Showing papers on "Particle published in 1989"

Gas-particle flow in a vertical pipe with particle-particle interactions

Abstract: A theory is presented for the fully-developed flow of gas and particles in a vertical pipe. The relation between gas pressure gradient and the flow rates of the two phases is predicted, over the whole range of cocurrent and countercurrent flows, together with velocity profiles for both phases and the radial concentration profile for the particles. The gas and the particles interact through a drag force depending on their relative velocity, and there are mutual interactions between pairs of particles through inelastic collisions. This model is shown to account for marked segregation of gas and particles in the radial direction, and the predicted relation between the pressure gradient and the flow rates of the two phases is surprisingly complex.

The Effect of Bubble Size on Fine Particle Flotation

Abstract: Expressions for the probability of collision (Pc) and adhesion (Pa) have been derived for fine particle flotation by calculating the trajectory of particles as they flow past a bubble in streamline How. Three different flow regimes have been considered in the present work, i.e., Stokes, potential and intermediate. For the intermediate flow conditions in which most flotation operations are carried out, the particle trajectories have been determined using an empirical stream function derived in the present work. For the case of a very hydrophobic coal sample, the values of the probability of collection (P) determined experimentally have been found to be in close agreement with the theoretically predicted Pc values over a range of bubble and particle sizes The expression for Pa has been derived by determining the time it takes for a particle to slide along the surface of a bubble after collision. It has been assumed that the bubble-particte adhesion occurs when the sliding time is equal to or exceed...

Aerosol particle deposition in numerically simulated channel flow

Abstract: The trajectories of rigid spherical particles in a turbulent channel flow are computed using a pseudospectral computer program to simulate the three‐dimensional, time‐dependent flow field. It is assumed that the channel is vertical so that gravity cannot directly cause the deposition of particles on the walls. The particles are assumed to be sufficiently small and widely separated so that their influence on the fluid velocity field can be ignored. It is found that when the particles are assigned random initial locations with initial velocities that are equal to the local fluid velocity, the particles tend to accumulate in the viscous sublayer. At the edge of the viscous sublayer, the particles that deposit on the wall typically possess normal components of velocity that are comparable in magnitude to the intensity of the normal component of the velocity in the core of the channel (i.e., of the order of magnitude of the friction velocity). A shear‐induced lift force having the form derived by Saffman for laminar flow is found to have virtually no effect on particle trajectories, except within the viscous sublayer where it plays a significant role both in the inertial deposition of particles and in the accumulation of trapped particles. The Reynolds number of the particles that deposit does not remain small compared with unity.

Particle bombardment effects on thin‐film deposition: A review

Abstract: In many atomistic film deposition processes, concurrent energetic particle bombardment (ions, atoms, molecules, atom clusters) may occur inadvertently and uncontrollably or bombardment may be used to deliberately modify film properties. These energetic particles can arise from (i) the acceleration of charged particles, (ii) high‐energy neutrals from reflection from bombarded surfaces, or (iii) charge exchange processes. Particle bombardment effects that can affect film formation and growth include (a) modifying the substrate surface (cleaning, defect formation), (b) momentum transfer processes in the surface region (sputtering, desorption, recoil implantation, defect formation), (c) addition of heat to the surface region, and (d) formation of secondary elelctrons that can affect chemical reactions. These in turn affect film properties such as adhesion, residual film stress, film morphology, density, grain size and orientation, surface coverage, pinhole density, and surface area. The understanding of these effects and how to use them advantageously is important to those utilizing processes where concurrent energetic particle bombardment is occurring or can be made to occur.

Fine particles in the global troposphere. A review

Abstract: The available body of data on fine particles (> 1 μm radius) in the troposphere is reviewed in relation to our understanding of sources, sinks and transformation processes of atmospheric aerosols. This review yields the following results. There are no data available to characterize the free troposphere above the boundary layer. Most of the boundary layer data suffer from the lack of a well-defined upper size limit of the samplers at about 1 μm radius and concurrent results on total fine particle mass. The fine particle composition data are consistent with our understanding of natural and anthropogenic sources of trace substances, which lead to fine particles. The chemical composition is presented for the three aerosol types: urban, non-urban continental and remote regions. Special attention needs to be focused on fine particle interaction with clouds for three reasons. Fine particles control number- and surface-distribution of the condensed matter; the state of precondensation haze and clouds provide the most important processes of fine particle elimination from the atmosphere. Furthermore, climatic effects of fine particles as expressed by their interaction with atmospheric radiation is strongest during their passage through clouds. Very little is known about the processing of aerosols through clouds. Consequently, there are large uncertainties about their climatic effect and their lifetime in the atmosphere. Finally, no single type of secular trend in particle concentration is found on a global scale. Depending on which component and which geographical region is monitored, both decreasing and increasing concentrations have been measured. The differences are consistent with our understanding of aerosol properties and their gas phase precursors. DOI: 10.1111/j.1600-0889.1989.tb00132.x

Dynamic simulation of electrorheological suspensions

Abstract: A simulation method is developed to investigate structure formation in electrorheological suspensions. The suspension is treated as polarizable, spherical particles in a nonconducting medium, with the spheres subject to electric polarization forces due to an applied electric field and to hydrodynamic resistance due to their motion through the continuous phase. The fibrous structures obtained from these simulations are independent of electric field strength and continuous phase viscosity in agreement with experimental observation. We have also found that the details of the simulated structures are sensitive to the treatment of the short‐range forces preventing particle overlap. When this force is represented by a form that accurately approximates a hard‐sphere interaction, the simulated structures agree well with those obtained experimentally, both with respect to their appearance and the time scale for structure formation.

Thermodynamics of Densification: I, Sintering of Simple Particle Arrays, Equilibrium Configurations, Pore Stability, and Shrinkage

Abstract: Equilibrium configurations for linear and closed arrays (rings and regular polyhedra containing a single pore) of identical particles (cylinders or spheres) were determined by minimizing the array's surface and grain-boundary energies with the assumption that each particle conserves its mass. The change in free energy between the initial and equilibrium configuration increases with dihedral angle (i.e., the equilibrium angle). More significantly, it is shown that pores will shrink to an equilibrium size if the number (n) of coordinating particles is greater than a critical value. The critical pore coordination number (nc) increases with the dihedral angle. Only pores with nnc are thermodynamically unstable during sintering. It is also shown that any mass-transport mechanism can lead to pore shrinkage while a connecting path to the pore surface remains open. The effective sintering “stress” (i.e., driving force) increases with the dihedral angle and decreases to zero as the equilibrium configuration is reached. Sintering stresses increase with decreasing coordination number. It is also shown that the shrinkage strain for closed arrays increases with the pore coordination number. Rearrangement phenomena within a powder compact are discussed with regard to resultant sintering forces on nonsymmetrically coordinated particles.

Monomer-addition growth with a slow initiation step: A growth model for silica particles from alkoxides

Abstract: A simplified monomer-addition model with a first-order activation step is developed to describe the dynamics of growth of silica particles from alkoxides. In the fimit of slow hydrolysis, we obtain expressions for the evolution of the particle mass and particle polydispersity, as well as an expression for the particle size as a function of the hydrolysis rate constant, the polymerization rate constant, and the initial concentration of the orthosilicate. We find that the formation of the particles is adequately modeled by a reaction limited growth. © 1989 Academic Press, Inc.

Process for the preparation of fluid loss additive and gel breaker

Abstract: A melted polymer of hydroxyacetic acid with itself or with other hydroxy-, carboxylic acid-, or hydroxycarboxylic acid-containing moieties is subjected to high shear and injected into an organic liquid which is inert to the polymer to form an organic liquid dispersion of very fine polymer particles. In one aspect the melted polymer is combined with the organic liquid and the combination is thereafter subjected to high shear to form the fine particle polymer dispersion. A dispersion agent may also be used to aid in dispersing the particles. The dispersion is used for fluid loss control and gel breaking in subterranean formations.

A General Equation for the Terminal Fall Speed of Solid Hydrometeors

Abstract: A comprehensive yet simple formula is presented for the terminal fall velocity of solid precipitation particles. It depends on three particle parameters: mass, the mean circumscribed area presented to the flow, and the mean effective projected area presented to the flow. This formula is deduced from a single mean Davies number-Reynolds number relation based on boundary layer theory and therefore includes environmental conditions (air density and temperature). Terminal velocity is predicted with errors ≲10% for a wide variety of particles, e.g., various planar and columnar crystals, rimed and unrimed aggregates, graupel (lump, conical, and hexagonal) and hail up to over 10cm in diameter. In view of its general form and the broad base of examination, the formula is felt to be generally applicable to all kinds of natural solid precipitation particles including shapes not tested in this study.

Selective Particle Deposition in Crossflow Filtration

Abstract: To study the mechanism of particle deposition in crossflow filtration, hydrodynamic forces exerted on a spherical particle touching the surface of filter medium are analyzed to derive the critical selective cut-diameter of the deposited particles under various crossflow velocities and filtration rates in a crossflow filtration system. Experimental data of turbulent crossflow filtration of dilute light calcium carbonate suspension agree with the prediction of this theory within 30% error under the crossflow velocity of from 0.57 to 1.14 m/s. Equations to estimate the characteristics of crossflow filtration, such as steady-state filtration rate and average specific resistance of cake, are also presented.

Hydrodynamics of Bubble-Mineral Particle Collisions

Abstract: This chapter surveys the hydrodynamic interactions between particles and bubbles in flotation. Some new approximate equations are given for collision efficiency. It is shown that collision processes of particles with bubbles are less effective than sliding processes because of their short duration and the strong deformation of the bubble at the collision point. Methods are suggested to estimate collision and sliding times. During contact between a particle and a bubble, the just forming thin liquid film must drain off and rupture. Therefore, possible ways of calculating film drainage time are discussed. Furthermore, possible experimental methods to determine these quantities are briefly described and recent experimental results presented.

Photochemistry of Colloidal Semiconductors. 30. HPLC Investigation of Small CdS Particles

Abstract: HPLC is used to determine the size distribution of CdS colloids These distributions agree with those obtained from electron microscopy The method is applied to recognize “magic” agglomeration numbers and to follow thermal particle growth and photodissolution In the thermal growth a gradual increase in size is ascribed to Ostwald ripening However, in the beginning a rather abrupt increase in size takes place that is ascribed to particle combination In the photodissolution of a sample containing two size distributions, the larger particles disappear more rapidly than the smaller ones, this effect being particularly pronounced when light of longer wavelengths is used where the small particles have little absorption

Measurement of the hydrodynamic surface roughness of noncolloidal spheres

Abstract: A new technique for measuring the surface roughness of noncolloidal spheres is presented. The time for a sphere initially in contact with a smooth surface to fall away under the influence of gravity through a viscous fluid is shown to be related to the largest scale of surface roughness of sufficient surface coverage to support the particle. The ratio of the time taken for a sphere to fall one particle diameter from a smooth mica plane to that for the sphere to fall between one radius and one diameter from the plane thus provides a means of measuring the effective hydrodynamic surface roughness of spheres. This technique was employed to measure the roughness of eight types of particles ranging from 43–6350 μm in diameter. The roughnesses were found to be on the order of 10−2 to 10−3 particle radii, and were in agreement with independent observations using a scanning electron microscope and an optical profilometer.

Particle dispersion in a turbulent, plane, free shear layer

Abstract: The particle dispersion mechanisms in an inhomogeneous, anisotropic, high Reynolds number, turbulent free shear flow are presented. Flow visualization, as well as laser attenuation and diffraction techniques, are used to characterize the evolution of the flow. It is shown that, at each downstream location, the spectral response of the particles to the coherent velocity field that develops in the layer as a result of a Kelvin–Helmholtz type of instability leads to a selective dispersion of the particles across the mixing layer. Furthermore, this particle dispersion layer is shown to be composed of a central core region, characterized by a small mean particle size and two external sublayers with considerably larger mean particle sizes. Finally, the experimental results suggest the dominant role played by the coherent, large scale vortical structure in the dispersion of the particles. Scaling arguments and simplified flow models supporting this hypothesis are also presented.

Method of toughening diamond coated tools

Abstract: of the Disclosure Method of toughening the structure of a diamond or diamond-like coated tool, by the steps of: (a) depositing, by low pressure CVD, a plurality of layers of separated diamond or diamond-like particles onto a nondiamond or nondiamond-like tool substrate (i.e., SiAlON, Si3N4, SiC, Si, Ti, Co cemented WC, TiC, Ni-Mo cemented TiCN), the substrate being selected to facilitate diamond or diamond-like deposition and to retain its strength-related properties after such CVD; and (b) interposing a mechanically adherent, planarized binding material (i.e. transition metals, silicon, boron) between and on said layers of particles and across the separated particles of each particle layer, said binding material being substantially devoid of diamond graphitizing or dissolution agents. A barrier layer is deposited onto said tool substrate prior to step (a) to prevent the egress of chemicals capable of graphitizing diamond or diamond-like particles. The total thickness of the coating structure is about 50-125 microns.

Structure of the Melting Layer in Mesoscale Convective System Stratiform Precipitation

Abstract: This study examines the aircraft observations and theoretical evolution of particles above, through, and below the melting layer in the stratiform region associated with a mesoscale convective system (MCS). The aircraft data were obtained from an advecting spiral descent where the descent rate approximately corresponded to the typical hydrometeor fall speeds. The microphysical and thermodynamic measurements not only allowed us to characterize the particle evolution, but also enabled us to compare them with the theoretical evolution of the particles in the melting layer and to quantify the associated heating and cooling rates. Even though complete melting requires a fairly deep layer, most of the mass melts, and thus most of the cooling occurs in a thin layer above the location of the radar bright band. Based upon the magnitude of vertical velocity fluctuations, the layers below the melting layer appear to be decoupled from those above. The ice water content above the melting layer is 2–3 times th...

The instability of a dispersion of sedimenting spheroids

Abstract: It is shown that hydrodynamic interactions between non-Brownian, non-spherical, sedimenting particles give rise to an increase in the number of neighbouring particles in the vicinity of any given particle. This result suggests that the suspension is unstable to particle density fluctuations even in the absence of inertia; a linear stability analysis confirms this inference. It is argued that the instability will lead to convection on a lengthscale (nl)−½, where l is a characteristic particle length and n is the particle number density. Sedimenting suspensions of spherical particles are shown to be stable in the absence of inertial effects.

Particle Orientation During Tape Casting in the Fabrication of Grain‐Oriented Bismuth Titanate

Abstract: Slurries containing platelike Bi4Ti3O12 particles have been tape cast to prepare green sheets with aligned particles. The slurries contain well-dispersed particles and show nearly Newtonian flow behavior. The effect of slurry composition and casting conditions on the particle orientation has been examined. The particle orientation in the green sheet is determined mainly by powder content; other parameters, such as binder content, casting speed, and blade opening, have little effect. The interaction between particles is a main cause for particle alingnment. The slurry with a large powder content is favorable for preparing dense grain-oriented ceramics.