Particle-particle Interaction Forces Research Articles

CFD-DDPM coupled with an agglomeration model for simulation of highly loaded large-scale cyclones: Sensitivity analysis of sub-models and model parameters

The Dense Discrete Phase Model coupled with an agglomeration model is developed and validated for the simulation of industrial cyclones with high solid loads. The performance of the model is influenced by sub-models, model parameters, and numerical parameters. To optimize the performance of the present CFD model, an extensive sensitivity analysis was performed, varying one sub-model or parameter at a time, and systematically assessing the effect on the results through comparisons with measured pressure drop and separation efficiency of a highly loaded pilot-scale cyclone. The investigation shows that the turbulence model and particle-particle restitution coefficient have the strongest influence. This study concludes with the recommendation of a set of sub-models, model parameters, and numerical parameters providing the best prediction of the hydrodynamics of large-scale highly loaded cyclones. In addition, the impact of some operating conditions on the performance of a large-scale highly loaded cyclone were examined. • Simulation of highly loaded cyclones: effects of sub-models and model parameters. • Particle-particle restitution coefficient and turbulence model influence the most. • The impact of particle-particle interaction force modeling is minor. • Most suitable sub-models, model parameters, and numerical parameters are suggested. • The impact of operating conditions on the performance of the cyclone was examined.

A volumetric-smoothed particle hydrodynamics based Eulerian-Lagrangian framework for simulating proppant transport

Different numerical methods have been applied to simulate the proppant transport in petroleum engineering, which can be roughly categorized as the Eulerian-Eulerian and Eulerian-Lagrangian models. Recently, a hybrid Eulerian-Lagrangian (E-L) approach, the multiphase particle-in-cell (MP-PIC) method, has been successfully applied to model large-scale proppant transport problems by introducing the concept of parcels (clusters of particle). In the MP-PIC method, particle-particle interaction force is expressed as the gradient of particle stress. The calculation of this gradient strongly depends on the interpolation between particle properties and Eulerian grids, which could lead to problems such as non-physical particle suspension, particle agglomeration and non-conserved interparticle interactions. In this study, a new method, the volumetric-smoothed particle hydrodynamics (V-SPH) method, is proposed to improve the calculation accuracy of the particle-particle interaction forces in the original MP-PIC method. In the V-SPH method, the calculation of the particle stress gradient no longer depends on the background Eulerian grids and the conservation of the interparticle stress is also guaranteed. In this paper, detailed introduction of the V-SPH based Eulerian-Lagrangian framework is provided. The reliability of the proposed V-SPH method is validated against both the numerical and experimental results in literature. By comparing with the original MP-PIC method, we observe that the non-physical particle agglomeration, as well as non-physical particle suspension problems can be well solved with the proposed new model. In addition, the impact of some key parameters in the V-SPH method on simulation results are also investigated. The choice of the PPP (number of particles per parcel) and the treatment of boundary particle deficiency are found to play important roles in model accuracy and efficiency. The V-SPH method proposed in this work can provide more accurate results than the original MP-PIC method, especially in the regions of dense proppant concentration, with comparable computing efficiency. With proper treatment of boundary deficiencies, it is promising to be used in more complex field-scale proppant transport problems.

Determination of Separation Performance in CFD-DEM Simulation Using Straw Particles in A Standard Cyclone

Although flow in biological materials sometimes behaves like a continuous one, it cannot be simulated with continuity-based modeling when it comes to discontinuous flow behavior. The Discrete Element Method (DEM) in combination with Computational Fluid Dynamics (CFD) is a computational method for modeling particles in fluid flow by tracking their motion. DEM is widely used in the field of engineering, and its use in the agricultural field is increasing. This study analyzes the CFD-DEM relationship of biological material in aerodynamic systems and reviews current applications. In the article, the definition of aerodynamic systems as a basic principle, particle-fluid and particle-particle interaction forces in the system, modeling of particle motions, CFD-DEM coupling method, and analysis applications of agricultural aerodynamic systems are examined. In this study, simulation experiments were carried out at 100 g/s and 200 g/s straw feeding values at each value of 18-15-12-10-8-6-4 m/s air and straw inlet velocities. The flow near the cyclone walls caused the straw particles to be directed towards the lower exit end of the cyclone. At feed densities of 100 g/s and 200 g/s, the least particle output was obtained at a rate of 18 m/s. The highest cyclone output efficiency was obtained at feed densities of 100 g/s and 200 g/s at a velocity of 12 m/s. The compatibility of the trial simulation results with the literature showed that the CFD-DEM application is an important approach to study the behavior of particulate matter in fluids.

On the Applicability of the Coarse Grained Coupled CFD-DEM Model to Predict the Heat Transfer During the Fluidized Bed Drying of Pharmaceutical Granules.

Fluidized bed dryer often used in the pharmaceutical industry for drying of wet granules. Coupled computational fluid dynamics (CFD) - discrete element method (DEM) is frequently used to model the drying process because of its ability to obtain the relevant information at the particle level. However, it becomes almost impossible to model the industrial scale fluidized bed dryer using the coupled CFD-DEM method because of the presence of large number of particles [Formula: see text]. To reduce the number of particles to be tracked in the simulation, coarse grained coupled CFD-DEM method was developed by researchers where a certain number of particles of the original system was represented by a relatively bigger particle in the coarse-grained system. The appropriate scaling of the particle-particle and particle-fluid interaction forces is necessary to make sure that the dynamics of the coarse-grained particles/parcels accurately represent the dynamics of the original particles. The coarse-graining of the drying process of pharmaceutical granules during fluidization needs systematic coarse-graining of the momentum, heat, and solvent vapor transfer process. A coarse grained coupled CFD-DEM method was used to model the momentum and heat transfer during the fluidization of pharmaceutical granules. It was shown that the heat transfer during the fluidization of large number of particles could be predicted by simulating a smaller number of bigger particles with appropriate scaling of particle-particle heat and momentum transfer, and particle-fluid heat and momentum transfer at significantly smaller computational time. This model can be further extended by including a coarse-grained moisture transport model in future.

Flow and force analysis on the formation of expanded beds in gas fluidization of fine ellipsoids

A flow regime called expanded beds in gas fluidisation exists for fine powders. Some attempts have been made in the past to analyse the formation mechanism of expanded beds, indicating that the essential condition for an expanded bed to exist is that the van der Waals force balances the contact force and the particle-fluid interaction force balances the weight of the bed at bulk scale. However, how particle shape affects the expanded bed formation is still not clear. In this work, CFD-DEM approach is applied to elucidate the particle-particle and particle-fluid interaction forces on the dynamic formation of expanded beds with fine ellipsoids. The results illustrate that at the start-up stage, the bulk disconnects into two major bulks with a lowest contact force on the axial direction, especially for fine ellipsoids. Spheres have a smoother expansion process than ellipsoids, and ellipsoids need longer start-up time. A slower gas introduction process has a more moderate change in bed height. The fractures gradually disappear when the cohesive forces become smaller. At the transition stage, when the aspect ratio deviates from 1.0, the pressure drop fluctuation significantly increases. At the stable stage, when the aspect ratio decreases or increases from 1.0, the contact forces and van der Waals forces increase, and the distributions of particle-fluid interaction forces become less uniform in the beds. Ellipsoidal particles undergo an orientation rearrangement during the expansion process.

Effect of particle-particle interaction on dielectrophoretic single particle trap in a sudden contraction flow

Dielectrophoretic (DEP) force is significant in manipulating tiny objects in micro/nano scale. To study the effect of electric interaction force on particle manipulation, a microstructure consisting of a pair of strip electrodes and a sudden contraction micro-channel was constructed. Besides DEP force and hydrodynamic force acting on single particle, the numerical model also involved electric interaction force and force moment on two particles. The analyses revealed that the particle-particle interaction force was in the same order as that of DEP force on single trapped particle. The interaction force resulted in trapping single particle failure under continuous DEP force. Thus, pulsed DEP force, turning on/off DEP force at a given time interval, was suggested. During the “off” period, the velocity difference of the two particles located at sudden contraction micro-channel enlarged the gap between them and further weakened the particle-particle interaction. By a proof-of-concept experiment, both the trapping behavior of single particle and that of two particles were in good agreement with the model. With carefully controlled parameters, the reliable function of retaining single particle was realized by pulsed DEP.

Direct Numerical Simulation of Particles in Spatially Varying Electric Fields †

A numerical scheme is developed to simulate the motion of dielectric particles in the uniform and nonuniform electric fields of microfluidic devices. The motion of particles is simulated using a distributed Lagrange multiplier method (DLM) and the electric force acting on the particles is calculated by integrating the Maxwell stress tensor (MST) over the particle surfaces. One of the key features of the DLM method used is that the fluid-particle system is treated implicitly by using a combined weak formulation, where the forces and moments between the particles and fluid cancel, as they are internal to the combined system. The MST is obtained from the electric potential, which, in turn, is obtained by solving the electrostatic problem. In our numerical scheme, the domain is discretized using a finite element scheme and the Marchuk-Yanenko operator-splitting technique is used to decouple the difficulties associated with the incompressibility constraint, the nonlinear convection term, the rigid-body motion constraint and the electric force term. The numerical code is used to study the motion of particles in a dielectrophoretic cage which can be used to trap and hold particles at its center. If the particles moves away from the center of the cage, a resorting force acts on them towards the center. The MST results show that the ratio of the particle-particle interaction and dielectrophoretic forces decreases with increasing particle size. Therefore, larger particles move primarily under the action of the dielectrophoretic (DEP) force, especially in the high electric field gradient regions. Consequently, when the spacing between the electrodes is comparable to the particle size, instead of collecting on the same electrode by forming chains, they collect at different electrodes.

Dielectrophoretic effect of nonuniform electric fields on the protoplast cell

In recent years, dielectrophoresis based microfluidics systems have been used to manipulate colloids, inert particles, and biological microparticles, such as red blood cells, white blood cells, platelets, cancer cells, bacteria, yeast, micro‌organisms, proteins, DNA, etc. In the current study the governing electric potential equations have been solved in the presence of cell for the purpose of studying particle-electric field dielectrophoretic interaction. Immersed Interface Method (IIM) which is a modified finite difference method is used to solve the governing 2D elliptic electrostatic equations with irregular boundaries. A neutral particle polarizes under the application of an electric field and causes local nonuniformity in electrostatic potential distribution. So cells experience electric stresses on its surface. The electric stress on cell surface is calculated by Maxwell Stress Tensor (MST) on both sides of cell. DEP force is calculated by integrating electric stress on particle surface. In the present study calculated electric stresses is validated by DEP force calculated using EDM method and exact solution. we neglect other electrokinetic effects such as electrophoresis and electro-osmosis. Electrophoresis can be neglected if the particles are not charged. The effect of applied voltage, dielectric constants of cells and cells orientation on particle-particle interaction force has been studied.

Regime of aggregate structures and magneto-rheological characteristics of a magnetic rod-like particle suspension: Monte Carlo and Brownian dynamics simulations

In the present study, we address a suspension composed ferromagnetic rod-like particles to elucidate a regime change in the aggregate structures and the magneto-rheological characteristics. Monte Carlo simulations have been employed for investigating the aggregate structures in thermodynamic equilibrium, and Brownian dynamics simulations for magneto-rheological features in a simple shear flow. The main results obtained here are summarized as follows. For the case of thermodynamic equilibrium, the rod-like particles aggregate to form thick chain-like clusters and the neighboring clusters incline in opposite directions. If the external magnetic field is increased, the thick chain-like clusters in the magnetic field direction grow thicker by adsorbing the neighboring clusters that incline in the opposite direction. Hence, a significant phase change in the particle aggregates is not induced by an increase in the magnetic field strength. For the case of a simple shear flow, even a weak shear flow induces a significant regime change from the thick chain-like clusters of thermodynamic equilibrium into wall-like aggregates composed of short raft-like clusters. A strong external magnetic field drastically changes these aggregates into wall-like aggregates composed of thick chain-like clusters rather than the short raft-like clusters. The internal structure of these aggregates is not strongly influenced by a shear flow, and the formation of the short raft-like clusters is maintained inside the aggregates. The main contribution to the net viscosity is the viscosity component due to magnetic particle-particle interaction forces in relation to the present volumetric fraction. Hence, a larger magnetic interaction strength and also a stronger external magnetic field give rise to a larger magneto-rheological effect. However, the dependence of the viscosity on these factors is governed in a complex manner by whether or not the wall-like aggregates are composed mainly of short raft-like clusters. An increase in the shear rate functions to simply decrease the effect of the magnetic particle-particle and the particle-field interactions.

Systematic study of the effect of particle density distribution on the flow and performance of a dense medium cyclone

Dense medium cyclone (DMC) is widely used to upgrade run-of-mine coal in modern coal preparation plants. The flow within it is very complicated, with multi-phases involved including air, water, coal and magnetic/non-magnetic particles of different sizes, densities and other properties. In this work, the effect of coal particle density distribution, one of the most important variables and highly related to coal type, is systematically studied using a combined approach of Computational Fluid Dynamics (CFD) and Discrete Element Method (DEM). In particular, the so called Johnson's SB function, which can describe a wide range of distributions, is employed to represent different particle density distributions. For a given density range, the function is characterized by two parameters: median particle density ρ0.5 and distribution parameter σj, with the latter describes the spread of the distribution. The effects of the two parameters on the flow and performance of a typical DMC are quantified. Moreover, their implication to practical operation, e.g. when the amount of near gravity material is high, is discussed. The results are also analysed in terms of the medium and particle flow fields, particle-fluid, particle-particle and particle-wall interaction forces to understand the fundamentals of the operation. The results obtained in this work should be useful to the design and control of DMC operations with different coal density distributions or coal types.

The influence of particle size and residual charge on electrostatic interactions between charged colloidal particles at an oil-water interface.

Electrostatic repulsive interaction forces between charged spherical colloidal particles at an oil-water interface are numerically studied by solving the standard three-dimensional Poisson-Nernst-Planck model. We directly compute the electrostatic force on a finite-size spherical particle and our results are applicable to all inter-particle distances without distinguishing short ranges and long ranges. The model successfully captures the scaling relationship of the force and the separation distance (d) between two charged particles at both short ranges (exponential dependence) and long ranges (∼d(-4)). The model also bridges these two ranges and provides quantitative information in the middle range. In addition, by assuming that there is a small residual electric charge at the particle-oil interface, the standard model is capable of quantitatively predicting the repulsive particle-particle interaction force over a large range of the separation distance between two particles. The favorable agreement between experiments and theoretical predictions also leads one to conclude that the standard model adequately describes the particle-particle interactions trapped at the oil-water interface.

Simulations of a Circulating Fluidized Bed Chemical Looping Combustion System Utilizing Gaseous Fuel

Numerical studies using Computational Fluid Dynamics (CFD) have been carried out for a complete circulating fluidized bed chemical looping combustor described in the literature (Abad et al., 2006 Fuel 85, 1174-1185). There have been extensive experimental studies in Chemical Looping Combustion (CLC), however CFD simulations of this concept are quite limited. The CLC experiments that were simulated used methane as fuel. A 2-D continuum model was used to describe both the gas and solid phases. Detailed sub-models to account for fluid-particle and particleparticle interaction forces were included. Global models of fuel and carrier chemistry were utilized. The results obtained from CFD were compared with experimental outlet species concentrations, solid circulation rates, solid mass distribution in the reactors, and leakage and dilution rates. The transient CFD simulations provided a reasonable match with the reported experimental data.

浮遊粒子と吸着剤粒子からなる2粒子分散系のブラウン動力学シミュレーション(流体工学,流体機械)

We have carried out Brownian dynamics simulations of sedimentation phenomena of a dispersion composed of two-kinds of spherical particles in water under the gravity field. This study may be the first step to develop a new technology which enables us to improve the visibility of rivers and lakes. In the present study, we have modeled sub-micrometer-dimension particles, or dirty particles in lakes as small spherical particles, and capturing particles as large spherical particles; these two kinds of particles conduct Brownian motion in water, and large particles adsorb small particles to sediment gradually toward the bottom in the gravity field. From the results of Brownian dynamics simulations, the influences of Brownian motion, particle-particle interaction forces, the size of each particle, and the gravity force on the performance of the adsorption of large particles have been discussed. In addition, we have discussed what the most appropriate situation of large particles is to accomplish the most effective adsorption rate or improve the visibility of water most effectively in terms of capturing particles. The most important conclusion derived from the present results is that, in order to improve the capturing performance, the Brownian motion of large particles have to be activated in an appropriate number density without losing the influence of the gravity.

Influence of particle‐particle interactions and particles rotational motion in traveling wave dielectrophoresis

Traveling wave dielectrophoresis provides an interesting method for the controlled movement of microsized particles in suspended mixtures, and as such is a promising tool in microfluidic technology. In this case, the electrostatic force acting on the particles has two components: one due to the spatially varying magnitude of the electric field and the other due to the spatially varying phase. The actual movement of the particle is determined by the combined effect of these two forces and corresponding torques, the viscous drag exerted by the fluid on the particle, and the electrostatic and hydrodynamic particle-particle interactions. This paper presents the first numerical simulations of the motion of particles subjected to all previous forces and torques. Our technique is based on a finite-element scheme in which the particles are moved using a direct simulation scheme respecting the fundamental equations of motion for both the fluid and the solid particles. The fluid-particle motion is resolved by the method of distributed Lagrange multipliers and the electrostatic forces are computed using the point-dipole approximation. Our simulations show that the particle behavior strongly depends on the mismatch of the dielectric properties between the particles and the fluid, and that the particle-particle interaction force as well as particles rotation speeds play crucial roles in the various regimes.

Dynamics of Electrorheological Suspensions Subjected to Spatially Nonuniform Electric Fields

A numerical method based on the distributed Lagrange multiplier method (DLM) is developed for the direct simulation of electrorheological (ER) liquids subjected to spatially nonuniform electric field. The flow inside particle boundaries is constrained to be rigid body motion by the distributed Lagrange multiplier method and the electrostatic forces acting on the particles are obtained using the point-dipole approximation. The numerical scheme is verified by performing a convergence study which shows that the results are independent of mesh and time step sizes. The dynamical behavior of ER suspensions subjected to nonuniform electric field depends on the solids fraction, the ratio of the domain size and particle radius, and four additional dimensionless parameters which respectively determine the importance of inertia, viscous, electrostatic particle-particle interaction and dielectrophoretic forces. For inertia less flows a parameter defined by the ratio of the dielectrophoretic and viscous forces, determines the time duration in which the particles collect near either the local maximums or local minimums of the electric field magnitude, depending on the sign of the real part of the Clausius-Mossotti factor. In a channel subjected to a given nonuniform electric field, when the applied pressure gradient is smaller than a critical value, the flow assists in the collection of particles at the electrodes, but when the pressure gradient is above this critical value the particles are swept away by the flow.

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Simulation of Particle Impact with a Wedge in Dilute Two-phase Flow

Dilute solid-fluid flow over a wedge in a stationary channel is numerically solved using one-way coupling between fluid and solid particles. The two-dimensional, steady, laminar carrierphaseflow is determined by Galerkin finite-element method using Newton's iteration for primitive variables, pressure, and velocity. Velocity is interpolated biquadratically and pressure is interpolated linearly. Parameter continuation is used to compute solutions for relatively large values of flow Reynolds number. Individual particles are tracked from specified inlet positionsby a fourth-order Runge-Kutta method applied to the equations of motion ofthe particle. Forces acting on the particle include drag, pressure, and inertia. Forces due to particle-particle interaction and Basset forces are neglected. Collisions with the wedge and the walls of the channel are modelled via assumed coefficients ofrestitution in both the normal and the tangential directions. The point of actual impact is determined by interpolation. Results are presented for various parameters, such as particle diameter, wedge angle, Reynolds number, particle density, etc.

Microscopic and macroscopic aspects of stick-slip motion in granular shear.

An annular shear cell has been used to investigate a number of factors known to influence stick-slip motion in an assembly of near monosized, spherical glass beads. In this paper, both the sample shear stress and volumetric strain were recorded, allowing new insights into the possible mechanics of stick-slip motion in a granular body. Rather than the commonly presented mechanism of sample dilation and fluidization accompanying the slip events, in the material studied here, sample dilation occurred during the preslip deformations of the granular body, while the slip event was accompanied by assembly contraction. Drive velocity and applied normal pressure were both found to influence the magnitude of the stick-slip spikes in a manner analogous to previous studies of stick-slip in assemblies of confined, near-spherical lubricant systems. Finally, atmospheric relative humidity was found to have a marked effect on the magnitude of the stick-slip motion. To investigate this mechanism more fully, the atomic force microscopy was employed to measure the particle-particle interaction forces as a function of atmospheric relative humidity. A water meniscus was found to form under all humidities, from less than 5% to greater than 95%. However, its influence on the adhesive forces varied by an order of magnitude. While most previous studies of stick-slip phenomena have attempted to remove atmospheric humidity as a variable, here we present a useful link between the role of relative humidity on particle-particle interaction forces and the macroscopic response of the granular assembly.

Microscopic and macroscopic effects of surface lubricant films in granular shear

Experiments were conducted to investigate the link between particle-particle interaction forces and the bulk properties of granular shear using an idealized system of near-spherical, monosized glass beads. The atomic force microscopy colloidal probe technique was employed to investigate the adhesion and friction between a single bead and a second glass surface, while the annular shear cell was used to measure the shear properties of the bulk granular material. A covalently bound monomolecular film of aliphatic chains was introduced to alter the tribological interactions between the particles. The atomic force microscope was used to measure the reduction in the particle-particle surface forces resulting from the addition of the boundary lubricant, while the shear cell showed that the effect of the lubricant film was to reduce the coefficient of internal friction and the dilation during shear. This is an experimental study to provide quantitative data linking particle-particle interaction forces and the shear properties of a granular body.

Surface chemistry–rheology relationships in concentrated mineral suspensions

The rheological behaviour of concentrated mineral suspensions can be controlled by understanding and regulating the net particle-particle interaction force. This paper summarizes the experimental results from studies showing how the yield stress of concentrated aqueous suspensions of alumina, zirconia and kaolin can be controlled through regulation of pH and the addition of inorganic electrolytes, ionic surfactants and polyelectrolytes (polymers). The various kinds of inter-particle forces acting in aqueous suspensions of fine particles are reviewed and used to interpret the experimental results.