Particle Wall Interaction Forces Research Articles

The gas-solid flow characteristics of cyclones

Cyclones have been widely used as separation devices during pneumatic conveying. The pipelines and cyclones inlet are connected by diameter change joints. The inlet cross section area is always larger than the pipeline. This results in the velocity of gas becoming lower than particle velocity when the gas-particles flow into the cyclone. The gas-solid flow in cyclones are investigated using FLUENT-EDEM method. The results show that the existence of particles had major effects on the gas flow of the cyclone within the inlet and conical separation parts. The tangential, axial, and radial velocities are only slightly changed when solid gas ratio at the same gas velocity/particle velocity ratio, while the velocities increase with the increasing of gas velocity/particle velocity ratios under the same solid gas ratio. The cyclone pressure drop decreases at a low solid gas ratio, and is maintained at an almost constant level with the high solid gas ratio. However, with the lower solid gas ratio, the particle strands are not clear, and the particle strand is found to become clearer and remained constant when the solid gas ratio is 6.42kg/kg at the same gas velocity/particle velocity ratio. The particle strands form early with the increasing of gas velocity/particle velocity ratios at the same solid gas ratio. At the same time, the particle-wall interaction force is determined to be the main reason for the particle breakage and wall attrition. The results could potentially provide support for the development of a pressure drop theory for cyclones.

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.

Systematic study of effect of particle size distribution in a dense medium cyclone by Johnson’s SB function

Dense medium cyclone (DMC) is a high-tonnage device that is widely used to upgrade run-of-mine coal in the 0.5–50mm size range. It is known that the performance of a DMC depends on the coal particle size distribution but quantitative relationships have not been established yet. In this work, the effect of the particle size distribution in a DMC is studied in detail using the combined Computational Fluid Dynamics (CFD) and Discrete Element Method (DEM). In particular, Johnson’s SB function, which is capable of representing a wide range of size distributions, is employed to describe the particle size distribution of coal. The function has two parameters, i.e., particle median size d0.5 and distribution parameter σj, for a given size range. It is found that for a constant σj, the operational head and medium differential decrease dramatically, and the separation efficiency deteriorates rapidly when d0.5 increases from 6 to 40mm. For a constant d0.5, the DMC performance is sensitive to σj, particularly when d0.5 and σj are small. Both the medium differential and split decrease at first and then almost remain constant as σj increases from 0.4 to 1.0, and the separation performance follows the similar trend as well. The simulation results are also analysed in terms of medium and particle flow patterns, particle–fluid, particle–particle and particle–wall interaction forces to elucidate the underlying mechanism. For example, the decrease of pressure gradient force and viscous drag force represents the loss of swirling energy and then contributes to the drop of operational pressure and worse separation efficiency. The results should be useful to better design and control DMC operations, particularly for various coal types with different particle size distributions.

Computational study of the multiphase flow in a dense medium cyclone: Effect of particle density

Dense medium cyclone (DMC) is widely used to upgrade run-of-mine coal in the coal industry. The flow within it is very complicated, with four phases (water, air, fine magnetite and coal) involved. To date, the underlying fundamentals are not well understood. In this work, the effect of particle density on the flow in a DMC is numerically studied to understand why coal type matters in DMC operation. The model used is a combined approach of discrete element method (DEM) and computational fluid dynamics (CFD). In the model, the motion of discrete mineral particles is obtained by DEM and the flow of medium (mixture of water, air and fine magnetites) phase by the traditional CFD. The simulated results are analysed in terms of medium and coal flow patterns, and particle–fluid, particle–particle and particle–wall interaction forces. It is shown that particles of different densities have significantly different effects on the flow in a DMC. The operational pressure, medium split and differential all decrease with the increase of particle density. The underlying mechanism is that different trajectories of particles of different densities lead to different spatial distributions of particle–fluid interaction forces which in turn yield different effects on the flow. The findings are useful to better understanding, designing and operating this complicated multiphase flow system.

Particle scale modelling of the multiphase flow in a dense medium cyclone: Effect of fluctuation of solids flowrate

Dense medium cyclone (DMC) is widely used to upgrade run-of-mine coal in the coal industry. The flow dynamics/fluctuation in a DMC is important but has not been studied previously. In this work, the dynamics is studied by numerically with special reference to the effect of the fluctuation of solid mass flowrate. The simulation is carried out by use of a combined approach of Computational Fluid Dynamics (CFD) and Discrete Element Method (DEM). In the model, the motion of discrete mineral particle phase is obtained by DEM which applies Newton’s equations of motion to every individual particle and the flow of medium (mixture of water, air and fine magnetites) phase by the traditional CFD which solves the Navier–Stokes equations at a computational cell scale. The simulated results are analysed in terms of medium and coal flow patterns, and particle–fluid, particle–particle and particle–wall interaction forces. It is shown that under high fluctuation frequency and current conditions, the performance of DMC is not sensitive to both the fluctuation amplitude and period of coal flow at the DMC inlet. However, under low fluctuation frequency, as fluctuation amplitude increases, the separation performance deteriorates slightly and the flow is obviously affected at the spigot. A notable finding is that the near-gravity particles that tend to reside at the spigot and/or have longer residence time in the DMC would be affected more than other particles. The work shows that this two-way coupled CFD–DEM model could be a useful tool to study the dynamics of the flow in DMCs.

Numerical investigation of the location of maximum erosive wear damage in elbow: Effect of slurry velocity, bend orientation and angle of elbow

A numerical simulation of fluid transportation process was carried out focused on the investigation into predicting the location of the puncture point location in elbow. The kinematics and trajectory of the discrete particles as well as the particle–particle interaction were described by discrete element method (DEM) while the hydrodynamic model of the fluid phase was based on the volume-averaged Navier–Stokes equations, and a fluid density-based buoyancy model was adopted to calculate the solid–fluid interaction force. In this paper, the spatial distribution of particle–wall interaction force was calculated and corresponded to the wear pattern in the elbow, the puncture point locations have been observed. In particular, the influences of slurry velocity, bend orientation and angle of elbow on the puncture point location were discussed.

Salt-induced changes of colloidal interactions in critical mixtures

We report on salt-dependent interaction potentials of a single charged particle suspended in a binary liquid mixture above a charged wall. For symmetric boundary conditions (BC) we observe attractive particle-wall interaction forces which are similar to critical Casimir forces previously observed in salt-free mixtures. However, in case of antisymmetric BC we find a temperature-dependent crossover from attractive to repulsive forces which is in strong contrast to salt-free conditions. Additionally performed small-angle x-ray scattering experiments demonstrate that the bulk critical fluctuations are not affected by the addition of salt. This suggests that the observed crossover can not be attributed alone to critical Casimir forces. Instead our experiments point towards a possible coupling between the ionic distributions and the concentration profiles in the binary mixture which then affects the interaction potentials in such systems.

CFD–DEM simulation of the gas–solid flow in a cyclone separator

In this work, a numerical study of the gas–solid flow in a gas cyclone is carried out by use of the combined discrete element method (DEM) and computational fluid dynamics (CFD) model where the motion of discrete particles phase is obtained by DEM which applies Newton’s equations of motion to every individual particle and the flow of continuum fluid by the traditional CFD which solves the Navier–Stokes equations at a computational cell scale. The model successfully captures the key flow features in a gas cyclone, such as the strands flow pattern of particles, and the decrease of pressure drop and tangential velocity after loading solids. The effect of solid loading ratio is studied and analysed in terms of gas and solid flow structures, and the particle–gas, particle–particle and particle–wall interaction forces. It is found that the gas pressure drop increases first and then decreases when solids are loaded. The reaction force of particles on gas flow is mainly in the tangential direction and directs mainly upward in the axial direction. The reaction force in the tangential direction will decelerate gas phase and the upward axial force will prevent gas phase from flowing downward in the near wall region. The intensive particle–wall collision regions mainly locate in the wall opposite to the cyclone inlet and the cone wall. Moreover, as the solid loading ratio increases, number of turns travelled by solids in a cyclone decreases especially in the apex region of the cyclone while the width of solid strands increases, the pressure drop and tangential velocity decrease, the high axial velocity region moves upwards, and the radial flow of gas phase is significantly dampened.

CFD-DEM modelling of multiphase flow in dense medium cyclones

Dense medium cyclone (DMC) is a high-tonnage device that is widely used to upgrade run-of-mine coal in coal preparation. It is simple in design but the flow pattern within it is complex due to the size and density distributions of the feed and process medium solids, and the turbulent vortex formed. This paper presents a mathematical model to describe this flow system by means of combining Discrete Element Method (DEM) with Computational Fluid Dynamics (CFD). The DEM is used to model the motion of discrete particles by applying Newton's laws of motion. The CFD is used to model the motion of slurry medium by numerically solving the local-averaged Navier–Stokes equations facilitated with the Volume of Fluid (VOF) and Mixture multiphase flow models. The approach is shown to be able to reproduce typical flow phenomena in DMCs. The effect of medium-to-coal ratio and the so-called “surging” phenomenon are analyzed. An attempt is made to explain the observed phenomena in terms of particle–particle, particle–fluid and particle–wall interaction forces. The approach offers a convenient way to examine the flow and performance of DMC in relation to geometric, material and operational conditions.

342 3D-TIRFMによるナノ粒子密度分布測定と溶液の電気的影響の評価(T06-4 マイクロ・ナノ熱流体システム(4) 計測,大会テーマセッション,21世紀地球環境革命の機械工学:人・マイクロナノ・エネルギー・環境)

The Particle motion at near-surface is greatly important for efficiency in micro chip. In this paper, concentration distribution of polyethylene latex particle (diameter 100 nm) at near-surface in micro-channel is evaluated by total internal reflection fluorescence microscopy. And particle-wall interaction forces (van der Waals, electrostatic, and lift force) are investigated. The results show the concentration is the largest at specific location from the wall surface. The location is consistent with balance position of theoretical particle-wall interaction force in static fluid. In flow field, the largest value location is far away from wall as increasing velocity. This impact is lower with high-ionic strength.

Characterizing the effect of substrate surface roughness on particle–wall interaction with the airflow method

The effect of surface roughness on particle–wall interaction was studied by the airflow method. Five kinds of monodispersed spherical particles ( D p50 = 11-41 μm) and six test pieces with different surface roughness ( Ra = 0.01-1.64 μm) were used in the experiments. The particles were dispersed on the test pieces to form a monolayer, and entrained in a rectangular air channel. The air velocity increased at a constant rate, and the entrained particles were detected with a laser dust monitor. Microscopic observations showed that particle entrainment occurred in discrete and intermittent events during experiment, thus a statistical parameter, i.e. the particle entrainment efficiency as a function of the air velocity, was defined for evaluating the particle–wall interaction force distribution. The experimental results showed that the air velocity for particle entrainment decreases with the increase of the surface roughness within submicron-scale and reaches a lower limit, while increases to some extent for micron-scale surface roughness. It was also found that the effect of the substrate surface roughness depends on the particle diameter.

Characterizing the effect of surface morphology on particle-wall interaction by the airflow method

AbstractParticle-wall interaction has been studied by the airflow method. The particles used were micrometer-size polymethyl methacrylate (PMMA) spheres treated with an additive, i.e. nanometer-size TiO2, Al2O3 or SiO2 particles. The method for the treatment was dry particle coating based on mechanofusion or simple mixing through shaking manually. The surface morphology of the PMMA particles was modified by changing the species of additives, its concentration and the treatment method. The particles after treatment were dispersed on the surface of a flat metal piece and an experiment on particle entrainment was carried out in an airflow channel to evaluate particle-wall interaction. Relationships between particle entrainment efficiency and air velocity, which corresponds to the distribution of the particle-wall interaction force, were obtained under various conditions, showing that the particle-wall interaction force tends to decrease with the increase in the concentration of added nanoparticles. Furthermore, analysis of the experimental results showed that the main factor affecting the particle-wall interaction force was the actual surface morphology of the particles after treatment.

Microdynamic analysis of the particle flow in a cylindrical bladed mixer

A microdynamic study of the particle flow in a vertical axis mixer with slowly rotating flat blades has been performed by means of a modified discrete element method. The conditions are comparable to recent experiments conducted using positron emission particle tracking, with a mixer being 249 mm in diameter, filled by 16,000 monosized spheres of 5 mm diameter, and two blades rotating at a speed of 19 rpm . The dependence of flow behaviour on particle–particle and particle–wall sliding and rolling frictions is quantified and the results are used to establish the spatial and statistical distributions of microdynamic variables related to flow and force structures such as velocity, porosity, coordination number, particle–particle and particle–wall interaction forces. While the geometry and operational conditions are relatively simple, the particle flow is shown to be very complicated. There is a three-dimensional zone in front of a blade where particles have a strong recirculating flow. Increasing sliding friction coefficient or decreasing rolling friction coefficient can promote the formation of this zone. The flow and force structures of particles in the mixer are not uniform, although macroscopically steady flow is reached readily. The results show that increasing the rolling friction coefficient and, in particular, the sliding friction coefficient can increase the bed porosity and decrease the mean coordination number. The recirculating flow and the mixing kinetics are promoted by increasing the sliding friction coefficient or decreasing the rolling friction coefficient. Furthermore force arching is strong in the particle bed, with large inter-particle forces concentrating near the bottom corner just in front of the blade and propagating into the bed. Increasing the sliding or rolling friction coefficient increases the potential energy of particles in the mixer, but the kinetic energy is not sensitive to these coefficients. The increased potential energy gives increased particle–particle and particle–wall interaction forces and hence an increased torque required to drive the system. The results highlight the capacity and usefulness of numerical simulation in developing an understanding of the interplay of structure, forces, velocities and mixing in granular systems.