SIZE DISTRIBUTION OF CLUSTERS INHERENT IN RANDOM DISPERSIONS OF EQUAL SPHERES WITH A COAGULATION RADIUS

Abstract

The spatial structure of a computer-simulated random dispersion of equal spheres is investigated. The sphere of influence, i.e. the coagulation radius, is assumed for each particle without any particle movement. If the distance between sphere centers lies within the coagulation radius, the spheres are regarded as being connected to each other to form a cluster. Various sizes of clusters exist inherently in the random dispersion. The effects of the coagulation radius and the bulk-mean particle volume fraction on the size distribution of clusters are discussed theoretically and experimentally.     

INTERACTION OF GRAVITY AND ELECTROSTATIC EFFECTS IN PIPE FLOW OF A GAS-PARTICLE SUSPENSION

ABSTRACT

The interaction of gravity and electrostatic effects in isothermal, fully developed, horizontal turbulent pipe flow of dilute suspensions has been examined. Experimental study has validated the fact that, in the presence of gravity, increased space charge associated with increased local concentration due to gravity accentuates the asymmetry in mass flux and particles density distributions in the vertical plane passing through the pipe centerline. The space charge effect on the particle mass flux distribution near the pipe bottom is enhanced by increased particle density. Measurements were made with air suspensions of monodispensed particles of alumina and latex with non-uniform particle charge in pipes of 51 mm and 127 mm diameters.     

MEASUREMENT OF THE SPECIFIC SURFACE AREA OF POWDERS BY THIN LAYER WICKING

ABSTRACT

The specific surface areas (A) of different clay mineral powders were measured by both the BET method and by thin layer wicking. The values of A for the BET and the wicking experiments coincided within a few percent. Thus, the simple and inexpensive thin layer wicking approach may well suffice to obtain reliable specific surface area values for most powders. From the wicking data it is also possible to obtain a rough estimate of the average particle size.     

A STUDY OF PARTICLE MOTION IN TURBULENT SHEAR FLOW OF A SUSPENSION

ABSTRACT

A report is made on the measurement of turbulent shear flows of a two-phase suspension of particles in a carrier fluid by the recently developed laser-Doppler anemometry particle sizing techniques, one for small particles (smaller than 240 μm) and one for large particles (larger than 240 μm). A good deal of insights of the dynamics at the individual particle level has thus been gained which defy the explanations offered by the conventional macroscopic theories.

These new experimental findings have stimulated the development of a series of new theoretical approaches which are based on the individual particle's dynamical frequency response characteristics in a turbulent flow. These new theories provide explanations to the measured peculiar features of flow behavior as well as a better understanding of the physics of such flows.     

A COMPARISON OF TWO NaCl TEST SYSTEMS USED FOR MEASURING FILTER EFFICIENCY

ABSTRACT

The filter efficiency of materials to be used in respiratory protective devices is commonly measured by using a solid NaCl particle challenge and a flame ionization detector. The Moore's BS 4400 Bench Rig is an example of such equipment.

Recently, TSI, Inc. introduced their Model 8110. This system also produces a NaCl particle challenge. It uses a laser diode detector.

The filter efficiencies of two different filter materials were determined in a comparative study utilizing several Moore's and TSI systems. The results indicate that the two test systems are relatable and that the between-machine consistency of the Model 8110 is better than the Moore's system.     

MEASUREMENT OF PARTICLE SIZE DISTRIBUTION IN COAL OIL WATER FUELS BY FOUR METHODS

ABSTRACT

Four methods were used to measure the solids particle size distribution in coal-ol1-water fuel. Both dry and wet screening were utilized for the coarser particles while four different instrumental methods were used to measure the finer particles in diluted liquid suspension. The wet stages of the analyses included both solvent-diluted organic suspensions and measurement after inversion to an aqueous system.

Consistent differences in the absolute values of particle size were observed between the four procedures. Possible reasons for these differences are discussed.     

THE ADHESION AND REMOVAL OF PARTICLES FROM SURFACES

ABSTRACT

The adhesion and removal of particles to and from substrates is a topic of great interest for both scientific and technological reasons. When a particle contacts a substrate, the adhesion-induced stresses cause deformations of the materials, which, in turn, affect adhesion. The size and nature of these deformations depend on both the interaction potential and mechanical properties, such as their elastic moduli and yield strengths of the contacting materials. Because of the number of interacting factors, particle adhesion and removal is a complex topic. Much of the present understanding of particle adhesion is based on the theoretical work of Johnson, Kendall, and Roberts, Derjaguin, Muller, and Toporov, and Maugis and Pollock, augmented by systematic experimentation. This paper will review the current understanding of particle adhesion and will illustrate effects of the particle-substrate interface using scanning electron micrographs.     

REMOVAL OF PARTICULATE CONTAMINANTS USING ULTRASONICS AND MEGASONICS: A REVIEW

ABSTRACT

Sonic cleaning is a commonly used technique for removal of particles from surfaces. The theory of particle removal using ultrasound is reviewed. Effects of cavitation and acoustic streaming are discussed. Experimental results using ultrasonics and megasonics (ultrasonic cleaning near 1 MH_z) from the literature are reviewed. Emphasis is placed on removal of fine particles from silicon surfaces for semiconductor manufacturing applications.     

EXPERIENCE WITH AND THEORETICAL MODELLING OF PARTICLE DISTRIBUTIONS FROM A CLASS 10,000 CLEANROOM

ABSTRACT

Particle data, engineering layouts, and theoretical modelling are presented for two class 10,000 cleanrooms with class 1,000 mini-environments used for advanced environmental research and development. Particle data generated from strategically selected sample points, is presented for background and controlled upsets in three dimensions in the clean room as well as in the intermediate changing room. The primary source of contamination in the clean room appears to be generated by the personnel. Although an increase in particle numbers is observed with increased traffic in the clean room, control over the environment is maintained by restricting high particle generating activities to within a separate changing room environment. A theoretical model based on application of a Gaudin-Schuhmann Law to the sparse matrices generated by standard industrial sampling methods is presented to show the particle distributions within the areas tested, as well as prediction of particle numbers outside the range sampled. Future work may include the refinement of the model to increase precision and prediction as well as expanding the model to predict particle distributions in three dimensions.     

MEASUREMENT OF PARTICLE VELOCITY IN PNEUMATIC TRANSPORT OF COAL USING CROSS-CORRELATION TECHNIQUE

ABSTRACT

Particle velocity has been determined experimentally in a solid-gas flow using the cross-correlation technique. Signals from two flow-monitoring devices, one based on the measurement of the dielectric constant of coal/air and the other based on the rate of static charge transfer technique, have been utilized to determine the cross-correlation function and hence the time delays between the signals. Other pertinent fluid dynamic parameters have been evaluated using experimentally determined particle velocities.     

EARLY-STAGE EVOLUTION OF PARTICLE SIZE DISTRIBUTION DUE TO GRAVITATIONAL COAGULATION

ABSTRACT

Evolution of the particle size distribution of an aerosol undergoing gravitational coagulation in early time stages was numerically investigated. The moment method was also applied in initiating a simple approach to the changes of particle size distribution parameters. Finally, an analytical solution was developed approximating the collision kernel into a form suitable for the analysis. The results underwent analysis by comparison. The derived analytical solution was found to be in good agreement with the numerical moment method. However, the results of the moment method do not exactly coincide with the results of the direct numerical integration because the particle size distribution does not attain the log-normal size distribution form.     

A NUMERICAL SIMULATION OF SWIRLING FLOW PNEUMATIC CONVEYING IN A HORIZONTAL PIPELINE

ABSTRACT

A numerical simulation for swirling and axial flow pneumatic conveying in a horizontal pipe was carried out with a Eulerian approach for the gas phase and a stochastic Lagrangian approach for particle phase, where particle-particle and particle-wall collisions were taken into consideration. The k-? turbulence model is used to characterize the time and length scales of the gas-phase turbulence. Models are proposed for predicting the particle source and additional pressure loss. The numerical results are presented for polyethylene pellets of 3.1 mm diameter conveyed through a pipeline of 13 m in length with an inner diameter of 80 mm, solid mass flow rate was 0.084 kg/s, and gas velocity was varied from 10 m/s to 18 m/s. The particle flow patterns, the particle concentration and the particle velocity, and additional pressure loss were obtained. It is found that the particle velocity and concentration has almost same value along flow direction in swirling flow pneumatic conveying. The profile of particle concentration for swirling flow pneumatic conveying exhibits symmetric distribution towards the centerline and the higher particle concentration appears in neighbor of wall in the acceleration region. At downstream, the uniform profile of particle concentration is observed. The particle velocity profile, on the other hand, is uniform for both swirling and axial flow pneumatic conveying. A comparison of the calculations with the measured data shows a good agreement within an average error of less than 15 percent.     

LASER LIGHT SCATTERING AS A METHOD OF DETECTING AND SIZING PARTICULATES ON SMOOTH PLANAR SURFACES

ABSTRACT

Currently available techniques for particulate detection on smooth planar surfaces using laser light scattering are discussed. A comparison of the response of these instruments using spherical and aspherical latex spheres on silicon wafers is presented. Actual measurement data for wafers with known particle size distributions will be used to illustrate the instrument response.

A first order model of the effects of substrate reflectivity on the measurement is developed. This model is verified with experimental data on silicon surfaces and oxide films.     

PREDICTED AND OBSERVED BEHAVIOR OF PLATELET AEROSOLS

ABSTRACT

A theory describing the aerodynamic behavior of triaxial particles was recently presented (D.L. Johnson and T.B. Martonen, Partic. Sci. & Technol. 12(2)). The formulation of particle motion was developed by regressing numerically predicted aerodynamic diameters on particle axial dimensions used in the numerical simulations, and provided an excellent fit (R² = 0.94) to geometric parameters. Aerodynamic diameters of cylindrical fibers calculated from the equation were within 5% of those calculated from Stöber's well known empirical equation, which was obtained for asbestos fibers settling in the sheared flow environment of the spiral duct aerosol centrifuge. In the present work, the centrifugal spectrometer was used to characterize platelets, i.e. microcrystal particles of non-circular cross-section. The experimental data are used to evaluate the theory for such an extreme triaxial particle shape. Predicted behavior is shown to be in good agreement with observed behavior in this and other works. The theory, therefore, may have important applications to air cleaning equipment, aerosol classification instrumentation, inhalation toxicology and aerosol therapy.     

EXPERIMENTAL STUDY OF DUST FILTRATION IN SURFACE-TYPE NONWOVENS

ABSTRACT

Surface type nonwovens are widely used in industrial dust control. Recently, they have been utilized in some engine air filtration applications as automotive filters, heavy-duty engine self-cleaning filters or safety filters. Because of their mechanical strength and regenerative ability they are a perfect material for applications where filter replacement is a problem. On the other hand, the random distribution of fibers and needle punching may result in pinhole formation during dust loading, especially at high aerosol velocities. As a result, the seepage mechanism is common in applications involving fine solid aerosols.

In the inertia dominated region, the collection efficiency of particles depends on the adhesion probability. When particle momentum increases, the efficiency decreases. In general, there is no agreement between filtration theory and experiment when the Stokes number is greater than one.

Filter efficiency increases with dust loading when the filter medium is a good dust cake supporter. In this case, dust reentrainment, causing seepage, may occur at high aerosol velocities and pressure drops. In contrast, reentrainment in nonwovens can take place even at lower aerosol velocities and dust loadings. It is difficult to predict conditions favorable for dust reentrainment and pinhole formation. This process depends on media geometry, dust particle size distribution, and aerosol flow parameters.

This paper discusses filter performance of surface-type nonwovens exposed to polydisperse dusts. Filter efficiency and pressure drop are discussed as functions of aerosol velocity, dust loading, and dust particle size distribution.     

PREDICTION OF PRESSURE DROP IN PNEUMATIC TRANSPORT SYSTEMS AT VARIOUS PIPE ORIENTATIONS USING AN EMPIRICAL CORRELATION FOR THE PARTICLE VELOCITY

ABSTRACT

A new empirical correlation for the particle velocity which incorporates the angle of inclination is proposed here. This correlation coupled with the expression for the solids friction factor obtained from the force balance on the particle was used to predict the pressure drops in the 0.0266 m and 0.0504 m systems held at various angles of inclination. Particles used in these systems included glass particles of 67, 450, and 900 µm weight mean diameter. The existence of minimum points in the predicted pressure drop curves as a function of gas velocity was corroborated by these two expressions.     

FIELD DEPENDENCE OF PARTICLE ELECTROPHORETIC MOBILITY IN NONPOLAR LIQUIDS

ABSTRACT

Contrary to earlier thought, recent studies have shown that particle electrophoretic mobility in nonpolar liquids is dependent on field strength at high fields. This study provides independent confirmation using a cross-flow electrofiltration technique and explores possible explanations for the phenomena. Experimental work was conducted using kaolin/Mi1-H-5606A petroleum oil systems with various concentrations of ionic additive. Electrophoretic mobility at different field strengths and additive concentrations were determined. It was found that mobility is independent of field strength at low and high fields but varies with field strength at intermediate fields. This dependence is explained on the basis of the relaxation effect and partial detachment of the diffuse double layer from the particle. In the field dependent region the mobility change with respect to field Increases with increasing surface charge and electrical double layer thickness. At high fields the double layer completely detaches and mobility behaves independently of field strength. Prior exposure of the particle to high fields appeared to temporarily increase mobility. This may be the result of selective deposition of low mobility particles and Increased particle charge due to field charge exchange or contact charging. Under the conditions of this study double layer detachment appeared to be the major source of field dependent behavior.     

EULERIAN AND LAGRANGIAN SIMULATIONS OF PARTICLE DEPOSITION FROM A POINT SOURCE IN A TURBULENT CHANNEL FLOW

ABSTRACT

Results comparing Eulerian and Lagrangian simulations of particle deposition from a point source in a channel are presented. The mean turbulent flow field is simulated using a two-equation k-? turbulence model. In the first, approach, diffusion of aerosol particles is studied by solving the corresponding advection-diffusion equation. Deposition of particles in the intermediate size range are analyzed by considering both the turbulent eddy diffusion and the eddy impaction processes, as well as the Brownian diffusion effects. In the second approach, the turbulence fluctuating velocity field are numerically simulated as a Gaussian random process. The Lagrangian trajectories of aerosol particles in the channel are then evaluated by solving the corresponding particle equation of motion. Effects of Brownian diffusion on particle motions are also included. A series of digital simulations for particles of various sizes which are released at different locations across the channel are carried out. Depositions of different size particles on the wall under a variety of conditions are analyzed. The relative significance of turbulence and Brownian effects are also discussed.     

EXPERIMENTAL AND NUMERICAL STUDY OF THE PARTICLE FLUCTUATING MOTION INDUCED BY COLLISIONS BETWEEN PARTICLES IN A VERTICAL GAS-SOLID FLOW

ABSTRACT

The effect of interparticle collisions on the gravitational motion of large particles in a vertical convergent channel is experimentally and numerically investigated. A probabilistic collision model is implemented in a three-dimensional Monte Carlo type Lagrangian simulation code. The numerical predictions are compared to the experimental results. It is shown that an interparticle collision model is necessary to reproduce the experimentally observed particle fluctuating motion characteristics. The simulation results using the present probabilistic collision model are found to yield satisfactory agreement with experimental observations, even though the collision frequency seems to be slightly overestimated. In particular, reduction of initial anisotropy of the particle fluctuating motion with increasing particulate mass flow rate is well reproduced by the simulation. A rather good agreement is also observed between experimental results and quantitative predictions of statistical properties of the flow such as particle axial and transverse velocity distributions and standard deviations.     

MODELING OF SOLID FLOWS IN A FLUIDIZED BED WITH SECONDARY TANGENTIAL AIR INJECTION IN THE FREEBOARD

Abstract

The Vortexing Fluidized-Bed Combustion (VFBC) technique was recently developed for small- and medium-scale coal-burning boiler applications. Experimental observations showed that the general solid flows in the freeboard of a vortexing fluidized bed consisted of three successive stages: (1) spirally ascending motion before colliding the freeboard wall, (2) bouncing on the wall, and (3) sliding on the wall and exiting the freeboard. This study attempts to model these three stages of solid flows. The dimensionless governing equations for particle motion in the swirling field were presented taking into account the interactions of particle inertia, centrifugal force, viscous fluid drag, and gravity. Numerical solutions of particle velocities and trajectories were pursued, and effects of particle momentum transfer number, Froude number, and particle-wall restitution coefficient were delineated. The experimental validation of solid flows in the swirling freeboard was furnished with an 18 cm ID laboratory fluidized bed.