Local Bubble Size Research Articles

Influence of impeller diameter on local gas dispersion properties in a sparged multi-impeller stirred tank

Vertical distributions of local void fraction and bubble size in air–water dispersion system were measured with a dual conductivity probe in a fully baffled dished base stirred vessel with the diameter T of 0.48m, holding 0.134m3 liquid. The impeller combination with a six parabolic blade disk turbine below two down-pumping hydrofoil propellers, identified as PDT+2CBY, was used in this study. The effects of the impeller diameter D, ranging from 0.30T to 0.40T (corresponding to D/T from 0.30 to 0.40), on the local void fraction and bubble size were investigated by both experimental and CFD simulation methods. At low superficial gas velocity VS of 0.0077m·s−1, there is no obvious difference in the local void fraction distribution for all systems with different D/T. However, at high superficial gas velocity, the system with a D/T of 0.30 leads to higher local void fraction than systems with other D/T. There is no significant variation in the axial distribution of the Sauter mean bubble size for all the systems with different D/T at the same gas superficial velocity. CFD simulation based on the two-fluid model along with the population balance model (PBM) was used to investigate the effect of the impeller diameter on the gas–liquid flows. The local void fraction predicted by the numerical simulation approach was in reasonable agreement with the experimental data.

Modelling of the Bubble Size Distribution in an Aerated Stirred Tank: Theoretical and Numerical Comparison of Different Breakup Models

Abstract The main topic of this study is the mathematical modelling of bubble size distributions in an aerated stirred tank using the population balance method. The air-water system consisted of a fully baffled vessel with a diameter of 0.29 m, which was equipped with a six-bladed Rushton turbine. The secondary phase was introduced through a ring sparger situated under the impeller. Calculations were performed with the CFD software CFX 14.5. The turbulent quantities were predicted using the standard k-ε turbulence model. Coalescence and breakup of bubbles were modelled using the MUSIG method with 24 bubble size groups. For the bubble size distribution modelling, the breakup model by Luo and Svendsen (1996) typically has been used in the past. However, this breakup model was thoroughly reviewed and its practical applicability was questioned. Therefore, three different breakup models by Martínez-Bazán et al. (1999a, b), Lehr et al. (2002) and Alopaeus et al. (2002) were implemented in the CFD solver and applied to the system. The resulting Sauter mean diameters and local bubble size distributions were compared with experimental data.

On the measurement of local gas hold-up, interfacial area and bubble size distribution in gas–liquid contactors via light sheet and image analysis: Imaging technique and experimental results

In this work a novel experimental technique for measuring local gas hold-up, interfacial area and bubble size distribution, in gas–liquid systems is proposed. The technique is based on advanced Image Processing coupled with experimental set-ups typically available for Particle Image Velocimetry. A fluorescent dye dissolved in the liquid phase allows to identify in-plane bubbles among all visible bubbles in the images. To this end, a suitable algorithm is proposed. The raw data so obtained are processed by previously developed statistical methods that result in a reliable reconstruction of actual dispersion properties.The technique is applied to the case of a gas-dispersed mechanically agitated vessel, and the data obtained are presented and discussed.

Two-Phase Flow-Induced Forces on Piping in Vertical Upward Flow: Excitation Mechanisms and Correlation Models

Momentum variation in two-phase flow generates significant low frequency forces, capable of producing unwanted and destructive vibrations in nuclear or petroleum industries. Two-phase flow-induced forces in piping were previously studied over a range of diameters from 6 mm to 70 mm in different piping element geometries, such as elbows, U-bends, and tees. Dimensionless models were then developed to estimate the rms forces and generate vibration excitation force spectra. It was found that slug flow generates the largest forces due to the large momentum variation between Taylor bubbles and slugs. The present study was conducted with a 52 mm diameter U-bend tube carrying a vertical upward flow. Two-phase flow-induced forces were measured. In addition, two-phase flow parameters, such as the local void fraction, bubble size and velocity, and slug frequency were studied to understand the relationship between the force spectra and the two-phase flow patterns. A new two-phase flow pattern map, based on existing transition models and validated using our own local void fraction measurements and force spectra, is proposed. This paper also presents a comparison of the present dimensionless forces with those of previous studies, thus covers a wide range of geometries and Weber numbers. Finally, a dimensionless spectrum is proposed to correlate forces with large momentum variations observed for certain flow patterns.

Prediction of air bubble dispersion in a viscous fluid in a twin-screw continuous mixer using FEM simulations of dispersive mixing

Bubble break-up was studied during mixing of a viscous Newtonian fluid in a co-rotating twin screw continuous mixer using experimental observations and finite element method (FEM) simulation of dispersive mixing. Bubble break-up was greatest with two paddle element configurations that had no stagger and a reverse stagger respectively while a third configuration with a forward element stagger showed the least break-up. The decrease in bubble break-up was found to be due to the decrease in the elongation flow density and an increase in local forward flow in the forward angle stagger configuration. Maximum stable bubble diameters and effective shear rate for break-up calculated at different locations in the mixer using the fundamental capillary number theory for drop and bubble break-up correlated well with the measured local mean bubble sizes and the mean local shear rates calculated using FEM simulations of the flow.

Coalescence efficiency of bubbles in bubble columns

AbstractBubble size distribution was modelled by employing the population balance equation (PBE). All three bubble coalescence mechanisms (turbulence, buoyancy and laminar shear) and the main bubble breakup mechanism (breakup due to turbulent eddies) were considered in the model. Local bubble size distributions at the top and bottom of the column were obtained by solving this PBE. The results were compared with the experimental data for seven independent multiphase systems (water/air, isomax diesel/air, kerosene/air and four other liquid mixture/air) at two diverse gas velocities. The experimental adjustable constant in the coalescence efficiency function was determined by fitting the population balance to the experimental bubble size distributions. An empirical correlation was proposed for the coalescence efficiency by the dimensional analysis, which includes Reynolds and Weber numbers. © 2011 Canadian Society for Chemical Engineering

Hydrodynamic stability of a bubble column with a bottom-mounted point air source

Bubble column is widely used in both industrial and environmental applications. In this study, we examine the flow dynamics and stability of a bubble column driven by a point air source centrally mounted at the bottom using Phase Doppler anemometry (PDA). The model cylindrical bubble column had an inner diameter of 152 mm and was filled with the liquid to about 1 m height, above the point air source, which was made of a 30-mm diameter perforated air stone. The bubble diameters were within the range of 400–1300 μm. A customized setup was developed for accurate PDA measurements of the two phases, and detailed turbulent characteristics of the liquid phase velocity, bubble diameter, bubble velocity and the slip velocity were collected throughout the column. The comprehensiveness of the data set enabled a close examination of the hydrodynamic stability inside the column. Measurements were taken at three different air rates, namely 0.13, 0.25 and 0.38 L/min (corresponding to average gas volume fractions of 0.0065, 0.0138 and 0.0197, respectively). The results illustrated a large-scale coherent liquid circulation pattern inside the column. The circulation pattern in the upper column was relatively steady, while the pattern in the lower column was strongly unsteady with the probability density functions (pdf) for both the liquid and bubble velocities showing distinct twin peaks. An analysis based on the determination of the bubble drag forces and transversal lift forces is performed by decomposing the twin-peaked pdfs into two separated Gaussian distributions, one for the upward flow due to the bubble rises and the other for the downward flow due to circulation. Through the decomposition, a stability criterion can then be established by choosing the local bubble size as the representative length scale for the turbulent eddies inside the column. The analysis with the criterion illustrates why a steady circulation pattern was achieved in the upper column, and at the same time shows that the instability at the bottom column was induced by the low frequency meandering of the bubble swarm. ► Phase Doppler anemometry (PDA) was used to measure the flow field in a bubble column. ► The two-phase flow fields with three different gas fractions were investigated. ► The results revealed the detailed turbulent characteristics. ► A stability criterion was proposed based on the drag and the lift forces for bubble swarm. ► The criterion can predict the transition from an unsteady to a steady flow pattern.

Three-dimensional CFD simulation of bubble–melt two-phase flow with air injecting and melt stirring

This paper reports on progress in developing CFD simulations of gas bubble–metallic melt turbulent flows induced by a pitched-blade impeller with an inclined shaft. Foaming process of aluminum foams, in which air is injected into molten aluminum composites and the melt is mechanical stirred by the impeller, has been investigated. A two-fluid model, incorporated with the Multiple Reference Frames (MRF) method is used to predict the three-dimensional gas–liquid flow in the foaming tank, in which a stirring shaft is positioned inclined into the melt. Locally average bubble size is also predicted by additively solving a transport equation for the bubble number density function, which accounts for effects of bubble breakup and coalescence phenomena. The computed bubble sizes are compared with experimental data from our water model measurement and reasonable agreements are obtained. Further, simulated results show that the volume averaged total and local gas fractions are generally increased with rising impeller speed and gas flow rate. The local averaged bubble size increases with increasing gas flow rate and orifice diameter and decreasing liquid viscosity, and decreases also with rising rotation speed of the impeller.

CFD simulations to portray the bubble distribution and the hydrodynamics in an annulus sparged air‐lift bioreactor

AbstractAir‐lift bioreactor (ABR) are applied in the pharmaceutical industry for those processes of low‐oxygen demand. The characteristics of the air bubbles in an ABR are important since they influence the overall performance of the bioreactor. In this article, two‐dimensional CFD simulations are carried out to portray the flow characteristics in an annulus‐sparged ABR with a low‐aeration rate, which is a scaled geometry of a 200 m3 annulus‐sparged ABR for the L‐ascorbic acid manufacturing. VOF model is applied firstly to capture the flow behaviour of the bubbles in the bioreactor, and the distributions of the bubbles and local mean bubble sizes are approximately obtained. With the local mean bubble size distribution, Eulerian model is adopted to investigate the performance of the bioreactor. The liquid velocity and the gas holdup are further discussed. The results show the existence of the local back mixing as well as the mal‐distributions of the velocity and the gas holdup along the radial direction in the annulus‐sparged ABR.

Local bubble size distribution, gas–liquid interfacial areas and gas holdups in an up-flow ejector

Particle image velocimetry (PIV) was used to measure local bubble size distributions (BSD), gas–liquid interfacial areas and gas holdups in an up-flow ejector, based on the water–air system with different liquid and gas flow rates under the presence/absence of the swirl body. The results show that the bubble flow patterns are different whether to add the swirl body into the nozzle, especially at low gas flow rate because the bubbles formed “bubble chain” in the ejector with swirl. The mean bubble sizes D 32 of the two are both related to the pressure drop between import and export, gas ratio and liquid flow. The interfacial area and D 32 are both mainly dependent on the local gas holdups. The mean bubble sizes in the absence of swirl body are smaller than that in the presence of swirl under different operating conditions. The gas holdups and interfacial area are larger with swirl than those without swirl. With the increase of the gas fraction, the differences of D 32, a t and e G become smaller.

Simulation of bubbly flows: Comparison between direct quadrature method of moments (DQMOM) and method of classes (CM)

In typical bubbly flow applications, bubbles can break or coalesce due to bubble–bubble and bubble–fluid interactions in presence of turbulence. Under this assumption, a fixed bubble size model might not be suitable for predicting correct multiphase flow behaviour in the gas–liquid system. For example, breakage and coalescence events produce very different bubble size distribution and then affects the interfacial interactions between the phases as heat and mass transfer and exchange forces, for example the drag and lift forces. In the present work, a rectangular bubble column and stirred tank reactor are modelled using an open-source computational fluid dynamics CFD package OpenFOAM. A population balance equation is introduced in the mathematical model to account the effects of bubble size distribution taking account the effect of coalescence and break-up phenomena on the hydrodynamic behaviour of multiphase flow. For solving the population balance equation an efficient numerical technique is integrated to enable the simulation of more complex flows that are encountered in industrial applications. Furthermore, the direct quadrature method of moments (DQMOM) and the method of classes (CM) are implemented and compared using an open source CFD package (OpenFOAM). An Eulerian–Eulerian approach with a standard k – ε model of turbulence is used. The momentum exchange between the bubbles and the continuous phase is taken into account with drag, lift and virtual mass forces. The predicted results are compared with measured data available in the scientific literature; they show that the gas volume fraction, velocity profiles and local bubble size are in good agreements.

Local void fraction and bubble size distributions in cold-gassed and hot-sparged stirred reactors

Vertical distributions of local void fraction, bubble size and gas–liquid interfacial area in air–water dispersions at 24 and 81 °C have been measured with a dual electric conductivity probe in a fully baffled dished base stirred vessel of 0.48 m diameter holding 0.145 m 3 liquid. The agitator was a hollow blade dispersing turbine below two up-pumping hydrofoils. The vertical distribution of the void fraction in the hot conditions is similar to that at ambient temperature though the void fraction is significantly lower in the hot system. The vertical distributions of bubble size show maxima with large bubbles above the bottom impeller, near the top impeller and close to the free surface. With given operating conditions, the overall Sauter means bubble size in the hot systems appears to be about 21% greater than when cold. Estimates of the local interfacial area show a maximum just above the level of the top impeller.

Gas holdup and bubble dynamics in a three-phase internal loop reactor with external slurry circulation

As modified three-phase fluidized reactors, loop reactors have been widely used in the area of chemical and energy processes. An external slurry circulation is introduced into a traditional internal loop reactor to improve the transfer between gas and slurry phases. Gas holdup and bubble dynamics are investigated by using the double-sensor conductivity probe technique in the present work. The results show that gas holdup inside the draft tube is greatly affected by the geometrical configuration and is much higher than that in the corresponding section of the annular region. Local, section-averaged, and overall gas holdups increase with increasing superficial gas velocity, while the effects of solid loading and external slurry circulation velocity are less significant than that of superficial gas velocity. Both local bubble size and bubble rise velocity vary significantly in different regions.

Advanced statistical analysis of Local Bubble Size Distributions in 2D gas fluidized beds

Advanced statistical analysis of Local Bubble Size Distributions in 2D gas fluidized beds

AREA-TO-VOLUME DATA TRANSLATION IN THE MEASUREMENT OF BUBBLE SIZE DISTRIBUTIONS VIA LASER SHEET AND IMAGE ANALYSIS

A nov el experimental technique for measuring local bubble size distributions in gasliquid systems, is proposed. The technique is based on laser sheet illumination of the gas-liquid dispersion and synchronized camera, i.e. on equipment typically available within PIV set-ups. By suitably arranging experiments and subsequent image analysis, bubbles intercepted by the laser sheet are clearly identified. The size distribution of observed intercept areas can therefore be assessed. This is however different from the actual bubble size distribution needed for modelling and interfacial area estimation purposes. In this paper a simple statistical correction procedure able to reconstruct actual bubble size distributions from relevant intercept size distributions is proposed and discussed. Preliminary data obtained in stirred gas-liquid dispersions confirm the technique viability.

Modelling of mass transfer in gas–liquid stirred tanks agitated by Rushton turbine and CD-6 impeller: A scale-up study

A combined computational fluid dynamics (CFD) and population balance model (PBM) approach has been applied to the simulation of gas–liquid stirred tanks agitated by (i) a Rushton turbine or (ii) a CD-6 impeller, operating at aeration numbers from 0.017 to 0.038. The multiphase simulations were realised via an Eulerian–Eulerian two-fluid model and the drag coefficient of spherical and distorted bubbles was modelled using the Ishii–Zuber equations. The effect of the void fraction on the drag coefficient was modelled using the correlation by Behzadi et al. [Behzadi, A., Issa, R.I. and Rusche, H., 2004, Modelling of dispersed bubble and droplet flow at high phase fractions, Chem Eng Sci, 59: 759–770]. The local bubble size distribution was obtained by solving the PBM using the quadrature method of moments (QMOM). The local k L a was estimated using both the Higbie penetration theory and the surface renewal model. The predicted gas–liquid hydrodynamics, local bubble sizes and dissolved oxygen concentration were in good agreement with experimental measurements reported in the literature. A slight improvement in the prediction of the aerated power number was obtained using the non-uniform bubble size distribution resulting from the coupled CFD–PBM simulation. Evaluation of the prospective scale-up approaches indicates a higher probability of maintaining a similar level of mass transfer in a larger tank by keeping the P g / V and VVM constant. Considering its predictive capability, the method outlined in this work can provide a useful scale-up evaluation of gas–liquid stirred tanks.

Container effects on the free drainage of wet foams

This work studies how drainage of a wet foam is affected by certain characteristics of its container: diameter, wettability and shape of the walls. Drainage is registered by three simultaneous techniques: (a) electrical conductance measurements for the evolution of the local liquid fraction, (b) close-up photos for the evolution of the local bubble size distribution and (c) volumetric measurements for the evolution of the global liquid fraction in the entire container. Electrical measurements are conducted at different heights along the foam with the aid of several pairs of non-intrusive ring electrodes. Three cylindrical Plexiglas containers of different diameter are used, before and after treatment of their walls to allow tests with hydrophobic and hydrophilic walls, respectively. To resemble the shape of common industrial containers, the lower part of the largest container has a gradually reducing diameter towards its bottom. Foam decay is slower in the hydrophobic containers, this being less evident as diameter increases. Moreover, the role of diameter and shape is complex since the highest drainage rate is measured at the largest container whereas the lowest one at the medium container. The possible effect of the container walls in promoting the macroscopic rigidity of the foam structure is discussed

Uniform Flow in Bubble Columns

Uniform bubbly flow in a 15 cm bubble column is investigated. We use a special needle sparger consisting of 559 separate needles, uniformly distributed over the bottom. With this sparger, we can ensure that all bubbles generated are of the same size and that the bubble injection is very uniform over the entire bottom of the column. Detailed experiments are reported, using optical glass fibers to measure the local gas fraction and bubble size and velocity and using laser Doppler anemometry to measure the liquid axial velocity field. We find that the homogeneous flow regime extends up to a gas fraction of 55% well beyond the predictions of theory. The superficial gas velocity at which the homogeneous regime looses its stability depends on the water quality: fresh water looses its stability much earlier than old water. However, the gas fraction as a function of the superficial gas velocity is in the homogeneous regime independent of the water quality. The overall gas fraction can be described by a Richardson...

Modelling mass transfer in an aerated 0.2m3 vessel agitated by Rushton, Phasejet and Combijet impellers

Abstract We assembled a set of models that allows investigation of local variables that are difficult to measure, validation of mechanistic physical models, and comparison of different numerical solutions. Population balances (PB) for bubbles were combined with local flow modelling in order to investigate G–L mass transfer in an air–water system. Performance of three different impeller geometries was investigated: Rushton (RT), Phasejet (PJ) and Combijet (CJ). Simulations were compared against experimental mixing intensity, gas hold-up, vessel-averaged volumetric mass transfer rates ( k L a ), and local bubble size distributions (BSDs). The simulations qualitatively predict k L a 's with different impellers at the fully dispersed flow region and gave new insight on how k L a is formed and distributed in the stirred vessels. The used bubble breakage and coalescence models are able to describe both air–water and viscous non-Newtonian G–L mass transfer. Difference between experimental mass transfer rates of the three impellers was within experimental error, even trough the flow patterns, gas distribution, and local BSDs differ considerably. The population balance for bubbles was modelled in two different ways, with multiple size groups (MUSIGs) and with the bubble number density (BND) approach. MUSIG calculations took over twice as much computational time than BND, but there was little difference in the results. The Rushton turbine k L a was described with best accuracy, which is not surprising since most phenomenological models are fitted based on RT experiments. We suggest that these models should be validated over a wider range of vessel geometries and operating conditions.

On the design of electrical conductance probes for foam drainage applications: Assessment of ring electrodes performance and bubble size effects on measurements

This work examines critical design aspects of electrical conductance probes for measuring the liquid fraction in foams. These include the electrodes’ size, shape, separation distance and intrusiveness as well as issues such as the excitation current frequency, multiplexing, data reduction to liquid fraction, etc. Measurements are performed with ring type electrodes, flush mounted on a test vessel wall at different heights to provide a measure of the longitudinal variation of liquid fraction in the foam. Runs are also performed with traditional disk and rod type electrodes immersed in the foam. The electrically determined local liquid fraction is compared with the photographically determined local bubble size distribution at the wall as well as with the volumetrically determined global liquid fraction in the entire vessel. For the examined protein-polysaccharide stabilized wet foam, drainage is negligible for a substantial initial period of time and only later a liquid fraction gradient emerges. In this no-drainage period, comparison of the local electrical signal with the local bubble size distributions reveals that the bubble size affects the liquid fraction. Moreover, a strong correlation is found between bubble size and onset of drainage.