Showing papers in "Aerosol Science and Technology in 1992"

Dispersion and Deposition of Spherical Particles from Point Sources in a Turbulent Channel Flow

Abstract: The dispersion and deposition of particles from a point source in a turbulent channel flow are studied. An empirical mean velocity profile and the experimental data for turbulent intensities are used in the analysis. The instantaneous turbulence fluctuation is simulated as a continuous Gaussian random field, and an ensemble of particle trajectories is generated and statistically analyzed. A series of digital simulations for dispersion and deposition of aerosol particles of various sizes from point sources at different positions from the wall is performed. Effects of Brownian diffusion on particle dispersion are studied. The effects of variation in particle density and particle-surface interaction are also discussed.

Measurement of Particle Density by Inertial Classification of Differential Mobility Analyzer–Generated Monodisperse Aerosols

Abstract: A density measurement technique based on the selection of a monodisperse aerosol with a differential mobility analyzer followed by classification according to aerodynamic diameter with an impactor has been designed and tested. Experimental results were obtained for several laboratory aerosols (dioctyl phthalate, (NH4)2SO4, NaCl, and H2SO4 at a range of humidities) by using four different microorifice uniform deposit impactor stages with aerodynamic diameter cut-offs of 0.12–0.56 Jim. The average error in measured particle densities is 4% and a maximum error of 8% is observed for all of the materials tested except NaCl, for which the measured effective density is 14% smaller than the true density. The discrepancy for NaCl is attributed to nonspherical particle shape. The system will be applied in the future to measure the densities of submicrometer atmospheric particles. This is Particle Technology Laboratory Report No. 817.

A Mathematical Model of the Impact and Adhesion of Microsphers

Abstract: A model is presented for the low velocity planar impact of a micrometer-sized sphere (microsphere) having an arbitrary angle of approach to a surface in the presence of arbitrary contact and external forces. This model, based upon classical impact dynamics and Hertzian theories, analytically relates the velocity change of the microsphere to the physical parameters of the microsphere and the surface and to the microsphere-surface adhesion forces. The model is based upon two fundamental assumptions, namely, that the energy losses due to the process of material deformation and the process of adhesion are independent, and that the energy loss due to the adhesion process occurs only during the rebound phase of the impact. No assumptions are made about the nature of inelastic deformations in the formulation of the model, permitting it to apply equally well to viscoelastic, elastic-plastic, or other materials or combinations thereof. The utility and accuracy of the model is assessed by comparing its predictions ...

Effect of Coating Thickness on Particle Bounce in Inertial Impactors

Abstract: A coating technique for applying uniform, thin layers of grease onto impactor substrates is described. The technique involves dissolving or diluting the grease in a solvent, applying the grease solution to the substrate, the spinning the substrate at a high speed. The coating thickness is controlled by the concentration of grease in the solution and by the spin speed. Apiezon-L vacuum grease and silicone oil were tested with this technique. Coating thicknesses between 0.3 and 9 μm were successfully generated on the polycarbonate film. The coated substrates and uncoated polycarbonate were tested for particle collection efficiency with polystyrene latex in two stages (0.56- and 1.0-μm cut-size) of a microorifice uniform deposit impactor. In the range of coating thicknesses tested, the collection efficiency of Apiezon-L was strongly related to the coating thickness; large coating thicknesses greatly improved particle collection. For the silicone oil, collection efficiency was high for all the coating thickne...

Response of Particle Measuring Systems Airborne ASASP and PCASP to NaCl and Latex Particles

Abstract: The response of a Particle Measuring Systems, Inc. (PMS) airborne Passive Cavity Aerosol Spectrometer Probe (PCASP)-100X and a PMS airborne Active Scattering Aerosol Spectrometer Probe (ASASP)-100X to nearly monodisperse aerosols of NaCl and polystyrene latex spheres in the size range 0.074–1.07 μm diameter (± 5%) are examined. Particles < 0.34 μm are size classified by electrical mobility using a Thermo System Inc. Electrostatic Classifier. The particles are also sized with the aid of a scanning electron microscope. Three equivalent diameters for the near cubic NaCl particles are considered: average of length and breadth, and cross section equivalent and volume equivalent diameters. For the linear diameter, the probes' response to latex and NaCl particles is quite similar. However, the use of cross section and volume equivalent diameter leads NaCl to be sized significantly less than the latex spheres consistent with reported differences between spherical (i.e., latex) and cubical geometry (i.e., NaCl). U...

Effect of Geometry on Particle Adhesion

Abstract: Forces between solids in air are predominantly attractive and cause adhesion of particles to each other and to surfaces. These forces become increasingly difficult for fine particles to overcome because the particle mass varies to the third power of the particle size. Very high accelerations are therefore required for particle deagglomeration or removal from surfaces. The particle adhesion phenomenon is important in a variety of scientific and engineering applications. Agglomeration, resulting from particle adhesion, affects the particle-size distribution and physical properties of aerosols and other particulate systems. In powders, tensile and shear strengths are determined by interparticle forces and they influence properties such as the dispersibility of powders in fluids and flow-through bins and hoppers. Effective particle removal by filtration depends on the ability of particles to remain on the collector surface. The objective of this study was to investigate the role of particle geometry on adhesi...

Dry Deposition of Atmospheric Contaminants: The Relative Importance of Aerodynamic, Boundary Layer, and Surface Resistances

Abstract: Measurements of dry deposition were conducted to potted plants at Claremont, CA, during the Southern California Air Quality Study and to patches on an equestrian statue at Gettysburg National Military Park, PA Simultaneous dry deposition measurements were also conducted on aerodynamically designed surrogate surfaces (symmetric airfoils) with predictable boundary layer characteristics at both locations Species of interest include gaseous SO2 and particulate SO2− 4, NO3−, Ca, and Pb The importance of each transport step in the deposition process was evaluated by combining the data from several types of surfaces with different surface characteristics The significance of particle resuspension was determined by simultaneous sampling with different exposure times The results suggest that the relative importance of aerodynamic, boundary layer, and surface resistances can vary greatly, depending on the depositing chemical species and the surface characteristics Of particular interest is the highly variable

Controlled Wind Tunnel Experiments for Particle Bounceoff and Resuspension

Abstract: The dynamics of particle rebound and resuspension were examined by using uranine particles, polymer microspheres, spores, and pollen in wind tunnel experiments. Particle diameters were 5–42 μm. Results show that both the fraction of rebound and the resuspension rate are strongly dependent on the free stream velocity, particle size, and relative humidity. The effects of relative humidity are more significant at lower windspeeds; a greater relative humidity appears to change the shape of the distribution of adhesion force, mainly affecting the lower range but not greatly affecting the upper end of the distribution. Resuspension rates decrease with time, essentially defining two regimes. The first regime lasts for < 1 min; after this time, the most easily resuspended particles have been removed, leaving only particles with much smaller resuspension rates for the second regime. At a windspeed of 6 m / s, the upper 20% of the distribution of turbulent fluctuations is responsible for ∼ 65% of the particle resus...

On the Combination Coefficient of Positive Ions with Ultrafine Neutral Particles in the Transition and Free-Molecule Regimes

Abstract: The charged fraction of ultrafine silver particles (5–50 nm) suspended in high-purity helium (99.998% grade) was measured as a function of particle size under different charging conditions (i.e., ion concentration and charging time). The charger used is a modified version of the one described in a previous study (Romay et al., 1991). Two radioactive sources of 250 μCi Po-210 are located in a cavity at the charger inlet. The cavity configuration is designed to control the range of the α rays. The modified charger provides a uniform electric field in the axial direction to separate the positive ions from the negative ions and electrons produced by the ionizing radiation. Thus, the aerosol particles are first exposed to a small region of electrons and positive ions, and are subsequently charged by positive ions in a much longer section of the charger. The ion concentration is estimated from the ion current collected in the exit electrode and measured with a picoammeter. The charging time is estimated from th...

A System to Measure the Hygroscopicity of Aerosol Particles

Abstract: An aerosol conditioner (wetted wall reactor) was modified to mimic the conditions inside the human respiratory tract, i.e., relative humidity > 99% and 37°C. By combining the wetted wall reactory with tandem differential mobility analyzer, a system was developed to study the hygroscopic properties of aerosols in submicrometer range. This paper describes the system, and the results of system tests using three compounds of known composition, NaCl, (NH4)2SO4, and (NH4)HSO4, are presented.

Flow Field and Pressure Drop of Filters with Rectangular Fibers

Abstract: A numerical study of viscous flow in a fibrous filter modeled as a staggered array of rectangular or cylindrical fibers arranged perpendicular to the flow direction was conducted to determine the fiber drag. The calculated dimensionless drag values for the case of the cylindrical fibers were found to be in excellent agreement with the analytical results of Kuwabara (1959). The effect of the fluid inertia on the flow around a cylindrical or rectangular fiber was found to be negligible for Reynolds number values 1 was found to be a strong function of the filter solid-volume fraction, which correlated well with the Kuwabara results for the case of cylindrical fibers. This is Particle Technology Laboratory Publication No. 740.

Counting Efficiencies for Alpha Particles Emitted from Wire Screens

Abstract: Counting efficiencies for alpha particles emitted from the front and the back of 30-, 105-, 200-, and 400-mesh wire screens were measured for ultrafine radon daughter aerosols deposited at face velocities in the range 5.1 to 30.8 cm s−1. Mean activity median diameters for the ultrafine 218Po, 214Pb, and 214Bi particles were 0.70 ± 0.16, 1.1 ± 0.3, and 1.0 ± 0.2 nm (0.062, 0.033, and 0.038 cm2 s−1), respectively, as determined from graded wire screen array analysis of the test atmosphere. For wire screen collection efficiencies 0.999. Alpha-particle losses within the screens (screen loss factors) were determi...

A Study of Particle Resuspension in a Turbulent Flow Using a Preston Tube

Abstract: A technique has been developed for the study of resuspension of particles from a flat surface by turbulent flow. In this study a Preston tube was used to determine the shear and flow forces required to resuspend particles from a surface. This technique, which allows a wide range of test configurations to be compared, was used to study the resuspension of glass beads at various free stream velocities. The results indicate that particles are resuspended according to a lognormal distribution with respect to any free stream velocity, surface shear stress, and shear velocity.

Multicomponent Aerosol Dynamics of the Pb-O2 System in a Bench Scale Flame Incinerator

Abstract: A study was carried out to understand the formation and growth of lead particles in a flame incinerator. A bench scale flame incinerator was used to perform controlled experiments with lead acetate as a test compound. A dilution probe in conjunction with real-time aerosol instruments was used to measure the evolution of the particle size distribution at different locations in the flame region. A multicomponent lognormal aerosol model is developed accounting for the chemistry of the lead-oxygen system, and various aerosol dynamic phenomena such as nucleation, coagulation, and condensation. Reasonable agreement is obtained between the predictions of the model using appropriate kinetic parameters and the experimental results.

Efficiency of Fibrous Filters with Rectangular Fibers

Abstract: A theoretical study of filtration characteristics of rectangular electret fibers arranged in a staggered array perpendicular to the flow direction was conducted assuming the Brownian diffusion and interception as the dominant particle capture mechanisms. The single-electret fiber efficiency due to pure diffusion was found to be proportional to -2/3, 0.278, and 1/2 powers of the Peclet number (based on the fiber hydraulic diameter), filter solid-volume fraction, and fiber aspect ratio, respectively. For pure interception, the single fiber efficiency was determined to be proportional to 1.55 and 0.54 powers of the interception parameter and filter solid-volume fraction, respectively. This is Particle Technology Laboratory Publication No. 741.

Evaluation of an Environmental Reaction Chamber

Abstract: A cylindrical, Pyrex glass chamber with stable temperature control was evaluated to determine the residence time of aerosol particles within an enclosure, and to compare our results to the theoretical expression derived by Crump and Seinfeld (1981) Monodisperse polystyrene latex particles with diameters between 004 and 3 μm were used to determine the particle deposition rates under different water temperature gradients (δTw) between the top and bottom water layers of the chamber The water layers and an insulating fiberglass jacket around the side wall helped to maintain a steady temperature gradient in the chamber and this was found to be very important in controlling particle deposition Results indicated that the minimal deposition losses occurred at δTw = 0°C for particles between 02 and 03 μm, with a residence time as long as 38 h Results also showed that particle deposition rate can be described by the theory of Crump and Seinfeld with an exponent n = 202 and a turbulence intensity k e = 0003

Experimental Study of Aerosol Filtration in the Transition Flow Regime

Abstract: An experimental study was carried out to investigate the performance of fibrous filters in the transition flow regime, where the fiber radius is of the same order of magnitude as the mean free path of gas molecules. Filter media with the mean fiber diameter of 0.21–0.72 μm and the packing density of 0.053–0.08 were used in this study. The transition flow conditions were achieved by reducing the gas pressure in the filter test apparatus. Experiments were performed in the pressure range of 0.1–1.0 atm using monodisperse particles of 0.04–0.45-μm diameter range. The particles were generated by the electrostatic classification technique. The particulate penetration through the filter and the corresponding pressure drop were measured at face velocities ranging from 6 to 19 cm/s. Experimental results showed that the penetration decreased by three to four orders of magnitude as the pressure was reduced from 1.0 to 0.1 atm. It was found that the most penetrating particle size increased slightly as the gas pressur...

Extraction of aerosol-size distributions from multispectral light extinction data

Abstract: An algorithm for inverting remote sensing measurements to determine particle concentration and size distribution, proposed by Nguyen and Cox in 1989, is described and evaluated. This determination uses extinction measurements at multiple wavelengths with the solution of the Fredholm integral equation. The solution represents the second derivative of the particle-size distribution as a linear combination of orthogonal functions chosen so that the resulting solution is smooth and positive everywhere. We used this algorithm to reconstruct concentrations and size distributions for simulated and experimental measurements of light extinction caused by different test aerosols. An “information content parameter” was used to measure how well each size distribution could be reconstructed from its corresponding measurements. For a wavelength range of 340–940 nm, aerosols in the accumulation mode (300 nm < d 50 < 2500 nm) were reconstructed well. The algorithm was good at reconstructing moderately polydisperse aeroso...

On the Determination of Continuous Submicrometer Liquid Aerosol-Size Distributions with Low Pressure Impactors

Abstract: The aspects associated with the determination of continuous submicrometer aerosol-size distributions using multijet low pressure impactors have been studied. Multiple sets of error-free and noisy, simulated data sets have been inverted, and impactors have been compared with the differential mobility particle-size analysis (DMA) method by using well-defined, laboratory-generated liquid oleic acid aerosols tagged with ammonium fluorescein. Impactors included in this study were a Berner-type impactor HAUKE 25/0.015 (BLPI), a modified University of Washington Mark 5 impactor (KLPI), and the impactor designed at the University of Florida (LLPI). The inversion of simulated error-free impactor data (i.e., the data with perfect kernel functions) for unimodal submicrometer aerosols with a small (2.5%) stage mass error estimate yields results very close to input distributions, when the method based on constrained regularization is used in the inversion. When the error estimate is increased, inverted spectra are fla...

Particle Motion in Two-Dimensional Confined Turbulent Flows

Abstract: This article introduces (1) the basic idea of a computer code on the basis of a Lagrangian model and (2) a method of computing particle concentration by the model. This computer program permits us to calculate the spatial distributions of particle velocity and concentration as well as the capture efficiency of the ventilation system in the two-dimensional confined turbulent flows. As a validation of the computer code, the experiment of Ruck and Makiola (1988), where the turbulent air flow passes a single-sided backward-facing step, is simulated. Then, we simulate a bounded turbulent field with a strong recirculation region to demonstrate the capability of the present program. The simulation is conducted for particles with diameters of 1, 60, and 100 μm and the predicted particle velocity and concentration distributions are given. The concentration of the 1-μm particles is also compared with that computed by the prediction program EOL (Fontaine et al., 1991).

A Stochastic Model for Particle Deposition and Bounceoff

Abstract: A new model for particle deposition and bounceoff that combines current knowledge of turbulent bursts with the stochastic properties of turbulent fluctuations is presented. The model predictions for deposition velocities agree with experimental results in the literature for dimensionless particle relaxation time τp + > 2. For τp + > 10, most of the particles delivered to the edge of the viscous sublayer are able to deposit onto the surface due to their inertia; the deposition velocity approaches an asymptotic value because the process becomes limited by the rate of turbulent delivery to the viscous sublayer. Because of the penetration of turbulent fluctuations into the viscous sublayer, the minimum values of vertical velocities needed for particles to deposit onto the surface are smaller than those predicted by the free flight model. Most of the deposition occurs from those turbulent fluctuations at the upper tail of the distribution of the vertical component of air velocity. In addition to the deposition...

Particle-Size Distribution of Chromium: Total and Hexavalent Chromium in Inspirable, Thoracic, and Respirable Soil Particles from Contaminated Sites in New Jersey

Abstract: Chromate slag is found mixed with soil at several sites in New Jersey. Previous analyses of the contaminated soil were limited to particle sizes > 30 μm. This study focuses on the inspirable, thoracic, and respirable particles that would enter the human respiratory tract, should the contaminated soil become airborne. This article discusses the soil-sampling procedure used at a slag-contaminated site and a site with visible hexavalent chromium crystals. In addition, the drying and sieving procedures and the techniques needed to resuspend the contaminated dust in a sealed chamber are described for aerodynamic size fractionation (30, 10, and 2.5 μm). The resuspended dust was collected on filters and analyzed for 38 elements by x-ray fluorescence (XRF). Wet chemistry techniques were used to measure total extractable and hexavalent chromium. It was found that 1.6% of the slag had a particle aerodynamic diameter (at 50% cut-point) d 50 = 30 μm, of that 1.1% was d 50 = 10 μm and 0.26% was d 50 = 2.5 μm. Total ch...

Experimental Evidence of the Importance of Rebound in Net Deposition of Particles

Abstract: Experiments were carried out in a low-speed wind tunnel to examine the processes of ideal deposition, net deposition, rebound, and reentrainment for large particles (lycopodium spores and ragweed pollen) using leaf surfaces (American elm and tulip poplar). Ideal deposition velocity was found to be linearly proportional to wind speed above a threshold level; below this level it was approximately the settling velocity. Net deposition velocity was close to the ideal deposition velocity at low speeds, but at higher speeds became significantly less than ideal deposition velocity because of rebound and reentrainment. Rebound appears to be more important than reentrainment at speeds above 3 m / s, but at low speeds reentrainment may be more important than rebound, although weak. Department of Land, Air and Water Resources, University of California, Davis, CA 95616.

Particle Suspension in a Rotating Drum Chamber When the Influence of Gravity and Rotation are Both Significant

Abstract: In recent years, rotating chambers have been found to be an effective method of retaining particles suspended in the air for an extended period of time. Rotating drum chambers have the potential of providing a stable atmosphere of well-characterized inhalable particles for periods lasting from hours to days for use in inhalation toxicology studies. To aid in planning for the use of rotating drum chambers in inhalation studies, we created a model that describes (a) the concentration of particles in the chamber under various conditions and (b) the particle sizes for which gravity and rotation influence particle dynamics. Previous publications describe the suspension / deposition of particles when the rotational effect is dominant, but do not describe particle suspension / deposition when gravitational settling is significant as occurs when such drum chambers are operated at optimal conditions for retaining the highest fraction of particles over time. By using the limiting trajectory of particles, the fracti...

Numerical Simulation of Two-Phase Gas-Particle Jet in a Crossflow

Abstract: A particle trajectory model to simulate the mean trajectories, velocities, and densities of a particle-laden gas jet injected into a uniform cross free stream has been developed. Governing equations were derived by considering a balance between forces acting on an elementary control volume of the crossjet along the trajectories of the gas and particle phases. The effects of particle size, particle loading ratio, injection angle, and the carrier gas to free stream velocity ratio and the particle-to-carrier gas velocity ratio at the jet exit on the average characteristics were investigated in this study. The computational results of the mean particle trajectories are in good agreement with available experimental observation. The present method should be useful to predict the average properties of a two-phase gas-particle jet in a crossflow.

May an Increase of the Effective Migration Rate Observed in Electrostatic Precipitators with Wider Plate Spacings or Faster Gas Streams Really Be Termed ``Non-Deutschian''?

Abstract: In recent years, experimental investigations concerning electrostatic precipitators have repeatedly confirmed an increase in the effective migration rate upon widening the plate-to-plate distance or raising the average gas flow velocity. An increase in the effective migration rate with increasing passage width or gas velocity is interpreted as an improvement in precipitator performance. This seems to contradict the traditional Deutsch equation. As a result, one often refers to these as “non-Deutschian phenomena.” It can be demonstrated that a certain part of the above experimental observations may, indeed, be explained within the scope of the Deutsch concept and hence must not be termed “non-Deutschian.” This is achieved by considering the influence of the particle size distribution when calculating the effective migration rates. These calculations are based on logarithmic Gaussian distributions, without any restriction of their global applicability. The effective migration rates calculated in this manner...

Ultrafine Combustion Aerosol Generator

Abstract: *This paper is a contribution of the National Institute of Standards and Technology and is not subject to copyright.

Optimization of Aerosol Penetration through Transport Lines

Abstract: When aerosol particles are transported through tubing systems for near-real-time monitoring purposes, there will be losses of particles on internal surfaces of the transport system that will render the monitoring process less accurate. Herein we consider three aspects of modeling particle penetration: an analytical solution for the effective particle depositional velocity in inclined tubes; the optimum diameter of a straight tube for maximum aerosol penetration; and the concept of maximum penetration, Pmax, between two points in space. Pmax is the upper bound, and the ratio of the actual penetration, P, to Pmax gives the scope for improving the transport line design to increase penetration. * Aerosol Technology Laboratory Publication 6441/02/01/91/NKA.

A Mathematical Study of the Sampling Characteristics of a Thin-walled Sampler Operating in Calm Air

Abstract: A study is made of the sampling characteristics of a thin-walled cylindrical sampler operating at various orientations in calm air. A mathematical approach is adopted to model the problem which gives an accurate description of the flow field in the vicinity of the sampling inlet. The paths of the particles in the air are then traced and the sampling efficiency investigated. In tracing the paths of the particles both inertial and gravitational effects are considered. The surface formed by the limiting particle trajectories, separating the sampled and unsampled particles, in the undisturbed flow determines the aspiration efficiency of the sampler. If the sampler is vertical, then the situation is symmetrical and the cross section of the surface formed by the limiting particle trajectories, e.g., A 0 is known to be circular. For all other orientations of the sampler the symmetry is lost, and the shape A 0 is then unknown and in this paper is investigated for various situations. The size range of particles in...

Microphysical measurements of fog formed in a turbulent jet

Abstract: An investigation of fog formation in the flow field of a turbulent jet was conducted in order to determine the processes responsible for the microscopic and macroscopic behavior of the fog. Profiles of number density, mean droplet diameter, and size distribution were measured by using a Phase Doppler Particle Analyzer, for two different conditions of the jet effluent saturated at 62° C and 85° C. The data allow the behavior of the jet to be characterized by the competing mechanisms of heterogeneous nucleation on entrained nuclei, growth of droplets, and turbulent diffusion. At the higher jet temperature, the competing mechanisms result in bimodal size distributions. This behavior was shown by a simple model in which condensation occurs simultaneously with entrainment and mixing. Microscopic measurements were also taken for vapor-air jets in which a water aerosol was added to the jet effluent. A comparison of this seeded condition with the latter condition shows that seeding moves the condensation towards ...