Density Segregation Research Articles

Modelling segregation of bidisperse granular mixtures varying simultaneously in size and density for free surface flows

Abstract

Hydrodynamic and separation performance of gas–solid separation fluidized bed with two-size-orifice distributor

The air distributor is an important part of the gas–solid separation fluidized bed, which directly determines the air distribution across the bed base. In this study, numerical simulation and experimental analysis are used together to study the influence of a two-size-orifice distributor on the fluidization quality and separation performance of the fluidized bed. Results show that the bed is prone to channeling when the air distributor contains 6mm orifices, and the values of fluidization parameters of the bed, i.e., pressure drop, voidage, and density stability, increase with decreasing orifice size. Owing to the redistribution of airflow caused by large orifices in the center, followed by the elimination of sidewall effects, the two-size-orifice distributor with 4mm inner orifices and 2mm outer orifices can improve the fluidization quality compared with other distributors and further promote the density segregation process of coal particles in the bed (ash separation degree Sash=21.62%). Finally, the coal particles are separated in a fluidized bed with the abovementioned two-size-orifice distributor. The yield of clean coal products is 54.83% with an ash content of 20.96%, while the yield of gangue is 45.17% with an ash content of 30.77%. Moreover, the possible error is as low as 0.10g/cm3, indicating the good separation effect of the gas–solid separation fluidized bed with the two-size-orifice distributor.

Shear-driven density segregation: an experimental study

Granular materials can segregate spontaneously due to differences in particle properties when subjected to vibrations, shear strain or because of the equipment geometries. Although the difference in particle size is the most critical factor that drives segregation, the effects of large density difference may also be detrimental for a lot of industries. In this work, we experimentally investigate density-driven segregation in bi-disperse mixtures of particles having the same size but different density when subjected to non-uniform shear rates. We found that the features of the segregation process are related to the density ratio as well as to the dimensionless loaded mass. The experimental outcomes are then compared with the solution of a simple density-driven segregation model. The model can successfully capture the main features of segregation driven by density for a range of density ratios.

Designing non-segregating granular mixtures

In dense flowing bidisperse particle mixtures varying in size or density alone, large particles rise (driven by percolation) and heavy particles sink (driven by buoyancy). When the two particle species differ from each other in both size and density, the two segregation mechanisms either enhance (large/light and small/heavy) or oppose (large/heavy and small/light) each other. In the latter case, an equilibrium condition exists in which the two mechanisms balance and the particles no longer segregate. This leads to a methodology to design non-segregating particle mixtures by specifying particle size ratio, density ratio, and mixture concentration to achieve the equilibrium condition. Using DEM simulations of quasi-2D bounded heap flow, we show that segregation is significantly reduced for particle mixtures near the equilibrium condition. In addition, the rise-sink transition for a range of particle size and density ratios matches the predictions of the combined size and density segregation model.

Resolved CFD–DEM coupling simulation using Volume Penalisation method

A resolved CFD–DEM coupling model for the simulation of particulate flows is proposed in this work. The Volume Penalisation (VP) method, which is a family of the continuous forcing Immersed Boundary (IB) method, is employed to express the particle–fluid interaction. A smooth mask function is used to avoid sharp transition between the solid (particle) and fluid domains that may cause numerical oscillation with moving particles. Optimal permeability is employed to reduce the model error associated with the VP method. It is determined as a function of only the interface thickness and fluid kinematic viscosity. The proposed model is accurate, easy to implement with any discretisation scheme, and only requires small computational overhead for particle–fluid interaction. Several simulations are performed to test the validity of the proposed model in various systems, i.e. from dilute to relatively dense flows, and the results show good agreement with the exact solution or empirical correlation. It is found that the error can be scaled with the ratio between the gap and interface thickness. The proposed model is also applied to predict the relative viscosity of suspensions and the density segregation in fluidised beds.

Upscaled DEM-CFD model for vibrated fluidized bed based on particle-scale similarities

Simulation based on discrete element method (DEM) coupled with computational fluid dynamics (CFD), coupled DEM-CFD, is a powerful tool for investigating the details of dense particle–fluid interaction problems such as in fluidized beds and pneumatic conveyers. The addition of a mechanical vibration to a system can drastically alter the particle and fluid flows; however, their detailed mechanisms are not well understood. In this study, a DEM-CFD model based on a non-inertial frame of reference is developed to achieve a better understanding of the influence of vibration in a vibrated fluidized bed. Because the high computational cost of DEM-CFD calculations is still a major problem, an upscaled coarse-graining model is also employed. To realize similar behaviors with enlarged model particles, non-dimensional parameters at the particle scale were deduced from the governing equations. The suitability and limitations of the proposed model were examined for a density segregation problem of a binary system. To reduce the computational costs, we show that the ratio between the bed width and model particle size can be reduced to a minimum value of 100; to obtain similar segregation behaviors, the ratio between the bed height and model particle size is considered unchanged.

COVID-19 cases in US counties: roles of racial/ethnic density and residential segregation

ABSTRACT Objective To investigate how racial/ethnic density and residential segregation shape the uneven burden of COVID-19 in US counties and whether (if yes, how) residential segregation moderates the association between racial/ethnic density and infections. Design We first merge various risk factors from federal agencies (e.g. Census Bureau and Centers for Disease Control and Prevention) with COVID-19 cases as of June 13th in contiguous US counties (N = 3,042). We then apply negative binomial regression to the county-level dataset to test three interrelated research hypotheses and the moderating role of residential segregation is presented with a figure. Results Several key results are obtained. (1) Counties with high racial/ethnic density of minority groups experience more confirmed cases than those with low levels of density. (2) High levels of residential segregation between whites and non-whites increase the number of COVID-19 infections in a county, net of other risk factors. (3) The relationship between racial/ethnic density and COVID-19 infections is enhanced with the increase in residential segregation between whites and non-whites in a county. Conclusions The pre-existing social structure like residential segregation may facilitate the spread of COVID-19 and aggravate racial/ethnic health disparities in infections. Minorities are disproportionately affected by the novel coronavirus and focusing on pre-existing social structures and discrimination in housing market may narrow the uneven burden across racial/ethnic groups.

Dry separation of fine particulate sand mixture based on density-segregation in a vibro-fluidized bed

Separation of fine particulate solid materials is one of most important unit operations in industry. Utilization of gas-solid fluidized beds has been considered where particulates are released from constraints due to contacts with surrounding particulates and segregation occurs according to density, size or combination of density and size. Addition of mechanical vibration to the gas-solid fluidized bed may improve dry solid separation. In this study, we investigated the dry separation characteristics of solid particulates using a vibro-fluidized bed especially focusing on the separation of fine particulate ores (≈100 μm) with small density differences. At first, we focused on the influence of fluidizing air velocity on the efficiency of segregation. Subsequently, the influence of vibration strength, vibration amplitude and frequency on segregation behavior was investigated. We found the density segregation does not occur with either gas-fluidization or vertical vibration alone. Only the combination of these effects produces density segregation. The fluidizing air velocity is an important factor to enhance the density-segregation of the particulates with small density difference.

Effects of sound fields on hydrodynamic and dry beneficiation of fine coal in a fluidized bed

The difficulty of forming a stable fluidized bed for fine coal (<1 mm) reduces the effectiveness of the density-segregation process and subsequent dry beneficiation of fine coal. Sound fields provide a new method of improving the bed stability of fine coal. In this study, the hydrodynamic and dry beneficiation of fine coal under a sound field was investigated by a numerical simulation and an experiment. The results indicate that when the sound frequency f (or sound pressure level SPL) is increased, the fluidization characteristics of the bed significantly improve, although the increased sound absorption coefficient weakens the positive effects of the sound field on the fluidization characteristics under high frequencies (f > 120 Hz). Moreover, the simulation results indicate that the introduction of the sound field eliminates large bubbles at a high gas velocity, resulting in an expansion of the gas velocity range for the efficient density segregation process of fine coal. Finally, under appropriate operating conditions (f = 120 Hz, SPL = 120 dB, u = Umf), fine coal was separated in a sound-assisted fluidized bed, with probable error E values as low as 0.11 g/cm3, where the clean coal yield was 52.20% with an ash content of 23.28%; the yield of gangue products was 47.80% with an ash content of 47.80%, demonstrating the efficient beneficiation of fine coal in a sound-assisted fluidized bed.

Study of Dry Mineral Separation Behavior in Dry Density Separators

Mineral dry density separation behavior (mineral beneficiation) involves segregation of high density particles and low density particles in the feed, i.e., in nutshell, mineral dry density beneficiation is density segregation in physics terms. In dry beneficiation, this density segregation is achieved using air as fluidized medium. Density segregation on dry separators can be achieved in two ways: (1) using minimum fluidization velocity differences and (2) using terminal velocity difference between different density particles. Dry separators based on minimum fluidization velocities follow two steps for dry beneficiation: (1) vertical density stratification and (2) horizontal segregation. Particles due to buoyancy, drag and gravitational forces get vertically density stratified and later frictional, gravity and external (vibrational, etc.) forces horizontally segregate different density particles. Dry separators based on terminal velocities follow single step for dry beneficiation of mineral, and air velocity in this type of dry density separators is maintained between terminal velocity of high density and low density particles. Velocity of air is higher for low density particles, which leads to higher drag and buoyancy forces to get pushed out of system with air and get collected at further distance than high density particles leading to density segregation. Mathematical models have been developed using force balance on particles and simulated in MATLAB for both type of separators to explain the physics of separation. Solutions to the simulated mathematical model equations are the particle trajectories of different density particles in the two dry separators. Iron ore is used as feed for studying separation futures in mathematical modeling on dry separators.

BIM-ABM simulation for emergency evacuation from conference hall, considering gender segregation and architectural design

ABSTRACT Human behavior simulation in emergency evacuation helps engineers identify design shortcomings and improve building performance. Conference halls, as a public assembly, are at a greater risk than ordinary buildings due to the number of participants, space density, and gender segregation. In this study, a BIM software (Revit) with the human behavior simulation engine (AnyLogic code) upon the agent-based modeling (ABM) approach was applied to simulate emergency evacuation from NIGC conference hall with 177 delegates. In order to consider gender segregation and architectural design on egress flow, three groups of people with special characteristics and behavioral rules were considered in five different egress scenario including different positions and dimensions of exit doors. Each simulated scenario was repeated 100 times to minimize error. Then, the model was verified through a real experimental test with the cooperation of 22 volunteers. Although the average evacuation time for all scenarios did not differ beyond 13%, the results showed that placing two symmetrical exit doors on both sides of the conference hall offers the most efficient and fastest egress pattern. Also, the critical evacuation time was 0.4–0.6 min., while 80% of people left the conference hall within 1.2–1.4 min. after alarm (excluding scenario 4). The results also indicated the impact of the location of sitting of low-speed people on the evacuation time and egress pattern. All these results help architectures and building interior designers to properly design and locate exit doors, corridors, number and distance between seats and rows in a conference hall.

Influences of fine powder on dynamic properties and density segregation in a rotating drum

In this paper, the influences of the addition of a small amount of fine powder and rotation speed on the dynamic properties and density-induced segregation behavior of granular matter in a quasi-two-dimensional rotating drum were experimentally investigated. An optical camera was applied to capture the motion of the particles in the drum. Image-processing technology and a particle tracking method were applied for determining the velocity, granular temperature and segregation index of granular materials. The results indicate that the addition of a small amount of fine powder has a significant effect on dynamic properties and density-induced segregation behavior. The average velocity and the average granular temperature are enhanced with the increase of the fine powder content because of the lubrication effect between particles. Additionally, the results indicate that density segregation is strengthened with the increase of fine powder.

Order from Chaos: Dynamics of density segregation in continuously aerated granular systems

Under continual disturbance such as vibration, tumbling, flow or aeration, granular or powder systems can display solid or fluid like behavior. Using a well-mixed system of same size (0.2 mm) non-cohesive glass beads and iron powder, we show that gentle aeration can completely segregate the components thereby reducing the entropy of mixing to create near total order from an initially chaotic mixture. We quantify the time dependence of the segregation process and identify two dynamic pathways that dominate depending on the intensity of the aeration. Such findings can facilitate the search for energy efficient methods to process granular systems in pharmaceutical, mining and waste recovery industries.

Effect of bend angle on granular size segregation in the chute flow under periodic flow inversion

Segregation of granular homogeneous mixtures is a common problem faced by many industries. In recent years, periodic flow inversions have shown to mitigate both size and density segregation. However, they are not fully explored. In this paper, we investigate the effect of bend angle (θ) in the chute flow using three-dimensional Discrete Element Method (DEM) simulations. We observed a significant difference in size segregation when changing the bend angle (θ) in terms of qualitative behavior and extent of segregation. For moderate and dense regimes, minimum segregation is observed for bend angle range of 120°–150°. Based on our results, we propose to use chutes with periodic 150° bends instead of vertical chutes, to reduce segregation and thus improve mixing.

In situ study on fracture behaviour of white etching layers formed on rails

Failure in engineering materials like steels is strongly affected by in-service deleterious alterations in their microstructure. White Etching Layers (WELs) are an example of such in-service alterations in the pearlitic microstructure at the rail surface. Cracks initiate in the rails due to delamination and fracture of these layers and propagate into the base material posing severe safety concerns. In this study, we investigate the microscale fracture behaviour of these WELs. We use in situ elastic-plastic fracture mechanics using J-integral to quantify the fracture toughness. Although usually assumed brittle, the fracture toughness of 21–25 MPa√m reveals a semi-brittle nature of WELs. Based on a comparison of the fracture toughness and critical defect size of WELs with the undeformed pearlitic steels, WELs are detrimental for rails. In the micro fracture tests, WELs show crack tip blunting, branching, and significant plasticity during crack growth due to their complex microstructure. The fracture behaviour of the WELs is governed by their microstructural constituents such as phases (martensite/austenite), grain size, dislocation density and carbon segregation to dislocations and grain boundaries. We observed dislocation annihilation in some martensitic grains in the WELs which also contributes to their fracture behaviour. Additionally, the strain-induced transformation from austenite to martensite affects the crack growth and fracture.

Parametric evaluation and performance optimization of fine coal separation in a vibrated gas-fluidized bed using response surface methodology

Fine coal separation in a vibrated gas-fluidized bed (VGFB) is based on the phenomenon of density segregation and provides an easy and efficient way to clean fine coal. In this study, the Response Surface Methodology (RSM) was used to study the influential significance of parameters, parametric interactions, and performance optimization of fine coal separation in a VGFB. The results show that the superficial air velocity, vibration frequency, and vibration amplitude have significant two-factor interactions. The synergy between vibration and airflow contributes to revealing the mechanisms of these interactions. The performance optimization using RSM was conducted and the optimal combination of parameter values with the best performance of density segregation was obtained as followed: U of 36.9 cm/s, f of 29.2 Hz, A of 1.88 mm, and T of 139 s. The verification experiments were carried out and the results were consistent with the results of performance optimization.

Experimental and Eulerian-Lagrangian-Lagrangian study of binary gas-solid flow containing particles of significantly different sizes

The coexistence of large particles (such as biomass or coal particles) and fine particles in gas-solid flow is common. In this study, an Eulerian-Lagrangian-Lagrangian method (EMMS-DPM-DEM) was developed to simulate the binary gas-solid flow containing particles of significantly different sizes, where fine particles were simulated using coarse grained discrete element method and the motion of large particles was captured using discrete element method. Experiments on density segregation of large particles were also carried out to validate the developed simulation method. It was shown that EMMS-DPM-DEM can predict the density segregation process of large particles reasonably well. Furthermore, the density segregation mechanism of large particles in a dense fluidized bed was explained at different scales, i.e., the Archimedes principle at macroscale, the entrainment and the global circulation pattern due to bubble motions at mesoscale and the particle-particle and gas-particle interactions at microscale. The method proposed here can be directly used to study the hydrodynamics of biomass thermochemical conversion in fluidized beds, although it was validated using segregation experiments of coal particles.

Dry beneficiation of fine coal deploying multistage separation processes in a vibrated gas-fluidized bed

ABSTRACTIn this study, multistage separation processes were introduced into fine coal beneficiation in a vibrated gas-fluidized bed in order to improve the separation performance. The changes of coal properties, operational parameters, and density segregation characteristics during multistage separation were systematically studied. The results showed that with an increase in the separation stage, the range of density was narrowed down and the required input energy decreased. The results showed that with the same yield, the ash content of clean coal decreased from 12.53% for single separation stage to 8.91% for three separation stages, indicating a significant improvement on separation accuracy.

Experimental study on mixing behaviors of wet particles in a bubbling fluidized bed

Mixing behavior of binary particles with different densities in a wet fluidized bed has been experimentally investigated in this work. Some important hydrodynamic characteristics during mixing, such as the minimum fluidization velocity, flow pattern, and flotsam particle distribution are measured. The differences between dry and wet particles are systematically compared. It is found that the minimum fluidization velocity Umf of wet particles is higher than that of dry particles due to the liquid bridge force between particles. Umf reaches its maximum when the liquid saturation S = 0.1, and then decreases, and does not change much when the liquid saturation in the region of S = 0.2–0.3. Liquid addition can bring two opposite effects on the mixing of a binary mixture of particles with different densities. First, the intensity of bubbling in the wet fluidized bed is lower than that in the dry fluidized bed at the same operation gas velocity Uf, which will weaken the mixing. Secondly, the liquid bridge forces between particles can restrain the density segregation, which is beneficial for mixing. A simple method is developed based on the analysis of forces on flotsam particles to evaluate the effect of liquid bridge force on mixing. A segregation index Se, defined as Se = Flift/Fresis, where Flift contains the drag force Fd and buoyancy force Fb, and Fresis contains the gravitation force Fg and liquid bridge force Flb, is used to indicate the segregation potential. The calculated results show that Se > 1 in the dry fluidized bed and Se < 1 in the wet fluidized bed, and dry particles are more inclined to segregate than wet particles.

Effect of pressure on segregation in granular shear flows.

The effect of confining pressure (overburden) on segregation of granular material is studied in discrete element method (DEM) simulations of horizontal planar shear flow. To mitigate changes to the shear rate due to the changing overburden, a linear with depth variation in the streamwise velocity component is imposed using a simple feedback scheme. Under these conditions, both the rate of segregation and the ultimate degree of segregation in size bidisperse and density bidisperse granular flows decrease with increasing overburden pressure and scale with the overburden pressure normalized by the lithostatic pressure of the particle bed. At overburdens greater than approximately 20 times the lithostatic pressure at the bottom of the bed, the density segregation rate is zero while the size segregation rate is small but nonzero, suggesting that different physical mechanisms drive the two types of segregation. The segregation rate scales close to linearly with the inertial number for both size bidisperse and density bidisperse mixtures under various flow conditions, leading to a proposed pressure dependence term for existing segregation velocity correlations. Surprisingly, particle stiffness has only a minor effect on segregation, although it significantly affects the packing density.