized by a random distribution of the two liquids with continually changing interface between them. This has been named as the churn turbulent flow pattern. The information thus obtained has been represented in the form of a flow pattern map.