Expansion behaviour of a binary solid-liquid fluidised bed (SLFB) system with different initial m... more Expansion behaviour of a binary solid-liquid fluidised bed (SLFB) system with different initial mass of solids was studied both experimentally and numerically. Three different sizes (3, 5 & 8 mm diameter) of borosilicate glass beads of equal density (2230 kgm) were used as fluidised particles. Three different combinations of particle size pairs of both equal and unequal mass ratios were used using a constant liquid (water) superficial velocity of 0.17 ms in all the cases. Numerically, a two dimensional Eulerian-Eulerian (E-E) CFD model incorporating kinetic theory of granular flow (KTGF) was developed to predict the bed expansion behaviour. It was observed that complete bed segregation occurred when the difference between the solid particle diameters was higher while lower difference in particle diameters led to partial bed segregation. The CFD model also predicted these behaviours which were in good agreement with the experimental data.
In this paper, the fluidization of 8 mm glass particles in water has been simulated using a new m... more In this paper, the fluidization of 8 mm glass particles in water has been simulated using a new methodology developed within the DEM framework. In this methodology, random liquid fluctuating velocities are used as direct input into the drag model. The specific aim of this study is to directly compute the granular pressure in a liquid fluidized bed. The granular pressure is defined using the particle-wall collision frequency and the corresponding particle momentum transport during the collision. Initially, we validated our model by comparing the relationship between superficial fluid velocity and bed expansion against the well-known Richardson-Zaki [1] equation. The results demonstrated a good agreement of our model. The granular pressure and temperature, as well as the particle-wall collision frequency, in the liquid fluidized bed were determined for superficial fluid velocities in the range between 0.08 and 0.32 m/s. The granular pressure exhibited a maximum (between 0.3-0.4 solid ...
The Australian Research Council has recently established an Industrial Transformation Research Hu... more The Australian Research Council has recently established an Industrial Transformation Research Hub in the area of iron ore, focussed on iron ore characterisation, beneficiation, materials handling and end-use functionality. The research hubs are part of a new initiative aimed at building stronger relationships between industry and universities, while addressing issues of national significance. "The ARC Research Hub for Advanced Technologies for Australian Iron Ore" will focus its attention on developing innovative approaches for creating enhanced value across the full value chain, through characterisation of different ore types, beneficiation, and materials handling and transport. This hub brings together five industrial organisations and a research team formed from well-established research groups based at the Newcastle Institute for Energy and Resources (NIER). These groups have a record of working closely with industry, creating opportunities. Examples include new beneficiation technologies such as the Reflux Classifier for achieving gravity separation, the expertise of TUNRA Bulk Solids in materials handling, and the Iron Making Centre at Newcastle, a capability maintained by BHP Billiton since the closure of its Technology Centre in 2009. The purpose of this paper is to provide an outline of the research hub, and its objectives, while providing some background on the research groups which now form the hub, and the strategy adopted for developing and implementing new technologies. The paper then describes a range of novel technologies that have the potential to be developed and applied to iron ore beneficiation.
Droplet evaporation in fluidized beds is of great interest in applications like fluidized catalyt... more Droplet evaporation in fluidized beds is of great interest in applications like fluidized catalytic cracking units. Although a significant number of analyses are available for modelling of droplet vaporization in a fluidized bed, very little work has been performed experimentally to measure the vapour concentration followed by numerical validation. In the present work, acetone droplet evaporation in a bubbling fluidized bed is studied experimentally as well as numerically. A liquid jet of acetone is injected into a hot bubbling fluidized bed kept well above saturation temperature of acetone. Nonintrusive Schlieren imaging, based on the difference in refractive index, is used to trace the acetone vapour concentration profile. The bubbling fluidized bed is modelled in an Eulerian framework using a simplistic porous media approach while the droplets are modelled in a Lagrangian framework. Intense interactions are observed between the evaporating droplets and hot particles during contac...
This paper reports on the collision interaction between a confined liquid film (water) and an imp... more This paper reports on the collision interaction between a confined liquid film (water) and an impacting hydrophilic glass particle of different diameters (1.1-2mm) and impact velocities (0.2-1 m/s). Depending on the impact Weber number (1.4-23.9) of the particles either complete capture of the particle inside the film or complete penetration through the film were observed. In the latter case a certain amount of liquid mass was found to adhere to the particle surface which again depended on the impact Weber number. A criterion was developed based on the energy balance approach to demarcate between these two regimes. Also, an analytical model was proposed to approximately determine the liquid mass attached to the particle. Computationally, a three dimensional CFD model was developed using the VOF approach and dynamic meshing technique which was found to be in good agreement with the experimentally observed dynamics in these two regimes.
This paper investigates the friction coefficient of a moving liquid marble, a small liquid drople... more This paper investigates the friction coefficient of a moving liquid marble, a small liquid droplet coated with hydrophobic powder and floating on another liquid surface. A floating marble can easily move across water surface due to the low friction, allowing for the transport of aqueous solutions with minimal energy input. However, the motion of a floating marble has yet to be systematically characterised due to the lack of insight into key parameters such as the coefficient of friction between the floating marble and the carrier liquid. We measured the coefficient of friction of a small floating marble using a novel experimental setup that exploits the non-wetting properties of a liquid marble. A floating liquid marble pair containing a minute amount magnetite particles were immobilised and then released in a controlled manner using permanent magnets. The capillarity-driven motion was analysed to determine the coefficient of friction of the liquid marbles. The "capillary charge" model was used to fit the experimental results. We varied the marble content and carrier liquid to establish a relationship between the friction correction factor and the meniscus angle.
This paper reports the direct and precise measurement of bubble coalescence in salt solutions usi... more This paper reports the direct and precise measurement of bubble coalescence in salt solutions using microfluidics. We directly visualized the bubble coalescence process in a microchannel using high-speed imaging and evaluated the shortest coalescence time to determine the transition concentration of sodium halide solutions. We found the transition concentration is ion-specific, and the capacity of sodium halide salts to inhibit bubble coalescence follows the order of NaF > NaCl > NaBr > NaI. The microfluidic method overcomes the inherent uncertainties in conventional large-scale devices and methods.
Langmuir : the ACS journal of surfaces and colloids, Jun 26, 2016
Numerous works have been reported on the evaporation of a sessile liquid marble, a liquid droplet... more Numerous works have been reported on the evaporation of a sessile liquid marble, a liquid droplet coated with particles, through geometric measurements. The lack of gravimetric measurement limits further understanding on the physical changes of a liquid marble during the evaporation process. Furthermore, the evaporation process of a marble containing a liquid binary mixture has not been reported. This paper studies the effective density and effective surface tension of an evaporating liquid marble which contains aqueous ethanol at relatively low concentrations. The effective density of an evaporating liquid marble is determined from the instantaneous mass and volume measurements. Subsequently, density measurements combined with surface profile fitting provides the instantaneous effective surface tension of the marble. We found that the density and surface tension of the evaporating marble is greatly affected by the particle coating.
in Wiley Online Library (wileyonlinelibrary.com) Hydrodynamics of collision interactions between ... more in Wiley Online Library (wileyonlinelibrary.com) Hydrodynamics of collision interactions between a particle and gas-liquid interface such as droplet/film is of keen interest in many engineering applications. The collision interaction between a suspended liquid (water) film of thickness 3.41 6 0.04 mm and an impacting hydrophilic particle (glass ballotini) of different diameters (1.1-3.0 mm) in low particle impact Weber number (We 5 q l v 2 p d p =r) range (1.4-33) is reported. Two distinct outcomes were observed-particle retention in the film at lower Weber number and complete penetration of the film toward higher Weber number cases. A collision parameter was defined based on energy balance approach to demarcate these two interaction regimes which agreed reasonably well with the experimental outcomes. It was shown that the liquid ligament forming in the complete penetration cases breaks up purely by "dripping/end pinch-off" mechanism and not due to capillary wave instability. An analytical model based on energy balance approach was proposed to determine the liquid mass entrainment associated with the ligament which compared well with the experimental measurements. A good correlation between the %film mass entrained and the particle Bond number (Bo 5 q l gd 2 p =r) was obtained which indicated a dependency of Bo 1.72. Computationally, a three-dimensional CFD model was developed to simulate these interactions using different contact angle boundary conditions which in general showed reasonable agreement with experiment but also indicated deficiency of a constant contact angle value to depict the interaction physics in entirety. The computed force profiles from computational fluid dynamics (CFD) model suggest dominance of the pressure force over the viscous force almost by an order of magnitude in all the Weber number cases studied.
A DEM-based model was developed and examined for simulation of aggregation in suspensions of α-al... more A DEM-based model was developed and examined for simulation of aggregation in suspensions of α-alumina nanoparticles. In the model, the random Brownian diffusion and the externally induced dielectrophoresis (DEP) motion were considered as the driving mechanisms for the transport of particles in colloidal suspension. To simulate particle interactions, the non-contact surface force and the contact force were taken into account using the well-known Derjaguin-Landau-Verway-Overbeek (DLVO) theory and the soft-sphere model, respectively. Specifically, the model was used to study the effects of pH, solid volume fraction and external AC electric field on α-alumina aggregate growth which was expressed in terms of coordination number, longest dimension, and fractal dimension. The simulations were carried out over a pH range of 4-10, solid volume fraction of 0.02-0.4, and a variety of AC electric fields. In relatively dilute suspensions, the aggregates predominantly exhibited chainlike structures, whereas at high solid volume fraction, aggregates with complex netlike structures were formed. It was also evident that, in concentrated colloidal suspensions, DEP had a negligible influence on aggregate growth over the examined conditions. The effect of DEP however, was found to be more noticeable on aggregate structure leading to the formation of more compact aggregates with a greater particle number density. The break-up and reattachment of sub-aggregates as well as the rearrangement of nanoparticles in the particle assemblies and subsequent curling of the loose network promoted by a strong AC electric field was deemed to be responsible for this structural transformation. Finally, the DEM-based model was used to predict the size of α-alumina aggregates over a range of pH. The predictions were found to be in good agreement with the published experimental data, particularly around the isoelectric point.
Multiphase systems are ubiquitous in industrial applications aimed at the generation of products ... more Multiphase systems are ubiquitous in industrial applications aimed at the generation of products either by chemical/biological reaction or physical separation based on density, electrical charge or surface properties such as hydrophobicity. The physical processing of these multiphase systems is carried out at all scales of operation and within an endless variety of vessel shapes and ancillary devices. Underpinning each process is a complex interaction between phases involving hydrodynamic, heat and mass transport. These phenomena are in turn governed largely by the nature of the flow, and in particular whether laminar or turbulent conditions prevail. In large-scale industrial processes the flows are almost always turbulent, whilst for microscale operations the flow will be laminar. Each condition provides its own challenge in being able to predict (and optimise) performance in terms of operational stability and efficiency of energy utilisation. Turbulent systems are particularly dif...
The success of many industrial processes largely depends on the structural characteristics of agg... more The success of many industrial processes largely depends on the structural characteristics of aggregates. In intensive aerobic digestion process for wastewater treatment applications, the structural characteristics namely aggregate shape, size and therefore the aggregate surface area strongly influence the transfer of dissolved oxygen from the aeration process to aggregates of harmful contaminants / microorganisms. The aim of this study was to apply discrete element modeling (DEM) techniques to the aggregation of suspended particles (microorganisms) to quantify the available surface area for convection and diffusion as a function of particles number concentration and surface charge. In this technique each solid particle was considered individually, thus accounting for its complex dynamics due to particle-particle and particle-fluid interactions. Periodic boundary conditions were adopted for all sides of the domain to minimize the computational requirements. The simulation inputs included particle and fluid characteristics such as particle size and density, solid concentration, suspension pH and ionic strength. A post processing method based on the Go-chess concept was developed to quantify the surface area of aggregate structure. The simulation results showed that whilst an increase in connection points increases the total surface area of the aggregate, this does not necessarily translate into an increase in the surface area available for oxygen transfer as combinations of open and close pores are formed. Aggregate surface area was directly determined by aggregate structural characteristics, and increased rapidly when the coordination number was below 3.5 and the fractal dimension was less than 1.5. A correlation for prediction of aggregate external surface area was also proposed as a function of aggregate structural characteristics in terms of fractal dimension and coordination number.
ABSTRACT In this study the transition from homogeneous to heterogeneous flow in a solid–liquid fl... more ABSTRACT In this study the transition from homogeneous to heterogeneous flow in a solid–liquid fluidized bed (SLFB) is examined both experimentally and numerically. The experimental apparatus comprised a refractive index-matched SLFB, comprising 5 mm diameter borosilicate glass and sodium iodine solution, which allowed for both instantaneous particle image velocimetry of the liquid flow field and solids hold-up measurements to be undertaken for superficial liquid velocities in the range of 0.06–0.22 m/s. The motion of individual, spherical steel balls (with diameters 6, 7, 8, 9 mm) was then tracked as it settled through the fluidized bed for differing superficial liquid velocities. It was observed that, for all the steel balls covered in this work, there was a change in slope in their respective classification velocity curves at a superficial liquid velocity of 0.08 m/s. This value was very close to the critical velocity of 0.085 m/s predicted from 1-D linear stability analysis; and therefore deemed to be the critical condition that marked the transition from homogeneous to non-homogenous flow. It is proposed that the change in slope of the classification velocity curve is due to the encounter of the settling foreign particles with liquid bubbles whose presence marks the onset of heterogeneous flow. Additional computational analysis, involving both Eulerian–Eulerian (E–E) and Eulerian–Lagrangian (E–L) approaches, is used to confirm the presence of liquid bubbles at a critical liquid hold-up of 0.54, which corresponds to that predicted from 1-D linear stability analysis. In summary, the study has highlighted that experimentally the transition condition for a SLFB can be obtained simply by observing the behavior of the classification velocity of a single foreign particle at different superficial liquid velocities. This transition condition was found to agree with the 1D linear stability criterion, Eulerian–Eulerian CFD (3D) and Eulerian–Lagrangian DEM (3D) approaches.
Forces acting on a single introduced particle in a solid-liquid fluidised BEd, Chemical Engineeri... more Forces acting on a single introduced particle in a solid-liquid fluidised BEd, Chemical Engineering Science,
The influence of primary particle size distribution (PPSD) on aggregation behaviour and the resul... more The influence of primary particle size distribution (PPSD) on aggregation behaviour and the resulting effect on yield stress of a concentrated colloidal suspension was investigated theoretically. The discrete element model (DEM) combined with the well-known DLVO theory was employed to obtain an insight into the aggregation process of nanoparticles with different PPSDs, where a modified version of the Flatt and Brown model [J. Am. Ceram. Soc. 89 (2006) 1244-1256] [9] was employed to calculate the corresponding suspension yield stress from the simulation results. Specifically, the aggregate growth and structure in terms of fractal dimension, coordination number and the longest dimension were examined. It was shown that at small PPSD variances, a netlike structure was formed with aggregate branches interconnected in multiple locations, whereas at large variances aggregates with more compact structure and smaller longest dimension were generated. The rate of aggregation and particle assemblage was found to be faster at broader PPSDs, in turn generating aggregates with narrower size distributions and more compact structures. The influence of PPSD on coordination number (CN) was found to be minor while a decrease in PPSD variances led to an increase in both the mass-equivalent size and the longest dimension of aggregates. Further, suspension yield stress decreased as PPSD became broader. The simulation results agreed well with the experimental measurements and the published data.
ABSTRACT Packed and fluidised beds in microfluidic devices offer the potential of enhanced heat a... more ABSTRACT Packed and fluidised beds in microfluidic devices offer the potential of enhanced heat and mass transfer capability at a scale where the process can be closely controlled. The knowledge of hydrodynamics of packed and fluidised beds in capillary tubes is essential for the design and optimization of such devices. This study experimentally examines the hydrodynamics of packed and fluidised beds in terms of pressure drop, bed expansion and minimum fluidisation velocity in tube sizes with inner diameters of 0.8, 1.2 and 17.1mm. Specifically the influence of the wall on the hydrodynamic characteristics of the beds was examined by changing the tube-to-particle diameter ratio.It was found that as the tube diameter reduces the bed voidage sharply increases leading to a reduction in the pressure drop across the bed. Also a pressure drop overshoot was observed at lower tube-to-particle diameter ratios which found to be associated with contact stresses due to wall friction.
An experimental and numerical study has been conducted to investigate the flow field in the vicin... more An experimental and numerical study has been conducted to investigate the flow field in the vicinity of a moving solid boundary that passes through a free surface into a liquid phase. Through the use of particle image velocimetry (PIV) techniques, the variation in the liquid velocity field in the vicinity of the three-phase contact line has been quantified for solid boundary velocities ranging between 0.12 and 1.01 m s Ϫ1. The experimental measurements provide good verification for a preliminary numerical model that predicts the bulk-bath flow patterns and boundary layer thickness.
Expansion behaviour of a binary solid-liquid fluidised bed (SLFB) system with different initial m... more Expansion behaviour of a binary solid-liquid fluidised bed (SLFB) system with different initial mass of solids was studied both experimentally and numerically. Three different sizes (3, 5 & 8 mm diameter) of borosilicate glass beads of equal density (2230 kgm) were used as fluidised particles. Three different combinations of particle size pairs of both equal and unequal mass ratios were used using a constant liquid (water) superficial velocity of 0.17 ms in all the cases. Numerically, a two dimensional Eulerian-Eulerian (E-E) CFD model incorporating kinetic theory of granular flow (KTGF) was developed to predict the bed expansion behaviour. It was observed that complete bed segregation occurred when the difference between the solid particle diameters was higher while lower difference in particle diameters led to partial bed segregation. The CFD model also predicted these behaviours which were in good agreement with the experimental data.
In this paper, the fluidization of 8 mm glass particles in water has been simulated using a new m... more In this paper, the fluidization of 8 mm glass particles in water has been simulated using a new methodology developed within the DEM framework. In this methodology, random liquid fluctuating velocities are used as direct input into the drag model. The specific aim of this study is to directly compute the granular pressure in a liquid fluidized bed. The granular pressure is defined using the particle-wall collision frequency and the corresponding particle momentum transport during the collision. Initially, we validated our model by comparing the relationship between superficial fluid velocity and bed expansion against the well-known Richardson-Zaki [1] equation. The results demonstrated a good agreement of our model. The granular pressure and temperature, as well as the particle-wall collision frequency, in the liquid fluidized bed were determined for superficial fluid velocities in the range between 0.08 and 0.32 m/s. The granular pressure exhibited a maximum (between 0.3-0.4 solid ...
The Australian Research Council has recently established an Industrial Transformation Research Hu... more The Australian Research Council has recently established an Industrial Transformation Research Hub in the area of iron ore, focussed on iron ore characterisation, beneficiation, materials handling and end-use functionality. The research hubs are part of a new initiative aimed at building stronger relationships between industry and universities, while addressing issues of national significance. "The ARC Research Hub for Advanced Technologies for Australian Iron Ore" will focus its attention on developing innovative approaches for creating enhanced value across the full value chain, through characterisation of different ore types, beneficiation, and materials handling and transport. This hub brings together five industrial organisations and a research team formed from well-established research groups based at the Newcastle Institute for Energy and Resources (NIER). These groups have a record of working closely with industry, creating opportunities. Examples include new beneficiation technologies such as the Reflux Classifier for achieving gravity separation, the expertise of TUNRA Bulk Solids in materials handling, and the Iron Making Centre at Newcastle, a capability maintained by BHP Billiton since the closure of its Technology Centre in 2009. The purpose of this paper is to provide an outline of the research hub, and its objectives, while providing some background on the research groups which now form the hub, and the strategy adopted for developing and implementing new technologies. The paper then describes a range of novel technologies that have the potential to be developed and applied to iron ore beneficiation.
Droplet evaporation in fluidized beds is of great interest in applications like fluidized catalyt... more Droplet evaporation in fluidized beds is of great interest in applications like fluidized catalytic cracking units. Although a significant number of analyses are available for modelling of droplet vaporization in a fluidized bed, very little work has been performed experimentally to measure the vapour concentration followed by numerical validation. In the present work, acetone droplet evaporation in a bubbling fluidized bed is studied experimentally as well as numerically. A liquid jet of acetone is injected into a hot bubbling fluidized bed kept well above saturation temperature of acetone. Nonintrusive Schlieren imaging, based on the difference in refractive index, is used to trace the acetone vapour concentration profile. The bubbling fluidized bed is modelled in an Eulerian framework using a simplistic porous media approach while the droplets are modelled in a Lagrangian framework. Intense interactions are observed between the evaporating droplets and hot particles during contac...
This paper reports on the collision interaction between a confined liquid film (water) and an imp... more This paper reports on the collision interaction between a confined liquid film (water) and an impacting hydrophilic glass particle of different diameters (1.1-2mm) and impact velocities (0.2-1 m/s). Depending on the impact Weber number (1.4-23.9) of the particles either complete capture of the particle inside the film or complete penetration through the film were observed. In the latter case a certain amount of liquid mass was found to adhere to the particle surface which again depended on the impact Weber number. A criterion was developed based on the energy balance approach to demarcate between these two regimes. Also, an analytical model was proposed to approximately determine the liquid mass attached to the particle. Computationally, a three dimensional CFD model was developed using the VOF approach and dynamic meshing technique which was found to be in good agreement with the experimentally observed dynamics in these two regimes.
This paper investigates the friction coefficient of a moving liquid marble, a small liquid drople... more This paper investigates the friction coefficient of a moving liquid marble, a small liquid droplet coated with hydrophobic powder and floating on another liquid surface. A floating marble can easily move across water surface due to the low friction, allowing for the transport of aqueous solutions with minimal energy input. However, the motion of a floating marble has yet to be systematically characterised due to the lack of insight into key parameters such as the coefficient of friction between the floating marble and the carrier liquid. We measured the coefficient of friction of a small floating marble using a novel experimental setup that exploits the non-wetting properties of a liquid marble. A floating liquid marble pair containing a minute amount magnetite particles were immobilised and then released in a controlled manner using permanent magnets. The capillarity-driven motion was analysed to determine the coefficient of friction of the liquid marbles. The "capillary charge" model was used to fit the experimental results. We varied the marble content and carrier liquid to establish a relationship between the friction correction factor and the meniscus angle.
This paper reports the direct and precise measurement of bubble coalescence in salt solutions usi... more This paper reports the direct and precise measurement of bubble coalescence in salt solutions using microfluidics. We directly visualized the bubble coalescence process in a microchannel using high-speed imaging and evaluated the shortest coalescence time to determine the transition concentration of sodium halide solutions. We found the transition concentration is ion-specific, and the capacity of sodium halide salts to inhibit bubble coalescence follows the order of NaF > NaCl > NaBr > NaI. The microfluidic method overcomes the inherent uncertainties in conventional large-scale devices and methods.
Langmuir : the ACS journal of surfaces and colloids, Jun 26, 2016
Numerous works have been reported on the evaporation of a sessile liquid marble, a liquid droplet... more Numerous works have been reported on the evaporation of a sessile liquid marble, a liquid droplet coated with particles, through geometric measurements. The lack of gravimetric measurement limits further understanding on the physical changes of a liquid marble during the evaporation process. Furthermore, the evaporation process of a marble containing a liquid binary mixture has not been reported. This paper studies the effective density and effective surface tension of an evaporating liquid marble which contains aqueous ethanol at relatively low concentrations. The effective density of an evaporating liquid marble is determined from the instantaneous mass and volume measurements. Subsequently, density measurements combined with surface profile fitting provides the instantaneous effective surface tension of the marble. We found that the density and surface tension of the evaporating marble is greatly affected by the particle coating.
in Wiley Online Library (wileyonlinelibrary.com) Hydrodynamics of collision interactions between ... more in Wiley Online Library (wileyonlinelibrary.com) Hydrodynamics of collision interactions between a particle and gas-liquid interface such as droplet/film is of keen interest in many engineering applications. The collision interaction between a suspended liquid (water) film of thickness 3.41 6 0.04 mm and an impacting hydrophilic particle (glass ballotini) of different diameters (1.1-3.0 mm) in low particle impact Weber number (We 5 q l v 2 p d p =r) range (1.4-33) is reported. Two distinct outcomes were observed-particle retention in the film at lower Weber number and complete penetration of the film toward higher Weber number cases. A collision parameter was defined based on energy balance approach to demarcate these two interaction regimes which agreed reasonably well with the experimental outcomes. It was shown that the liquid ligament forming in the complete penetration cases breaks up purely by "dripping/end pinch-off" mechanism and not due to capillary wave instability. An analytical model based on energy balance approach was proposed to determine the liquid mass entrainment associated with the ligament which compared well with the experimental measurements. A good correlation between the %film mass entrained and the particle Bond number (Bo 5 q l gd 2 p =r) was obtained which indicated a dependency of Bo 1.72. Computationally, a three-dimensional CFD model was developed to simulate these interactions using different contact angle boundary conditions which in general showed reasonable agreement with experiment but also indicated deficiency of a constant contact angle value to depict the interaction physics in entirety. The computed force profiles from computational fluid dynamics (CFD) model suggest dominance of the pressure force over the viscous force almost by an order of magnitude in all the Weber number cases studied.
A DEM-based model was developed and examined for simulation of aggregation in suspensions of α-al... more A DEM-based model was developed and examined for simulation of aggregation in suspensions of α-alumina nanoparticles. In the model, the random Brownian diffusion and the externally induced dielectrophoresis (DEP) motion were considered as the driving mechanisms for the transport of particles in colloidal suspension. To simulate particle interactions, the non-contact surface force and the contact force were taken into account using the well-known Derjaguin-Landau-Verway-Overbeek (DLVO) theory and the soft-sphere model, respectively. Specifically, the model was used to study the effects of pH, solid volume fraction and external AC electric field on α-alumina aggregate growth which was expressed in terms of coordination number, longest dimension, and fractal dimension. The simulations were carried out over a pH range of 4-10, solid volume fraction of 0.02-0.4, and a variety of AC electric fields. In relatively dilute suspensions, the aggregates predominantly exhibited chainlike structures, whereas at high solid volume fraction, aggregates with complex netlike structures were formed. It was also evident that, in concentrated colloidal suspensions, DEP had a negligible influence on aggregate growth over the examined conditions. The effect of DEP however, was found to be more noticeable on aggregate structure leading to the formation of more compact aggregates with a greater particle number density. The break-up and reattachment of sub-aggregates as well as the rearrangement of nanoparticles in the particle assemblies and subsequent curling of the loose network promoted by a strong AC electric field was deemed to be responsible for this structural transformation. Finally, the DEM-based model was used to predict the size of α-alumina aggregates over a range of pH. The predictions were found to be in good agreement with the published experimental data, particularly around the isoelectric point.
Multiphase systems are ubiquitous in industrial applications aimed at the generation of products ... more Multiphase systems are ubiquitous in industrial applications aimed at the generation of products either by chemical/biological reaction or physical separation based on density, electrical charge or surface properties such as hydrophobicity. The physical processing of these multiphase systems is carried out at all scales of operation and within an endless variety of vessel shapes and ancillary devices. Underpinning each process is a complex interaction between phases involving hydrodynamic, heat and mass transport. These phenomena are in turn governed largely by the nature of the flow, and in particular whether laminar or turbulent conditions prevail. In large-scale industrial processes the flows are almost always turbulent, whilst for microscale operations the flow will be laminar. Each condition provides its own challenge in being able to predict (and optimise) performance in terms of operational stability and efficiency of energy utilisation. Turbulent systems are particularly dif...
The success of many industrial processes largely depends on the structural characteristics of agg... more The success of many industrial processes largely depends on the structural characteristics of aggregates. In intensive aerobic digestion process for wastewater treatment applications, the structural characteristics namely aggregate shape, size and therefore the aggregate surface area strongly influence the transfer of dissolved oxygen from the aeration process to aggregates of harmful contaminants / microorganisms. The aim of this study was to apply discrete element modeling (DEM) techniques to the aggregation of suspended particles (microorganisms) to quantify the available surface area for convection and diffusion as a function of particles number concentration and surface charge. In this technique each solid particle was considered individually, thus accounting for its complex dynamics due to particle-particle and particle-fluid interactions. Periodic boundary conditions were adopted for all sides of the domain to minimize the computational requirements. The simulation inputs included particle and fluid characteristics such as particle size and density, solid concentration, suspension pH and ionic strength. A post processing method based on the Go-chess concept was developed to quantify the surface area of aggregate structure. The simulation results showed that whilst an increase in connection points increases the total surface area of the aggregate, this does not necessarily translate into an increase in the surface area available for oxygen transfer as combinations of open and close pores are formed. Aggregate surface area was directly determined by aggregate structural characteristics, and increased rapidly when the coordination number was below 3.5 and the fractal dimension was less than 1.5. A correlation for prediction of aggregate external surface area was also proposed as a function of aggregate structural characteristics in terms of fractal dimension and coordination number.
ABSTRACT In this study the transition from homogeneous to heterogeneous flow in a solid–liquid fl... more ABSTRACT In this study the transition from homogeneous to heterogeneous flow in a solid–liquid fluidized bed (SLFB) is examined both experimentally and numerically. The experimental apparatus comprised a refractive index-matched SLFB, comprising 5 mm diameter borosilicate glass and sodium iodine solution, which allowed for both instantaneous particle image velocimetry of the liquid flow field and solids hold-up measurements to be undertaken for superficial liquid velocities in the range of 0.06–0.22 m/s. The motion of individual, spherical steel balls (with diameters 6, 7, 8, 9 mm) was then tracked as it settled through the fluidized bed for differing superficial liquid velocities. It was observed that, for all the steel balls covered in this work, there was a change in slope in their respective classification velocity curves at a superficial liquid velocity of 0.08 m/s. This value was very close to the critical velocity of 0.085 m/s predicted from 1-D linear stability analysis; and therefore deemed to be the critical condition that marked the transition from homogeneous to non-homogenous flow. It is proposed that the change in slope of the classification velocity curve is due to the encounter of the settling foreign particles with liquid bubbles whose presence marks the onset of heterogeneous flow. Additional computational analysis, involving both Eulerian–Eulerian (E–E) and Eulerian–Lagrangian (E–L) approaches, is used to confirm the presence of liquid bubbles at a critical liquid hold-up of 0.54, which corresponds to that predicted from 1-D linear stability analysis. In summary, the study has highlighted that experimentally the transition condition for a SLFB can be obtained simply by observing the behavior of the classification velocity of a single foreign particle at different superficial liquid velocities. This transition condition was found to agree with the 1D linear stability criterion, Eulerian–Eulerian CFD (3D) and Eulerian–Lagrangian DEM (3D) approaches.
Forces acting on a single introduced particle in a solid-liquid fluidised BEd, Chemical Engineeri... more Forces acting on a single introduced particle in a solid-liquid fluidised BEd, Chemical Engineering Science,
The influence of primary particle size distribution (PPSD) on aggregation behaviour and the resul... more The influence of primary particle size distribution (PPSD) on aggregation behaviour and the resulting effect on yield stress of a concentrated colloidal suspension was investigated theoretically. The discrete element model (DEM) combined with the well-known DLVO theory was employed to obtain an insight into the aggregation process of nanoparticles with different PPSDs, where a modified version of the Flatt and Brown model [J. Am. Ceram. Soc. 89 (2006) 1244-1256] [9] was employed to calculate the corresponding suspension yield stress from the simulation results. Specifically, the aggregate growth and structure in terms of fractal dimension, coordination number and the longest dimension were examined. It was shown that at small PPSD variances, a netlike structure was formed with aggregate branches interconnected in multiple locations, whereas at large variances aggregates with more compact structure and smaller longest dimension were generated. The rate of aggregation and particle assemblage was found to be faster at broader PPSDs, in turn generating aggregates with narrower size distributions and more compact structures. The influence of PPSD on coordination number (CN) was found to be minor while a decrease in PPSD variances led to an increase in both the mass-equivalent size and the longest dimension of aggregates. Further, suspension yield stress decreased as PPSD became broader. The simulation results agreed well with the experimental measurements and the published data.
ABSTRACT Packed and fluidised beds in microfluidic devices offer the potential of enhanced heat a... more ABSTRACT Packed and fluidised beds in microfluidic devices offer the potential of enhanced heat and mass transfer capability at a scale where the process can be closely controlled. The knowledge of hydrodynamics of packed and fluidised beds in capillary tubes is essential for the design and optimization of such devices. This study experimentally examines the hydrodynamics of packed and fluidised beds in terms of pressure drop, bed expansion and minimum fluidisation velocity in tube sizes with inner diameters of 0.8, 1.2 and 17.1mm. Specifically the influence of the wall on the hydrodynamic characteristics of the beds was examined by changing the tube-to-particle diameter ratio.It was found that as the tube diameter reduces the bed voidage sharply increases leading to a reduction in the pressure drop across the bed. Also a pressure drop overshoot was observed at lower tube-to-particle diameter ratios which found to be associated with contact stresses due to wall friction.
An experimental and numerical study has been conducted to investigate the flow field in the vicin... more An experimental and numerical study has been conducted to investigate the flow field in the vicinity of a moving solid boundary that passes through a free surface into a liquid phase. Through the use of particle image velocimetry (PIV) techniques, the variation in the liquid velocity field in the vicinity of the three-phase contact line has been quantified for solid boundary velocities ranging between 0.12 and 1.01 m s Ϫ1. The experimental measurements provide good verification for a preliminary numerical model that predicts the bulk-bath flow patterns and boundary layer thickness.
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Papers by Geoffrey Evans