Packing of Ternary Mixtures of Nonspherical Particles

An experimental investigation of the packing of some ternary mixtures of nonspherical particles has been conducted. The double extrapolation method is used to determine the packing density of a particle mixture in infinite space. The application of experimental design to particulate mixture problems is demonstrated. The results indicate that the similarity between the packing systems of spherical and nonspherical particles can be readily depicted in terms of specific volume variation.     

Dynamic Shape Factor of Nonspherical Aerosol Particles in the Diffusion Regime

Diffusional losses of monodisperse polysterene spheres and doublets from laminar aerosol flows and stagnant aerosols in cylindrical tubes were measured. The losses of spheres were used to determine their diameter (range 69–120 nm) and the volume equivalent diameter of their doublets. The losses of the doublets were used to determine their dynamic shape factor which for randomly oriented doublets was established to be 1.127 ± 0.085 for Knudsen numbers between 1.1 and 2.0.     

Porosity Calculation of Binary Mixtures of Nonspherical Particles

An investigation has been made of the packing of binary mixtures of either spherical or nonspherical particles. It is shown that this binary packing system can satisfactorily be described by the Westman equation. By use of the concept of "equivalent packing diameter," nonspherical particle packing may be related to spherical particle packing. The porosity of binary mixtures of nonspherical particles can then be predicted by means of a model developed for spherical particles. This approach is verified by the good agreement between the calculated and experimental results.     

Recent Advances in Light Scattering Calculations for Nonspherical Particles

Methods for calculating the light scattering properties of nonspherical particles are reviewed, and applications to particle characterization are considered. Angular scattering results are given for particles in fixed and random orientations. These include the Mueller matrix representation and quasi-three-dimensional computer generated plots. Spectral (wavelength-dependent) scattering results are shown for spheres, a spheroid, and a glass fiber. Applications of spectral scattering to characterizing polydisperse suspensions are discussed.     

Calculation of the Velocity of Free Settling of Solid Particles in a Newtonian Liquid

A cubic equation for the velocity of free settling of solid particles in a Newtonian liquid is derived on the basis of a simple approximate expression for the drag coefficient of spherical particles as a function of the Reynolds number using the concept of the effective diameter of a particle of arbitrary shape, which characterizes the surface, volume, and midsection of the particle. The exact and approximate solutions are compared, and some experimental data are analyzed.     

Aerosol Generation of Nonspherical Particles by Desublimation of Copper Phthalocyanine

A novel generator for a defined test aerosol consisting of nonspherical particles was developed based on the desublimation of copper phthalocyanine during adiabatic cooling. Employing a brush disperser, copper phthalocyanine powder is dosed and dispersed in a nitrogen flow and sublimated in a tube furnace. Downstream the furnace new particles are formed due to the adiabatic expansion and the desublimation of the material in a laval nozzle. The generated particles were characterized employing a scanning mobility particle sizer and an aerosol particle mass analyzer to determine the size distribution and the dynamic shape factor. For the operating parameters of the generator examined here, particles with a mobility diameter between 30 and 600 nm were generated. The measured values for the dynamic shape factor of the particles were confirmed by scanning electron microscopy analysis.     

液-固流化床中液速分布与颗粒循环流动

通过理论分析和实验研究考察了液一固平板流化床中颗粒流和液流的运动规律,提出了将分散的颗粒流连续介质化的假设和基于容积通量的流休力学表达方式,建立了液体流动和颗粒循环流动的数学模型并定义了颗粒流与液流的有效粘度。理论计算表明,液体通量的径向分布为抛物线,液流有效粘度和液含率与表观液速有关：颗粒通量的径向分布也为抛物线且颗粒上流区与回流区的分界点在0.577D,循环流动强度取决于液含率和液体密度,但颗粒循环分界点的位置与颗粒类型和操作液速无关。实验观察支持模型预测结果。     

Measurement of the Size of Fine Nonspherical Particles with a Light-Scattering Particle Counter

To characterize flow separation and classification processes, particle size distributions must be measured in the airborne state without affecting the state of dispersion or disturbing the flow. Light-scattering devices with an optically defined measuring volume are specially designed for this purpose. However, the light-scattering device must be calibrated using nonideal particles which are present within the multiphase flow, preferably on an equivalent diameter based on settling rate. The cyclone cut size calibration can solve this problem. By means of this aerodynamic calibration technique, it is possible to measure size distributions of nonspherical particles with a light-scattering particle counter in industrially relevant cases.     

The Collision of Nonspherical Aerosol Particles with Small Evaporating Water Drops

Studies in the collision efficiencies of aerosol particles with water drops have generally assumed that the particles are spherical, even though ambient aerosol particles may tend to be nonspherical. The present theoretical study estimates the effects of nonspherical particle rotation in shear flow and shape dependency of the thermophoretic force in the computation of collision efficiencies of small evaporating water droplets. The results obtained show considerable differences between collision efficiencies of spherical and nonspherical particles of equivalent Stokes radii.     

Distinguishing Between Spherical and Nonspherical Particles by Measuring the Variability in Azimuthal Light Scattering

Azimuthal variabilities in scattering of monochromatic, circularly polarized light by individual spherical and nonspherical particles were measured using the DAWN-A (Wyatt et al. Appl. Opt. 27:2405–2421, 1988) differential light scattering detector. Measured aerosols included polystyrene latex spheres (PSL), quartz, and sodium chloride particles of 0.576, 0.741, 0.966, and 1.250 μm diameter. Signals from eight detectors at different azimuthal angles at a polar angle of 55° showed that variabilities for nonspherical particles significantly exceeded values for the spherical PSL. The probability that a quartz or sodium chloride particle would be incorrectly identified as a sphere are less than about 5% for all sizes investigated.     

Determination of the Scalar Friction Factor for Nonspherical Particles and Aggregates Across the Entire Knudsen Number Range by Direct Simulation Monte Carlo (DSMC)

The friction factor of an aerosol particle depends upon the Knudsen number (Kn), as gas molecule–particle momentum transfer occurs in the transition regime. For spheres, the friction factor can be calculated using the Stokes–Millikan equation (with the slip correction factor). However, a suitable friction factor relationship remains sought-after for nonspherical particles. We use direct simulation Monte Carlo (DSMC) to evaluate an algebraic expression for the transition regime friction factor that is intended for application to arbitrarily shaped particles. The tested friction factor expression is derived from dimensional analysis and is analogous to Dahneke's adjusted sphere expression. In applying this expression to nonspherical objects, we argue for the use of two previously developed drag approximations in the continuum (Kn → 0) and free molecular (Kn → ∞) regimes: the Hubbard–Douglas approximation and the projected area (PA) approximation, respectively. These approximations lead to two calculable geometric parameters for any particle: the Smoluchowski radius, R _S, and the projected area, PA. Dimensional analysis reveals that Kn should be calculated with PA/πR _S as the normalizing length scale, and with Kn defined in this manner, traditional relationships for the slip correction factor should apply for arbitrarily shaped particles. Furthermore, with this expression, Kn-dependent parameters, such as the dynamic shape factor, are readily calculable for nonspherical objects. DSMC calculations of the orientationally averaged drag on spheres and test aggregates (dimers, and open and dense 20-mers) in the range Kn = 0.05–10 provide strong support for the proposed method for friction factor calculation in the transition regime. Experimental measurements of the drag on aggregates composed of 2–5 primary particles further agree well with DSMC results, with differences of less than 10% typically between theoretical predictions, numerical calculations, and experimental measurements.

Copyright 2012 American Association for Aerosol Research     

On the Inertial Separation of Nonspherical Aerosol Particles from Laminar Flows. I. The Cylindrical Case

The inertial separation of cylindrical particles from laminar flows was studied theoretically and experimentally. The calculational method introduced was based on the simultaneous solution of the equations of particle translation and rotation, in which the expression for the operating fluid-dynamic force and torque was obtained from slender body theory. The collecting obstacles chosen for the exemplification of the method were a long circular cylinder and a nuclepore hole. The necessary undisturbed flow fields in the vicinity of these obstacles were derived by a numerical solution of the full Navier-Stokes equation. Particle trajectories, specifying location and orientation, were generated with the aid of a computer.

The experiments consisted of the determination of the motion of macrocylinders within a liquid flow channel near models of the above two obstacles. Both particle location and orientation could be ascertained by video photography from two directions of sight, an elaborate subsequent recording, and data processing.

A comparison of theoretical and experimental trajectories for identical conditions was made, whereby a satisfactory agreement was observed. Consequently, trajectories of aerosol particles were calculated, and efficiencies of collision with a cylindrical fiber and in a nuclepore system were found.

In general, the motion of particles in this study was characterized by a pronounced rotation, a tendency of alignment on a time basis along the flow lines, and a strong dependence on the initial orientation, which is typical of short residence time phenomena.     

Research on the Motion of Solid Particles in a Hydrocyclone

Abstract

By using a new type of laser surveying instrument named the particle dynamics analyzer, the radial and axial velocity components and the size and concentration of solid particles at selected positions within a transparent hydrocyclone were measured directly, and profiles of the solid particle flow field were obtained. Some new observations were made, such as that the maximum concentration was at the loci of zero vertical velocity, and there was separation of some particles in the inner helical flow.     

Behavior of Compact Nonspherical Particles in the TSI Aerodynamic Particle Sizer Model APS33B: Ultra-Stokesian Drag Forces

The aerodynamic behavior of aggregates consisting of uniform polystyrene latex (PSL) spheres and unaggregated cuboidal Natrojarosite particles in a TSI aerodynamic particle sizer (Model APS33B) has been studied. In initial tests, monodisperse PSL micro-spheres ranging from 0.3 to 7 μm in geometric diameter were generated from aqueous suspensions using a Lovelace nebulizer. APS33B responses for these uniform-sized particles showed multiple peaks. The major (primary) peak, which resulted from the smallest particle, corresponded to the unaggregated single spheres (singlets); the second, third, and fourth peaks were identified as doublets, triangular triplets, and tetrahedral quadruplets, respectively. Both doublets and triplets moved with their long axes in perpendicular (maximum drag) orientation to the flow direction in the APS33B. In contrast, the tetrahedral particles were isometric and had the same dynamic shape factor (drag resistance) for all three primary orientations. The particle Reynolds numbers (Re _p) for these particles were calculated and ranged from 0.2 to 30 in the sensing volume of the APS33B detector (i.e., ultra-Stokesian conditions). Ultra-Stokesian drag forces for all three types of aggregates were, therefore, estimated and expressed as a function of an empirical factor (1 + aRe ^b _p) to the Stokesian drag force. The ultra-Stokesian drag of a Natrojarosite particle was measured in the range 20 Re _p < 50 and could be described with a similar expression. This approach facilitates the study of the dynamic behavior of nonspherical particles and yields new information about the characteristics of drag forces in the ultra-Stokesian regime     

选粉机分级室内的颗粒运动特性

在分析SLK5500型选粉机结构和分级机理的基础上,通过构建选粉机分级室模型,根据计算流体力学理论,运用DPM模型与非稳态算法求解颗粒运动轨迹,阐述了不同粒径的颗粒群在分级室内的运动特性. 考察了选粉机分级室内的流场速度分布,并捕捉颗粒计算了风量和转速对细粉颗粒产率和80 mm筛余的影响,模拟结果与实验结果基本相符,验证了模型的可靠性.     

Development of a Pulsed-Field Differential Mobility Analyzer: A Method for Measuring Shape Parameters for Nonspherical Particles

For a nonspherical particle, a standard differential mobility analyzer (DMA) measurement yields a mobility-equivalent spherical diameter, but provides no information about the degree of sphericity. However, given that the electrical mobility for nonspheres is orientation-dependent, and that orientation can be manipulated using electric fields of varying strength, one can, in principle, extract some type of shape information through a systematic measurement of mobility as a function of particle orientation. Here, we describe the development of a pulsed-field differential mobility analyzer (PFDMA) which enables one to change the peak E-field experienced by the particle to induce orientation, while still maintaining the same time-averaged field strength as a standard DMA experiment. The instrument is validated with polystyrene latex (PSL) spheres with accurately known size, and gold rods with dimensions accurately determined by transmission electron microscopy (TEM). We demonstrate how the instrument can be used for particle separation and extraction of shape information. In particular, we show how one can extract both length and diameter information for rod-like particles. This generic approach can be used to obtain dynamic shape factors or other multivariate dimensional information (e.g., length and diameter).

Copyright 2014 American Association for Aerosol Research     

熟料颗粒角速度实验及预测研究

在水泥熟料流动状态换热研究中,一般认为颗粒的旋转运动对整体换热效果具有一定影响.在所建立的颗粒角速度实验装置上,利用姿态传感器采集颗粒转角信号,对多工况下颗粒的旋转角速度进行了实验测定.研究发现角速度随推料速度的增大而增大,颗粒的推程运动角速度大于返程运动角速度,在一个推程内,颗粒自旋角度占颗粒圆周的17％ ～25％,对颗粒堆积体的换热有较大影响.基于神经网络理论对实验数据进行处理,建立了颗粒转角的BP神经网络模型,预测值相对于实验值平均误差为14％,该模型为考虑颗粒旋转的水泥熟料流态换热研究提供了分析及验证基础,同时对于其他颗粒的角速度研究具有一定的借鉴意义.     

同轴对称撞击流中的颗粒运动行为孙志刚

采用高速相机对同轴对称撞击流中颗粒运动行为进行了实验研究,采用图像处理软件对获得的图片进行分析处理,得到了颗粒在同轴对称撞击流中的运动轨迹、运动速度、最大渗入距离和旋转速度等参数.考察了喷嘴出口气速、颗粒大小、颗粒出口速度以及喷嘴间距对颗粒运动行为的影响.结果表明:颗粒在同轴对称撞击流中的运动有直接射出模式和振荡模式,其中振荡模式占70%;出口气速的增加使颗粒轴线速度和旋转速度变大;随着颗粒相对出口速度、颗粒Stokes数和喷嘴间距的增加,颗粒的最大渗入距离变大.     

Interaction of Silica Particles Through a Liquid

Measurements of the destruction time of the sediment column structure were carried out for different fractions of silica particles in NaCl and CaCl₂ solutions whose concentrations were varied from 0.25 M to 2.0 M. On the basis of this study, linear relationships between the reciprocal of the destruction time of the silica sediment column structure and average diameter of the silica fraction, the density and the work of cohesion of the solutions were found. From these relationships the critical values of the particle diameter, the cohesion work and density of the solutions were determined for the destruction time equal to infinity (1/t = 0). The detachment force of one particle from another, which resulted from gravitational interaction, was calculated using the above critical parameters. For the system whose components have critical values the detachment force is found to be equal to the attachment force resulting from the interfacial interaction. Next, on the basis of our earlier study and Derjaguin's approximation, the radius of the contact plane and the attachment force between two silica particles were calculated. The detachment force decreased with increasing salt concentration, but the attachment force was nearly constant. It was found that, for a given salt concentration, the relationship between the reciprocal of the destruction time and the difference between the detachment and attachment forces was also linear. It appeared that for the silica column structure having critical parameters the values of the difference between the detachment and attachment forces were negative. This means that in the calculation of the attachment force between silica particles in the studied solutions an additional interaction should be taken into account.