Geoffrey Evans | The University of Newcastle (original) (raw)

Papers by Geoffrey Evans

Procedia Engineering, 2015

Froth flotation is a process that utilizes the surface properties of mineral particles to separat... more Froth flotation is a process that utilizes the surface properties of mineral particles to separate them from unwanted gangue materials. Usually, the mineral particles are either naturally hydrophobic, or surfactant-treated hydrophobic and thus, they are prone to adhere to bubbles. Once attached, the capillary force is dominant in the control of the particle bubble aggregate stability. Capillary force is a function of surface tension and contact angle, which is a measure of particle hydrophobicity. In this paper, the Discrete Element Method has been applied to simulate the interaction between particles and a central bubble in a quiescent liquid with the aim of describing: (1) the motion of the particle sliding over the bubble surface, and (2) the formation of the three phase contact line. Simulation results for particle-bubble separation distance versus time showed excellent agreement with experimental observations. Moreover, the simulations correctly predicted the time when the particle contacted the bubble. Additional simulations were carried out for multi-particle attachment that highlighted the interaction between particles, including collision and rebounding phenomena. These simulations correctly captured experimental observations reported in the literature and have provided valuable insight into the particle loading capacity of bubbles being utilized in mineral flotation devices.

International Journal of Multiphase Flow

ABSTRACT

Frontiers in chemistry, 2018

Dynamic surface wetting of particles in contact with droplet is a complex phenomenon ubiquitously... more Dynamic surface wetting of particles in contact with droplet is a complex phenomenon ubiquitously encountered in many multiphase systems of industrial importance. In this study, we address this aspect by investigating impact behavior of a water droplet (diameter = 2.9 ± 0.1 mm) in the Weber number () range from ~4 to 104 on a stationary spherical brass particle (diameter = 10 mm) with and without heat transfer using a combination of high speed imaging and computational fluid dynamics (CFD) modeling approach. In cold state interactions (20°C), droplet exhibited oscillatory interfacial motion comprising periodic spreading and recoiling motion. Interactions involving heat transfer were studied in film boiling regime (350°C) and two outcomes were noted-droplet rebound and disintegration. A coupled Level Set and Volume of Fluid (VOF) approach based multiphase CFD model was utilized to predict the dynamic spread ratio and transient evolution of droplet shape during the interaction. To cap...

Scientific Reports, 2016

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.

The Canadian Journal of Chemical Engineering, 2014

ABSTRACT Turbulence plays a critical role in detachment process of bubble from a solid surface. T... more ABSTRACT Turbulence plays a critical role in detachment process of bubble from a solid surface. To investigate this effect, detachment process of a stationary air bubble from a nozzle in both quiescent and turbulent liquid field was studied. A stationary vertical (flat-ended) needle of ID 1.24 mm was used as a nozzle to generate a bubble which was anchored to the needle tip. Different sizes of bubbles were generated in quiescent liquid. Volume and contact angle for these bubbles were measured precisely using microscopic imaging technique and correlated. In the quiescent case experiments, a constant contact angle of 90o and bubble diameter of 3.05 ± 0.004 mm were obtained consistently. A simple force balance approach was proposed assuming bubble in equilibrium to determine this maximum bubble diameter during detachment. The detached bubble size calculated using this approach agreed fairly well with the experimental results. An oscillating grid device capable of operating at different frequencies was then applied to generate a homogeneous, near-isotropic turbulent velocity field around the anchored bubble. It was observed that for detachment of smaller bubbles, higher turbulence intensity was indeed necessary. The turbulent flow field was quantified using particle image velocimetry (PIV) technique and resolved into flow structures (eddies) of different length scales using a Gaussian filter. It was concluded that smaller eddies perturbed the bubble interface whilst the larger eddies contributed to weakening of the capillary force causing the bubble detachment. Energy dissipation profile obtained from the PIV images indicated significant energy dissipation near the bubble compared to the bulk fluid which supported the fact that strong interactions between bubble and eddies were indeed responsible for bubble detachment.

In this study, the penetration behavior of a particle impacting a gas-liquid interface has been i... more In this study, the penetration behavior of a particle impacting a gas-liquid interface has been investigated. High speed video has been used to examine the interface response within the impact zone, and to record the motion of the particle and three-phase contact line (TPCL) under the critical and non-critical conditions. It has been found that the correlation of velocity of particle and TPCL with particle position can be expressed as a quadratic function. The differences in cavity profile under different conditions have also been identified. In addition, the effect of individual factors on the critical impact velocity has been analysed. The results have shown that the surface tension force dominates the penetration process. The potential maximum size of floatable particles has been suggested theoretically.

Advanced Powder Technology, 2014

The capture of solid particles by air bubbles is a complex process influenced by hydrodynamic and... more The capture of solid particles by air bubbles is a complex process influenced by hydrodynamic and surface forces. Computer simulation provides an alternative way to experimental approaches to give an insight into the phenomenon of particle capture. In this paper, the threedimensional Discrete Element Method (DEM) has been applied to simulate the interactions between fine particles and a central bubble in a quiescent liquid. A system, consisting of 200 monodisperse silica particles and a stationary air bubble, was considered. Drag, buoyancy, hydrodynamic resistance, hydrophobic and gravitational forces have been included in the simulations. The attractive hydrophobic forces between the particles and the bubble was estimated through a single exponential decay law which depends on two parameters, K and λ. K is related to the maximum strength of the hydrophobic force, while λ is the length that indicates how fast the hydrophobic force decays with particle-bubble distance. The results have shown that a decrease in the ratio of maximum hydrophobic force to particle weight from 5.2×10 6 to 5.2 produced a decrease in the capture efficiency by 42.6% for λ=1 μm and by 12.5% for λ=1 nm.

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...

Chemical Engineering Research and Design, 2015

ABSTRACT Vaporization of atomised feedstock is one of the critical processes in Fluid Catalytic C... more ABSTRACT Vaporization of atomised feedstock is one of the critical processes in Fluid Catalytic Cracking (FCC) risers; which is more often ignored in most of the FCC riser modelling studies. In this study, two different vaporization mechanisms of feedstock namely homogeneous mode and heterogeneous mode were studied. Different homogeneous models duly validated for various pure component droplets were applied to predict the vaporization time of the feed droplets typically expected in FCC feed vaporization zone. A new physical model for heterogeneous vaporization considering droplet-particle collision mechanics was also developed in the present study which compared well with the other existing heterogeneous modelling approaches. Comparison of the two vaporization modes indicates that under typical operating conditions of FCC riser, vaporization time of feed droplets predicted by heterogeneous mode is always lower than the homogeneous mode at least by an order of magnitude due to significant increase in heat transfer coefficient which accounts for droplet-particle contact. It is expected that actual vaporization time of feed droplets in an industrial FCC riser should lie in the range predicted by these two vaporization mechanisms which actually set the two limiting modes of vaporization. Obtained results predicted by the models could be used to aid design of the FCC feed vaporization zone.

Minerals Engineering, 2014

ABSTRACT A flotation detachment model is developed by considering energy balance in the process. ... more ABSTRACT A flotation detachment model is developed by considering energy balance in the process. Energies concerned are surface energy increment and kinetic energy supplied by turbulent liquid motion. Surface energy increment is the work of adhesion by surface forces which is reflected by surface tension and contact angle. What makes this model outstanding from other detachment models of energy balance perspective is more accurate account of kinetic energy supplied from turbulent liquid motion. Eddies in the same scale as attached particles are considered accountable for particle detachment in the close vicinity. In this way, detachment probability is written as a function of energy dissipation rate. Predictions from different models are compared to experimental results. It is demonstrated that previous models overestimate the influence from turbulent liquid motion. Notably, with more accurate account of eddies’ influence, the new model predicts particle detachment in accordance with experimental results.

Advanced Powder Technology, 2015

ABSTRACT The present study investigates the collision behaviour of a smaller particle into a larg... more ABSTRACT The present study investigates the collision behaviour of a smaller particle into a larger stationary droplet – a phenomenon related to many process engineering applications. Experimentally, the collision process was studied using glass ballotini particles (diameter: 1.13 ±0.02 mm) and a supported stationary water droplet (diameter 3.41 ± 0.01 mm) at different particle impact velocities (Weber number range: 0.2 – 13.5) using high speed imaging technique. A transition from partial to complete penetration was observed with decrease in sinking time and significant shape deformation of the droplet when Weber number was increased. Numerically, a one dimensional transient force balance approach was adopted which included contributions of six major forces such as gravity, capillary force, fluid drag, buoyancy, pressure force and added mass force during the penetration process. Of all the forces, capillary force was found to be controlling such interaction process. Recognizing the limitation of one dimensional model to capture the details of the collision physics especially the movement of three phase contact line (TPCL) on the particle surface, a 3D computational fluid dynamics (CFD) model was developed using multiphase volume of fluid (VOF) method combined with the dynamic meshing technique. The proposed CFD model agreed well in predicting the sinking time of the particle and overall collision dynamics including shape deformation of the droplet when compared with the experiments.

ISIJ International, 2012

Blast gas discharge from the taphole in the course of the blast furnace hearth drainage was exper... more Blast gas discharge from the taphole in the course of the blast furnace hearth drainage was experimentally studied using a packed bed cold model. It was found that gas breakthrough time was strongly influenced by the furnace operating conditions and coke bed structure. Gas breakthrough time decreases with (a) increasing draining rate; (b) decreasing slag and iron levels in the hearth; and (c) increasing slag viscosity. It increases with an increase in the coke-free layer depth and coke-free space width. Under certain conditions, the gas-liquid interface in the region directly above the taphole becomes unstable, leading to viscous finger formation and subsequently early blast gas discharge from the taphole. The amount of blast gas entrained into the taphole due to viscous fingering, when it occurs, is sufficient to cause a splashy taphole stream.

Advanced Powder Technology, 2015

ABSTRACT The success of many industrial processes largely depends on the structural characteristi... more ABSTRACT 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 Modelling (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. 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.

Procedia Engineering, 2014

Transport phenomena occur frequently in industrial problems. Most of the turbulent transport prop... more Transport phenomena occur frequently in industrial problems. Most of the turbulent transport properties can be directly associated with the turbulent energy dissipation rate; hence it is a very significant parameter in the design of chemical processing equipment. To develop a better chemical processing equipment design, a thorough knowledge of the effect flow structure on local turbulence parameters like turbulent kinetic energy, eddy diffusivity and the energy dissipation rate are required. Turbulence is heterogeneous in most of the process equipment. Hence, the use of spatial average energy dissipation rate causes error in modelling of turbulent transport processes. In this present work, particle image velocimetry (PIV) is used to obtain the energy spectrum from grid generated homogeneous turbulence velocity data. The model of energy spectrum given by Kang et al. (2003) has been fitted to this energy spectrum using energy dissipation rate. A different approach, based on a third order structure function and velocity gradient technique has been used to compute the energy dissipation rate. The model predictions have been verified by experimental PIV velocity data from oscillating grid apparatus.

Procedia Engineering, 2015

Froth flotation is a process that utilizes the surface properties of mineral particles to separat... more Froth flotation is a process that utilizes the surface properties of mineral particles to separate them from unwanted gangue materials. Usually, the mineral particles are either naturally hydrophobic, or surfactant-treated hydrophobic and thus, they are prone to adhere to bubbles. Once attached, the capillary force is dominant in the control of the particle bubble aggregate stability. Capillary force is a function of surface tension and contact angle, which is a measure of particle hydrophobicity. In this paper, the Discrete Element Method has been applied to simulate the interaction between particles and a central bubble in a quiescent liquid with the aim of describing: (1) the motion of the particle sliding over the bubble surface, and (2) the formation of the three phase contact line. Simulation results for particle-bubble separation distance versus time showed excellent agreement with experimental observations. Moreover, the simulations correctly predicted the time when the particle contacted the bubble. Additional simulations were carried out for multi-particle attachment that highlighted the interaction between particles, including collision and rebounding phenomena. These simulations correctly captured experimental observations reported in the literature and have provided valuable insight into the particle loading capacity of bubbles being utilized in mineral flotation devices.

International Journal of Multiphase Flow

ABSTRACT

Frontiers in chemistry, 2018

Dynamic surface wetting of particles in contact with droplet is a complex phenomenon ubiquitously... more Dynamic surface wetting of particles in contact with droplet is a complex phenomenon ubiquitously encountered in many multiphase systems of industrial importance. In this study, we address this aspect by investigating impact behavior of a water droplet (diameter = 2.9 ± 0.1 mm) in the Weber number () range from ~4 to 104 on a stationary spherical brass particle (diameter = 10 mm) with and without heat transfer using a combination of high speed imaging and computational fluid dynamics (CFD) modeling approach. In cold state interactions (20°C), droplet exhibited oscillatory interfacial motion comprising periodic spreading and recoiling motion. Interactions involving heat transfer were studied in film boiling regime (350°C) and two outcomes were noted-droplet rebound and disintegration. A coupled Level Set and Volume of Fluid (VOF) approach based multiphase CFD model was utilized to predict the dynamic spread ratio and transient evolution of droplet shape during the interaction. To cap...

Scientific Reports, 2016

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.

The Canadian Journal of Chemical Engineering, 2014

ABSTRACT Turbulence plays a critical role in detachment process of bubble from a solid surface. T... more ABSTRACT Turbulence plays a critical role in detachment process of bubble from a solid surface. To investigate this effect, detachment process of a stationary air bubble from a nozzle in both quiescent and turbulent liquid field was studied. A stationary vertical (flat-ended) needle of ID 1.24 mm was used as a nozzle to generate a bubble which was anchored to the needle tip. Different sizes of bubbles were generated in quiescent liquid. Volume and contact angle for these bubbles were measured precisely using microscopic imaging technique and correlated. In the quiescent case experiments, a constant contact angle of 90o and bubble diameter of 3.05 ± 0.004 mm were obtained consistently. A simple force balance approach was proposed assuming bubble in equilibrium to determine this maximum bubble diameter during detachment. The detached bubble size calculated using this approach agreed fairly well with the experimental results. An oscillating grid device capable of operating at different frequencies was then applied to generate a homogeneous, near-isotropic turbulent velocity field around the anchored bubble. It was observed that for detachment of smaller bubbles, higher turbulence intensity was indeed necessary. The turbulent flow field was quantified using particle image velocimetry (PIV) technique and resolved into flow structures (eddies) of different length scales using a Gaussian filter. It was concluded that smaller eddies perturbed the bubble interface whilst the larger eddies contributed to weakening of the capillary force causing the bubble detachment. Energy dissipation profile obtained from the PIV images indicated significant energy dissipation near the bubble compared to the bulk fluid which supported the fact that strong interactions between bubble and eddies were indeed responsible for bubble detachment.

In this study, the penetration behavior of a particle impacting a gas-liquid interface has been i... more In this study, the penetration behavior of a particle impacting a gas-liquid interface has been investigated. High speed video has been used to examine the interface response within the impact zone, and to record the motion of the particle and three-phase contact line (TPCL) under the critical and non-critical conditions. It has been found that the correlation of velocity of particle and TPCL with particle position can be expressed as a quadratic function. The differences in cavity profile under different conditions have also been identified. In addition, the effect of individual factors on the critical impact velocity has been analysed. The results have shown that the surface tension force dominates the penetration process. The potential maximum size of floatable particles has been suggested theoretically.

Advanced Powder Technology, 2014

The capture of solid particles by air bubbles is a complex process influenced by hydrodynamic and... more The capture of solid particles by air bubbles is a complex process influenced by hydrodynamic and surface forces. Computer simulation provides an alternative way to experimental approaches to give an insight into the phenomenon of particle capture. In this paper, the threedimensional Discrete Element Method (DEM) has been applied to simulate the interactions between fine particles and a central bubble in a quiescent liquid. A system, consisting of 200 monodisperse silica particles and a stationary air bubble, was considered. Drag, buoyancy, hydrodynamic resistance, hydrophobic and gravitational forces have been included in the simulations. The attractive hydrophobic forces between the particles and the bubble was estimated through a single exponential decay law which depends on two parameters, K and λ. K is related to the maximum strength of the hydrophobic force, while λ is the length that indicates how fast the hydrophobic force decays with particle-bubble distance. The results have shown that a decrease in the ratio of maximum hydrophobic force to particle weight from 5.2×10 6 to 5.2 produced a decrease in the capture efficiency by 42.6% for λ=1 μm and by 12.5% for λ=1 nm.

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...

Chemical Engineering Research and Design, 2015

ABSTRACT Vaporization of atomised feedstock is one of the critical processes in Fluid Catalytic C... more ABSTRACT Vaporization of atomised feedstock is one of the critical processes in Fluid Catalytic Cracking (FCC) risers; which is more often ignored in most of the FCC riser modelling studies. In this study, two different vaporization mechanisms of feedstock namely homogeneous mode and heterogeneous mode were studied. Different homogeneous models duly validated for various pure component droplets were applied to predict the vaporization time of the feed droplets typically expected in FCC feed vaporization zone. A new physical model for heterogeneous vaporization considering droplet-particle collision mechanics was also developed in the present study which compared well with the other existing heterogeneous modelling approaches. Comparison of the two vaporization modes indicates that under typical operating conditions of FCC riser, vaporization time of feed droplets predicted by heterogeneous mode is always lower than the homogeneous mode at least by an order of magnitude due to significant increase in heat transfer coefficient which accounts for droplet-particle contact. It is expected that actual vaporization time of feed droplets in an industrial FCC riser should lie in the range predicted by these two vaporization mechanisms which actually set the two limiting modes of vaporization. Obtained results predicted by the models could be used to aid design of the FCC feed vaporization zone.

Minerals Engineering, 2014

ABSTRACT A flotation detachment model is developed by considering energy balance in the process. ... more ABSTRACT A flotation detachment model is developed by considering energy balance in the process. Energies concerned are surface energy increment and kinetic energy supplied by turbulent liquid motion. Surface energy increment is the work of adhesion by surface forces which is reflected by surface tension and contact angle. What makes this model outstanding from other detachment models of energy balance perspective is more accurate account of kinetic energy supplied from turbulent liquid motion. Eddies in the same scale as attached particles are considered accountable for particle detachment in the close vicinity. In this way, detachment probability is written as a function of energy dissipation rate. Predictions from different models are compared to experimental results. It is demonstrated that previous models overestimate the influence from turbulent liquid motion. Notably, with more accurate account of eddies’ influence, the new model predicts particle detachment in accordance with experimental results.

Advanced Powder Technology, 2015

ABSTRACT The present study investigates the collision behaviour of a smaller particle into a larg... more ABSTRACT The present study investigates the collision behaviour of a smaller particle into a larger stationary droplet – a phenomenon related to many process engineering applications. Experimentally, the collision process was studied using glass ballotini particles (diameter: 1.13 ±0.02 mm) and a supported stationary water droplet (diameter 3.41 ± 0.01 mm) at different particle impact velocities (Weber number range: 0.2 – 13.5) using high speed imaging technique. A transition from partial to complete penetration was observed with decrease in sinking time and significant shape deformation of the droplet when Weber number was increased. Numerically, a one dimensional transient force balance approach was adopted which included contributions of six major forces such as gravity, capillary force, fluid drag, buoyancy, pressure force and added mass force during the penetration process. Of all the forces, capillary force was found to be controlling such interaction process. Recognizing the limitation of one dimensional model to capture the details of the collision physics especially the movement of three phase contact line (TPCL) on the particle surface, a 3D computational fluid dynamics (CFD) model was developed using multiphase volume of fluid (VOF) method combined with the dynamic meshing technique. The proposed CFD model agreed well in predicting the sinking time of the particle and overall collision dynamics including shape deformation of the droplet when compared with the experiments.

ISIJ International, 2012

Blast gas discharge from the taphole in the course of the blast furnace hearth drainage was exper... more Blast gas discharge from the taphole in the course of the blast furnace hearth drainage was experimentally studied using a packed bed cold model. It was found that gas breakthrough time was strongly influenced by the furnace operating conditions and coke bed structure. Gas breakthrough time decreases with (a) increasing draining rate; (b) decreasing slag and iron levels in the hearth; and (c) increasing slag viscosity. It increases with an increase in the coke-free layer depth and coke-free space width. Under certain conditions, the gas-liquid interface in the region directly above the taphole becomes unstable, leading to viscous finger formation and subsequently early blast gas discharge from the taphole. The amount of blast gas entrained into the taphole due to viscous fingering, when it occurs, is sufficient to cause a splashy taphole stream.

Advanced Powder Technology, 2015

ABSTRACT The success of many industrial processes largely depends on the structural characteristi... more ABSTRACT 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 Modelling (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. 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.

Procedia Engineering, 2014

Transport phenomena occur frequently in industrial problems. Most of the turbulent transport prop... more Transport phenomena occur frequently in industrial problems. Most of the turbulent transport properties can be directly associated with the turbulent energy dissipation rate; hence it is a very significant parameter in the design of chemical processing equipment. To develop a better chemical processing equipment design, a thorough knowledge of the effect flow structure on local turbulence parameters like turbulent kinetic energy, eddy diffusivity and the energy dissipation rate are required. Turbulence is heterogeneous in most of the process equipment. Hence, the use of spatial average energy dissipation rate causes error in modelling of turbulent transport processes. In this present work, particle image velocimetry (PIV) is used to obtain the energy spectrum from grid generated homogeneous turbulence velocity data. The model of energy spectrum given by Kang et al. (2003) has been fitted to this energy spectrum using energy dissipation rate. A different approach, based on a third order structure function and velocity gradient technique has been used to compute the energy dissipation rate. The model predictions have been verified by experimental PIV velocity data from oscillating grid apparatus.