加速粉尘凝聚减少爆破拆除扬尘的理论与实践

通过对尘粒起动风速与尘粒粒径之间关系的研究,认为可以通过加速粉尘颗粒凝聚,从而生成大尘粒的办法来减少爆破拆除中的扬尘.通过对尘粒间物理凝聚力的大小及其生成条件的研究,发现在干燥情况下促使尘粒凝聚的主导作用力是范德华力;在湿润条件下促使尘粒凝聚的主导作用力是液体桥联力,并且液体桥联力比范德华力要大得多.因而认为用物理方法创造液体桥联力发挥作用的条件,生成大而牢固的尘粒,减少爆破拆除中的扬尘是合适的.通过对尘粒间化学作用力的研究,认为用化学方法增大尘粒间的凝聚力,减少爆破拆除中的扬尘是可行的和重要的.用现场实践数据支持了上述结论.     

Numerical study on compression processes of cohesive bimodal particles and their packing structure

In this study, the compression characteristics of bimodal cohesive particles were investigated using a discrete element method (DEM) simulation. The compression and packing processes were simulated under different conditions of size ratios of 1–4 and fine particle mixing ratios of 0–0.5. The cohesive force was expressed using the surface energy proposed by the Johnson-Kendall-Roberts (JKR) cohesion model having a surface energy of 0–0.2 J/m². The calculated results demonstrated that even in the case of cohesive particles, an increase in the particle size ratio reduced the void fraction of the powder bed during the packing and compression processes. In addition, it was found that the cohesive force decreased the contact number, especially the coarse-coarse contacts, although it had little impact on the void fraction. Our DEM simulations suggested that it is necessary to evaluate the contact numbers even under similar void fractions, which will be essential in the case of different material mixtures, such as all-solid-state batteries.     

超细粉体分级填料塔的结构设计

从气固流态化原理出发介绍了分级填料塔的工作原理,借助波纹填料塔的一些计算方法,进行了超细粉体分级填料塔的结构设计。固体颗粒由于颗粒大小的不同,其重量亦不同,使其上升的曳力亦不同,故在固定风速下,不同粒径的颗粒在分级填料塔内由上至下实现分层,团聚的颗粒受到填料的不断冲击而达到分散,提高了整体的分级效果。将该设备用于固体颗粒的分级,取得了显著的分级效果,展现了广阔的应用前景。     

Fluidity and Flow Properties of Fine Powders

ABSTRACT

Fluidization and/or flow properties of many fine powders (d₅₀ < 50 μm), including pharmaceutical powders, toners, powder paints, and ceramic powders, are of critical importance. Particles in this range behave as cohesive powder because of the relatively large inter-particle forces (electrostatic, van der Waals', and liquid bridge forces), compared to the hydrodynamic force exerted on the particles by the fluid flowing around the particles. Flow additives, mechanical agitation, and other forces such as acoustic and electromagnetic, are often applied for good fluidization and uniform dilute phase flow. In this paper, we present a brief discussion and experimental data on fluidization properties, fluidity, and flow behavior of several fine powders as functions of particle size distribution, relative humidity, relative concentration of flow additives, and the frequency and amplitude of mechanical agitation. Electrostatic charging, dependent upon the chemical composition and electrical conductivity of the particles, and its influence upon the flow properties are also presented.     

CFD-DEM numerical study on air impacted packing densification of equiaxed cylindrical particles

A CFD-DEM model was developed to reproduce the packing densification process of mono-sized equiaxed cylindrical particles under air impact. The effects of operating parameters on packing density were firstly studied. Then various microscopic properties of packing structures such as coordination number (CN), contact types, particle orientations, pore features were characterized and compared. And corresponding densification mechanisms were analysed based on particle motion behaviour, local structure evolution, and forces. Results indicate that the air impact can realize the packing densification of cylindrical particles under appropriate conditions. The pore size distribution in the packing of cylindrical particles shows a tail at larger pore sizes compared with that in the packing of equal spheres. Both the size and the sphericity of the pores decrease in the final dense packing; also, more surface-surface and less surface-edge contacts between two particles therein can be formed. More cylindrical particles tend to be in parallel or perpendicular contact with each other to form more stable local structures during air impact. Most particles at higher position move down (direction of gravity/air impact) with about one particle length during the densification process and most particles exhibit translational motion to realize the local rearrangement for pore filling through air impact induced inter-particle forces.     

Granular packings of cohesive elongated particles

We report numerical results of effective attractive forces on the packing properties of two-dimensional elongated grains. In deposits of non-cohesive rods in 2D, the topology of the packing is mainly dominated by the formation of ordered structures of aligned rods. Elongated particles tend to align horizontally and the stress is mainly transmitted from top to bottom, revealing an asymmetric distribution of local stress. However, for deposits of cohesive particles, the preferred horizontal orientation disappears. Very elongated particles with strong attractive forces form extremely loose structures, characterized by an orientation distribution, which tends to a uniform behavior when increasing the Bond number. As a result of these changes, the pressure distribution in the deposits changes qualitatively. The isotropic part of the local stress is notably enhanced with respect to the deviatoric part, which is related to the gravity direction. Consequently, the lateral stress transmission is dominated by the enhanced disorder and leads to a faster pressure saturation with depth.     

Discrete element modeling of solid formation during electrophoretic deposition

Electrophoretic deposition (EPD) of colloidal ZrO₂ ceramic powder was examined with respect to the internal colloidal forces and the external electrical field. The influence of electrolytic dissociation of water close to the deposition electrode (cathode) on the electrostatic interaction between the particles and the local electric field is discussed. The discrete element method (DEM) was used to get an insight into the kinetics determining particle packing and density gradient microstructures. The simulation indicates that high particle concentrations combined with low electric field strength result in coagulated flocs and a low packing density in the deposit layer. Tentative phase diagrams for various colloidal forces and electrical field strengths were established.     

加速粉尘凝聚降低爆破拆除扬尘的理论研究

通过分析爆破拆除中扬尘的特性以及尘粒的起动风速与尘粒粒径之间的关系,认为可以通过加速粉尘颗粒的凝聚,从而生成大尘粒的办法来减少爆破拆除产生的爆破扬尘.通过对尘粒间的物理凝聚力和化学凝聚力的比较和分析,认为用物理方法增大尘粒粒径,从而达到减少爆破拆除中产生的爆破扬尘是合适的;同时用化学方法增大粉尘颗粒间的凝聚力,减少爆破拆除中的扬尘是可能的.     

Structural features of two-component dusty plasma Coulomb balls

A binary mixture of dust particles in plasma occurring in an external electrostatic spherical confinement field has been numerically simulated. Calculations have been performed within the framework of the molecular dynamics method, with the interparticle forces described in terms of an isotropic Yukawa pair potential. It is shown that the particles form a shell structure, each particular shell consisting of particles of the same kind. The structural properties of such a binary mixture of particles and the specific features of their segregation under the conditions of recent experiments on the formation of Coulomb balls have been studied.     

Analysis of the packing structure of wet spheres by Voronoi–Delaunay tessellation

The structure of a packing of narrowly sized wet spheres with packing density 0.435 is analysed against the well-established random close packing with packing density 0.64 by means of the Voronoi and Delaunay tessellation. The topological and metric properties of Voronoi polyhedra, such as the number of faces, perimeter, area and volume of a polyhedron, the number of edges, perimeter and area of a polyhedron face, have been quantified. Compared to the well established random close packing, the distributions become wider and more asymmetric with a long tail at the higher values. The volume and sphericity of each Delaunay cell have also been quantified. Their distributions are shown to be wider and more asymmetric than those for the random close packing, but the peaks are almost the same. For the wet particle packing, the correlations between Voronoi polyhedron size and shape and between Delaunay cell size and shape are more scattered. The topological and metric results are also shown to be consistent with those obtained for the packing of fine particles, although the dominant forces in forming a packing differ. The results should be useful to the quantitative understanding of the structure of loosely packed particles.     

DEM simulations: mixing of dry and wet granular material with different contact angles

In solid mixing the raw materials typically differ at least in one material property, such as particle size, solid density and wetting properties, which in turn influence particle mobility. For example, smaller particles can percolate through the voids of larger ones under the influence of strain and gravity. This may produce fine particle accumulation at the bottom of the mixing vessel which results in undesired, inhomogeneous final products. When wet particles with different wetting properties need to be mixed, heteroagglomeration may occur as another segregation mechanism. We present a new capillary bridge force model to study segregation in moist cohesive mixing processes using DEM. New analytical equations of best fit are derived by solving the Young–Laplace equation and performing a regression analysis, in order to investigate discontinuous mixing processes of dry and moist materials with different particle sizes and different contact angles. Compared to a dry mixing process, mixing efficiency is improved by the addition of a small amount of liquid. While percolating segregation is reduced, heteroagglomerates occur in the wet mixing process.     

Enhancing fine particle separation by hybrid-electrostatic-turbulence coagulation: An experimental and numerical investigation

The electrostatic precipitator(ESP) has low efficiency in removing sub-micron particles. Coagulation technology, as a fine particle pretreatment technology, uses an external effect to agglomerate and grow fine particles, increase the average particle size, and make it easier to remove by subsequent dust removal equipment. However, the coagulation efficiency of a single coagulation technology is limited. Aiming at the particle charging mechanism and coagulation mechanism in the electric/turbulent composite coagulation process of fine particles, this paper uses a combination of numerical simulation and experiment to study the effects of different structural parameters, discharge parameters and flue gas parameters on corona discharge and particle charge. On this basis, the coagulation characteristics of charged particles in the turbulent flow field are studied. The results show that, when the Angle between the tip of the arista electrode is 90°, the corona discharge effect is the best. With the increase of supply voltage and temperature, the charge of particles increases. When the applied positive voltage is 29 kV and negative voltage is ?35 kV, the total coagulation coalescence efficiency of fine particles reaches the maximum. The coagulation efficiency increases with the increase of temperature, but decreases with the increase of inlet flow rate.     

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.     

On the motion of macroparticles in inhomogeneous magnetic field

The behavior of macroparticles that form a near-wall dust in tokamaks is described with allowance for the force of gravity, force of entrainment by plasma ion flux, and Lorentz force acting on the motion of charged macroparticles in an inhomogeneous magnetic field. It is shown that, in some cases, the effect of induced currents that arise in the particles during this motion must in some cases be taken into account.     

Effect of dust-neutral collisions on the dust characteristics in a magnetized plasma sheath

The characteristics of dust in a plasma sheath are investigated in the presence of an external magnetic field and taking into account the dust-neutral collision force. The continuity and momentum equations of ions and dust particles are solved numerically with various magnitudes of collision force by using the fluid model. The numerical results have revealed that the collision force reduces the dust gyro radius, changes the positions of the extrema of the dust density and the velocity in the depth direction. It is shown that the collision force reduces the dust kinetic energy which has no fluctuation even in a strong external magnetic field.     

选粉机分级区水泥粉体颗粒运动及受力分析

通过建立选粉机分级区气固两相力学模型,对水泥颗粒进行拟动力学分析与运动分析,明确认为选粉机分级区颗粒流体在宏观上为过渡区流态(即处于层流与紊流之问),颗粒在选粉机分级区所受作用力中,离心力最大,空气作用在颗粒上的阻力次之,重力较小,浮力最小,而且还发现了选粉机的结构参数、工艺参数对重力、浮力的影响比较小,在设计及操作中通常可不考虑其影响等。     

Granular packing: numerical simulation and the characterisation of the effect of particle shape

The packing of granular particles is investigated using a combined finite-discrete element approach. One of the aims of this paper is to present an application of a recently improved numerical simulation technique for deformable granular material with arbitrary shapes. Our study is focused on the influence of the effect of the particle shape on (1) the emergent properties of a granular pack (packing density, coordination number, force distribution), and on (2) the spatial distribution of the stress. A set of simulations that mimick the sedimentation process is carried out, with varying input parameters, such as contact friction and particle shape. It is shown that the eccentricity of the particles not only significantly influences the final density of the pack but also the distribution of the stress and the contact forces. The presence of surface friction increases the amount of disorder within the granular system. Stress heterogeneities and force chain patterns propagate through the particles more efficiently than for the frictionless systems. The results also suggest that for the monodisperse systems investigated the coordination number is one of the factors that controls the distribution of the stress within a granular medium.     

Cohesion of lactose powders at low consolidation stresses

The flow characteristics of a powder system are known to be influenced by particle size distribution, particularly the content of fine particles, and interparticle forces. This paper reports an investigation that has identified and quantified links between physical properties, viz size distribution, bulk density and particle density, and cohesion in compacted beds of powder. An annular shear cell was used in the determination of the cohesion of cohesive and free-flowing milled lactose powders at low consolidation stresses in the range 0.31–4.85 kPa and under ambient conditions. Following consideration of the compaction and shearing processes, it was postulated and confirmed that cohesion could be expressed as a function of powder surface area per unit volume and dimensionless preconsolidation stress. It was shown that care is needed in the measurement of surface–volume mean diameter when applying correlations developed from the experimental data.     

Mechanomics and Physicomics in Gravisensing

Sensing gravity by ‘non-specialized’ cells is still puzzling. We don’t know where or by which mechanism such cells sense gravity. These questions in ‘gravisensing’ are not much different from questions in general mechanobiology. Numerous studies have been reported in this field in the last couple of decades. What are the mechanical properties of a cell? Are there differences in mechanical properties between cell types and if so why? How are forces perceived and transduced to a meaningful biological event. Novel techniques such as optical and magnetic tweezers, atomic force microscopy, magnetophoresis and computer modeling make the field of mechano-sensing or perhaps physicomics accessible. A similar approach should also be applied for gravity-related research. This paper addresses the current techniques used in mechanosensing and exemplifies how a cell could sense the relatively weak force of gravity.     

Experimental and computational study on the separation performance of an electrostatic precipitator with curved transvers collecting plates

Aiming at the problems of low collection efficiency of fine particulate matter and large area occupied by existing electrostatic precipitators (ESP), a new type of horizontal electrode ESP is proposed. It has the advantages of accelerating turbulent coalescence, increasing the effective dust collecting area and increasing the particle driving speed. The performance of the new type of ESP is systematic studied through simulation and experiment at the same time, and the results matches well. By comparing the dust removal effect of the horizontal electrode ESP and the conventional ESP, it can be concluded that the horizontal electrode ESP has a better dust removal efficiency, and can still maintain a better dust removal effect under high air velocity. The dust removal efficiency of new ESP can reach above 98% under the experimental conditions.