Quantitative analysis of agglomerates levitated from particle layers in a strong electric field

The electrification, agglomeration, and levitation of particles in a strong electric field were analyzed experimentally and theoretically. Particle layers of glass, alumina, and ferrite were formed on a plate electrode and an external voltage was applied. Microscopic observations of the agglomerates levitated from the particle layers revealed that the number of primary particles constituting an agglomerate is affected by particle diameter and electrical resistance, but not by the applied electric field. The electric field distributions in the system were calculated by considering the charges and geometries of the agglomerates formed on the particle layers. The charges of the agglomerates were obtained experimentally. All forces acting on the agglomerates (i.e., gravitational forces, Coulomb forces, interaction forces between polarized particles, image forces, and gradient forces) were analyzed under different conditions, including various electric field distributions and charges of agglomerates. Furthermore, the critical conditions for the levitation of the agglomerates were evaluated using a force balance.     

Effects of various parameters on ultrasonic separation of inclusions from magnesium alloy melt

The acoustic radiation force generated by ultrasonic standing wave in the flow media can make solid particles suspending in the liquid agglomerate at the nodal planes of the waves and then realize their separation, which is also known as ultrasonic agglomeration in chemical industry. In this paper, ultrasonic waves were employed to promote and accelerate the separation of inclusions from magnesium alloy melt, and the effect of acoustic radiation forces on oxide inclusions removal from magnesium alloy melts were studied by numerical calculation. The agglomeration behavior of the inclusions was also obtained by solving the equations of motion for inclusions. Finally, parametric studies, usually very helpful for continued optimization and design efforts, were carried out to evaluate the effects of various parameters such as ultrasonic power, ultrasonic treating time, particle size and density difference between particle and melt on the inclusions distribution. The results indicate that when a moderate ultrasonic power was applied, most of inclusions could agglomerate at wave nodes in a short time which finally enhanced and accelerated the separation of inclusions from magnesium alloy melt.     

The particle finite element method: a powerful tool to solve incompressible flows with free‐surfaces and breaking waves

Particle Methods are those in which the problem is represented by a discrete number of particles. Each particle moves accordingly with its own mass and the external/internal forces applied to it. Particle Methods may be used for both, discrete and continuous problems. In this paper, a Particle Method is used to solve the continuous fluid mechanics equations. To evaluate the external applied forces on each particle, the incompressible Navier–Stokes equations using a Lagrangian formulation are solved at each time step. The interpolation functions are those used in the Meshless Finite Element Method and the time integration is introduced by an implicit fractional‐step method. In this manner classical stabilization terms used in the momentum equations are unnecessary due to lack of convective terms in the Lagrangian formulation. Once the forces are evaluated, the particles move independently of the mesh. All the information is transmitted by the particles. Fluid–structure interaction problems including free‐fluid‐surfaces, breaking waves and fluid particle separation may be easily solved with this methodology. Copyright © 2004 John Wiley & Sons, Ltd.     

Discrete element modeling of non-linear submerged particle collisions

Coupling of the Discrete Element Method with Computational Fluid Dynamics (DEM–CFD) is a widely used approach for modeling particle–fluid interactions.Although DEM–CFD focuses on particle–fluid interaction, the particle–particle contact behavior is usually modeled using a simple Kelvin–Voigt contact model which may not represent realistic interactions of particles in high viscosity fluids. This paper presents an implementation of a new user-defined contact model that accounts for the effects of lubrication of fluid between two approaching particles while maintaining all other DEM–CFD particle–fluid interaction phenomena. Theoretical model that yields a non-linear restitution coefficient for submerged particle collisions, which was developed by Davis et al. (J Fluid Mech 163:479–497, 1986), is implemented in a DEM–CFD code. In this model, the behavior of particles at a contact depends on fluid properties, particle velocities and distance between particle surfaces. When two particles approach each other in a fluid, their kinetic energy decreases gradually because of a lubrication effect associated with the thin fluid layer between the particles. Particle post-collision behavior is governed by a simplified elastic contact law. With lubrication, it is possible that particles are not able to rebound if the approaching velocity is completely damped by lubrication, and in this case the particles agglomerate in the fluid. Tangential surface friction-slip forces are activated as in the case of dry particle contact. The lubrication model represents an advanced submerged particle collision approach that permits improved accuracy when modeling problems with high particle concentrations in a fluid. An application of the new model is shown in a simple sediment transport problem.     

Effect of turbulent flow field properties on improving the removal of coal-fired fine particles in chemical-turbulent agglomeration

Chemical-turbulent agglomeration is a promising coupling agglomeration method to improve the removal of fine particles, in which the turbulent flow field plays an important role on the collision of chemical droplets and fine particles. However, there is no specific study about the effect of turbulent flow field properties on the agglomeration and removal of fine particles. In this work, three kinds of turbulent vortex sheets with different structures were designed to generate vortexes with different scales and generation dimensions, the particle agglomeration effect and characteristics, as well as the particle removal effect by ESP with different turbulence generators were investigated. The results demonstrated that the turbulence generator with large-scale and two-dimensional vortexes in flow field had the best effect on improving the agglomeration and removal of fine particle. Besides, the motion trajectories and the turbulent agglomeration kernel of chemical droplets and fine particles were calculated to further explore the interaction mechanism of particle agglomeration and flow field properties. The results proved that the turbulent flow field containing large-scale and two-dimensional vortexes can effectively enlarge the capture area of fine particles by chemical droplets and promote the collision probability of them, thus improving the agglomeration and removal of fine particles.     

Numerical investigation on deposition of solid particles in a lid-driven square cavity with inner heated obstacles

The deposition of aerosol particles in laminar mixed-convection flow in a lid-driven cavity with two heated obstacles is investigated numerically by an Eulerian–Lagrangian method. Lagrangian particle transport calculations are carried out to track 2000 particles that initially exerted with random distribution in flow regime and also assumed that the effect of particles on the fluid is neglected. All the affecting forces on particle equation of motion, such as Brownian, thermophoresis, drag, lift and gravity are considered. The main goal is to study the effective parameter on deposition of particles such as free convection, distance and size variation of obstacles. Numerical results showed that free convection is an effective parameter that affected the deposition. As a main result, it is observed that deposition decreases with increasing in the Richardson number. Results showed that by increasing obstacles distance, deposition increases. Finally, it is revealed that the size of obstacles has a great effect on particle deposition, such that by increasing the obstacles size, the deposition increases.     

Drag,lift and torque acting on a two-dimensional non-spherical particle near a wall

Gas-solid granular flows with non-spherical particles occur in many engineering applications such as fluidized beds. Such flows are usually contained by solid walls and always some particles move close to a wall. The proximity of a wall considerably affects the flow fields and changes the hydrodynamic forces and torque acting on particles moving near the wall. In this paper, we numerically investigate the drag, lift and torque acting on a non-spherical particle in the vicinity of a planar wall by means of lattice Boltzmann simulations. To gain an exhaustive understanding of the complex hydrodynamics and study the influence of various geometrical and flow parameters, a single 2D elliptical particle is selected as our case study. In the simulations, the effect of particle Reynolds number, distance to the wall, orientation angle and aspect ratio on drag, lift and torque is studied. Our study shows that the presence of a wall causes significant changes in hydrodynamic forces, with increasing or decreasing drag and lift forces, depending on the distance from the wall. Even the direction of lift and torque may change, depending on both the distance from the wall and particle orientation angle. Also, an ellipse with higher AR experiences larger hydrodynamic forces and torque whatever the gap size and orientation angle.     

Stability analysis of optofluidic transport on solid-core waveguiding structures

Optofluidic transport involves the use of electromagnetic energy to transport nanoparticles through the exploitation of scattering, adsorption and gradient (polarization) based forces. This paper presents a new approach to stability analysis for a system of broad applicability to such transport, namely the optical trapping of dielectric particles in the evanescent field of low index (polymer) and high index (silicon) solid-core waveguide structures integrated with microfluidics. Three-dimensional finite element based simulations are used to determine the electromagnetic and hydrodynamic field variables for the system of interest. The net force acting on particles is determined through evaluation of the full Maxwell and flow shear stress tensors, and a trapping stability number is obtained by comparing the work required to remove a particle from the waveguide with available random thermal energy. These forces are correlated to controllable experimental parameters such as particle size, fluid velocity, and channel height, and a series of trapping stability diagrams is produced which detail the conditions under which optofluidic transport is possible.     

Understanding granular media: from fundamentals and simulations to industrial application

Academic and industrial research on the behaviour of granular material is increasingly supported by numerical simulations. Such simulation environments are not only a vital tool in design and analysis, but also a key link between physicists and engineers. This Topical Collection includes a range of papers that use simulation to study granular matter from the micro- and macroscopic behaviour of particles to the behaviour of large industrial or geomechanical systems. These contributions enhance our understanding of basic physical effects and processes such as heat transfer, agglomeration and screening but also give essential parameters for the simulation of industrial systems, or, at least, guide the strategies required to provide realistic simulation results. The collection focuses on the presentation of (a) new coupled simulation methods to consider the interaction of granular bodies with structural or fluid systems (b) new findings for the consideration of particle shape and particle size distributions within simulations, (c) acoustic wake agglomeration, and (d) gravitational flow in geomechanics.     

Agglomeration process of dry ice particles produced by expanding liquid carbon dioxide

Formation of dry ice particles and their agglomeration process have been studied experimentally. The dry ice particles were produced by expanding liquid carbon dioxide at room temperature and pressure, and then introduced into an additional tube acting as an agglomeration chamber. In the experiments, the temperatures of the jet flow and the tube wall were measured by thermocouples, and dry ice particles in the jet flow were observed by a high speed camera with a zoom lens. It was found that two stages of temperature reduction occurred in the jet flow, corresponding to the agglomeration process. It was also found that the particle size of the agglomerates increased and the particle velocity decreased with increasing tube diameter. The agglomeration process of dry ice particles can be explained by the particle deposition and reentrainment, i.e. dry ice particles of several micrometers are deposited on the tube wall and form a deposition layer; then, agglomerates are reentrained from the layer into the jet flow.     

An Euler–Lagrange particle approach for modeling fragments accelerated by explosive detonation

In this paper, a method is proposed for modeling explosive‐driven fragments as spherical particles with a point‐particle approach. Lagrangian particles are coupled with a multimaterial Eulerian solver that uses a three‐dimensional finite volume framework on unstructured grids. The Euler–Lagrange method provides a straightforward and inexpensive alternative to directly resolving particle surfaces or coupling with structural dynamics solvers. The importance of the drag and inviscid unsteady particle forces is shown through investigations of particles accelerated in shock tube experiments and in condensed phase explosive detonation. Numerical experiments are conducted to study the acceleration of isolated explosive‐driven particles at various locations relative to the explosive surface. The point‐particle method predicts fragment terminal velocities that are in good agreement with simulations where particles are fully resolved, while using a computational cell size that is eight times larger. It is determined that inviscid unsteady forces are dominating for particles sitting on, or embedded in, the explosive charge. The effect of explosive confinement, provided by multiple particles, is investigated through a numerical study with a cylindrical C4 charge. Decreasing particle spacing, until particles are touching, causes a 30–50% increase in particle terminal velocity and similar increase in gas impulse. Copyright © 2015 John Wiley & Sons, Ltd.     

SIMULATION OF FLUX DECLINE DUE TO PARTICULATE FOULING IN ULTRAFILTRATION

The flux decline in thin-channel and tubular ultrafiltration (UF) modules due to fouling by dilute suspensions is presented for a wide range of operating conditions. The dynamics of fouling is simplified by viewing the particle deposition on the membrane surface at discrete time as a steady state event and formulating the problem as an infinite series of successive events. Only inertial effects are considered and it is assumed that for a dilute system, particle-particle interactions and forces of interaction between particles and membrane walls are insignificant. Further, at such low concentration, the motion of fluid and particle may be taken independent of each other. The equations of motion for the particles are solved by Fourthorder Runge-Kutta method, where the fluid flow is obtained from the finite difference solution of Navier-Stokes equation. The present theoretical calculations of flux decline due to particulate fouling at typical UF operating conditions indicate that the inertial effects are important and under positive wall permeation flux conditions, particles are encouraged to migrate to the membrane walls.     

The influence of storage relative humidity on aerosolization of co-spray dried powders of hygroscopic kanamycin with the hydrophobic drug rifampicin

The purpose of this study was to investigate the influence of storage humidity on in vitro aerosolization and physicochemical properties of co-spray dried powders of kanamycin with rifampicin. The powders were stored for one-month in an open Petri dish at different relative humidities (RHs) (15%, 43%, and 75%) and 25?±?2?°C. The in vitro aerosolization (fine particle fraction, FPF) of the powders was determined by a next generation impactor (NGI). The moisture content, particle morphology and crystallinity of the powders were determined by Karl Fischer titration, scanning electron microscopy, and X-ray powder diffractometry, respectively. At all RH, the FPF of hydrophobic rifampicin-only powder was unaffected but the FPF of hygroscopic kanamycin-only powder significantly decreased even at 43% RH. The kanamycin-only particles fused together, crystallized and formed hard cakes at 75% RH. The aerosolization of kanamycin and rifampicin in the combination powders remained unaffected at 15% and 43% RH, but aerosolization significantly decreased at 75% RH. Enrichment of the surface of the particles with hydrophobic rifampicin did not protect the combination powders from moisture uptake but it prevented particle agglomeration up to 43% RH. At 75% RH, the moisture uptake led to agglomeration of the particles of the combination powder particles and consequently an increase in aerodynamic diameter. Further studies are required to investigate how rifampicin enrichment prevents particle agglomeration, the possible mechanisms (e.g. particle interactions due to capillary forces or electrostatic forces) for the changes in the aerosolization and changes in surface composition during storage.     

Separation of Micron-Scale Particles by Size Using Helium II

Here we report on a new technique we are developing to classify micron scale particles, i.e., separate them by size, using superfluid ⁴He (He II). The technique is based on the unique property of He II, that is its two fluid nature which includes the viscous normal fluid component that flows in the direction of a heat current. The gravitational settling of small particles of a particular size can be balanced by the viscous drag from the normal fluid in a bath of He II. Further a non-uniform heat flux can allow a range of particle sizes to be suspended and collected at different locations within a He II counterflow channel. To demonstrate this principle, we have built a prototype particle separator and tested the process with particle diameters in the range 1 μm to 10 μm. The motion of the particles in this separator was measured using the Particle Image Velocimetry technique. Separation of a mixture of particles into two different size categories was achieved demonstrating the validity of this concept to classify particles. Particle agglomeration was also observed and is an issue particularly for particles with a diameter smaller than a few microns.     

Effect of carrier fluid viscosity on retention time and resolution in gravitational field-flow fractionation

Gravitational field-flow fractionation (GrFFF) is a useful technique for fast separation of micrometer-sized particles. Different sized particles are carried at different velocities by a flow of fluid along an unobstructed thin channel, resulting in a size-based separation. They are confined to thin focused layers in the channel thickness where force due to gravity is exactly opposed by hydrodynamic lift forces (HLF). It has been reported that the HLF are a function of various parameters including the flow rate (or shear rate), the size of the particles, and the density and viscosity of the liquid. The dependence of HLF on these parameters offers a means of altering the equilibrium transverse positions of the particles in GrFFF, and hence their elution times. In this study, the effect of the viscosity of the carrier fluid on the elution behavior (retention, zone broadening, and resolution) of micrometer-sized particles in GrFFF was investigated using polystyrene (PS) latex beads as model particles. In order to change the carrier liquid viscosity without affecting its density, various amounts of (hydroxypropyl) methyl cellulose (HPMC) were added to the aqueous carrier liquid. It was found that particles migrate at faster rates as the carrier viscosity is increased, which confirms the dependence of HLF on viscosity. At the same time, particle size selectivity decreased but peak shape and symmetry for the more strongly retained particles improved. As a result, separation was improved in terms of both the separation time and resolution with increase of carrier viscosity. A theoretical model for plate height in GrFFF is also presented, and its predictions are compared to experimentally measured values.     

Experimental and numerical determination of the lubrication force between a spherical particle and a micro-structured surface

In many particulate processes suspensions need to be handled. Hydrodynamic forces in presence of a liquid as a surrounding continuum medium can significantly affect the particle collision behaviour. When particles approach a wall, lubrication force can become dominant with decreasing distance. This force was described analytically by different authors for a smooth flat wall. Roughness was found to be an important factor in this context, but the mechanisms are still not fully understood. In this work, the effects of topology on the lubrication force were studied using a regular prismatic micro-structured titanium surface produced by micro-milling. A nanoindentation setup was modified for the direct measurement of this force during the particle approach to polished and micro-structured surfaces in liquid. For a more detailed insight on the behaviour of the fluid in the decreasing gap between particle and surface microstructure, resolved computational fluid dynamics (CFD) simulations were performed using an overset mesh method. The comparison of simulation results with nanoindentation tests and analytical solution showed a good agreement. The effects of structure size and particle contact location at various approaching velocities on the lubrication force were investigated.     

Effect of fluid–particle-interactions on dispersing nano-particles in epoxy resins using stirred-media-mills and three-roll-mills

Fibre reinforced composites are indispensable in the field of modern lightweight structures, such as used in aerospace, automotive industry or in wind power plants. Those materials provide high weight savings and increase the efficiency of a structure significantly. Therefore, various efforts are made to continuously improve the quality of the matrix and the fibres. By embedding nano-particles into the epoxy matrix, the mechanical properties as well as the electrical and thermal characteristics can significantly be improved [1]. In most cases these nano-sized particles are produced as dry powders not as single primary particles but rather as particle collectives consisting out of several primary particles. For the application in reinforced composites the particles must be suspended in epoxy resin as separately dispersed primary particles or in a certain aggregate size. Generally, the influencing parameters to break up the aggregates in a dispersion process can be divided into the stress mechanism, the intensity and the frequency of the dispersing machine itself, the properties of the dispersed particles (e.g. the particle–particle interactions) the properties of the homogenous phase and the particle–resin-interactions. Besides the effect of the chosen dispersing machine the optimization of the dispersing process was investigated by applying modified particle surfaces and varying the fluid properties. The results show that the surface properties of the particles must fit to the epoxy resin properties and the attractive forces between the primary particles must be reduced or the stabilization improved, respectively. An indication for an improved stabilization and adjustment of the particles surface properties to the fluid properties can be obtained by measurements of the contact angle and the rheological properties. Generally, an increase of viscosity and mass fraction of the product leads to a higher energetic efficiency of the dispersion process in the stirred media mill and three-roll-mill.     

Characterization of magnetic forces by means of suspended particles in paramagnetic solutions

A new technique, whereby the so-called "iso-directional-force lines" may be illustrated directly, is proposed. The method is based on the levitation forces acting on a particle immersed in paramagnetic fluid in nonhomogeneous magnetic field. An ordered set of capillaries, each containing a small particle suspended in the fluid, may be used to depict the iso-directional-force lines by the equilibrium positions of the particles. Once the field is mapped (by a selected standard system), its characteristics are known, with respect to any other fluid particle combinations. The latter is of significance for research and technology.     

Numerical simulation of particulate cake formation in cross-flow microfiltration: Effects of attractive forces

We present a two-dimensional simulation model to explore cake formation in cross-flow filtration. The model uses the lattice Boltzmann method (LBM) for fluid computation and the discrete element method (DEM) for particle computation; they were fully coupled with the smoothed profile method. We verified our model by simulating filtration under different transmembrane pressures. We then investigated the effects of attractive forces and particle concentration on the cake formation mechanism. Generally, as the attractive interaction and particle concentration increased, the particles formed a cake layer with a looser body and rough surface, due to the decrease in the mobility of the particles in contact with the cake surface. It is concluded that the effects of particle concentration are affected by the different conditions of attractive interactions between the particles.     

Experimental study on the fluidization discharging characteristics of Geldart-C kaolin powders in a blow tank with pulsed gas

Kaolin powders have been suggested to be able to adsorb heavy metal vapor from coal-fired flue gas. However, due to the influence of inter particle forces, such as liquid bridge force, it is difficult to realize stable pneumatic conveying. In the present work, the fluidization characteristics of kaolin powders were investigated. A series of unstable flow phenomena such as agglomeration, channeling, and slugging occurred during the fluidization process. Also, the fluidization discharging characteristics of kaolin powder in an optimized blow tank were experimentally studied. The results indicated that the introduction of pulsed gas can effectively destroy agglomeration and thus improving the stability of discharging. Visual experiments in pseudo-2D fluidized bed were also confirmed the destructive effect of pulsed gas on agglomeration. With an increase in either fluidization gas velocity U_f or pulsed gas velocity v_pulsed, the mass flow rate of kaolin powder G first increased and then decreased. Finally, drying experiments demonstrated that there is free water on the surfaces of the kaolin powders. The analysis of forces indicated that the liquid bridge force F_lb between particles is much larger than the particle gravity F_g. The liquid bridge force might be one of key reasons for kaolin powder agglomerating.