Diffusional Particle Loss Upstream of Isokinetic Sampling Inlets

Isokinetic sampling, in which a subsample is extracted from the center of laminar aerosol flow, is routinely used to collect representative particles for analysis. Isokinetic sampling minimizes wall effects, including particle loss due to Brownian diffusion to the tube wall. This particle diffusion is analogous to the heat transfer problem originally posed by Graetz in 1883. Analytical solutions to the Graetz problem have been applied to calculate particle loss averaged over the entire main flow. However, these solutions overestimate diffusional particle loss near the center of the main flow. In the present solution, confluent hypergeometric functions are used to solve analytically for particle concentration as a function of radius. The solution is integrated near the center of the main flow to determine particle loss for isokinetically sampled aerosols. Sampling efficiencies valid down to nanometer-sized particles are presented in terms of dimensionless parameters. Diffusional particle loss for isokinetically sampled aerosol can be 1.8 times less than that from the main flow aerosol. The present results can be used to design isokinetic sampling systems and to assess particle loss in these systems. For 5 nm diameter particles sampled isokinetically from a laminar flow tube (0.318 cm tube radius, 10 m length) into an ultrafine condensation particle counter, sampling efficiency is strongly affected by main flow Reynolds number, Re; sampling efficiency increases from 4.9%at Re=100 to 99%at Re=1500.     

Influence of gas velocity on the particle collection and reentrainment in an air-cleaning electrostatic precipitator

Particle deposition and reentrainment experiments were performed in a two-stage electrostatic precipitator (ESP), consisting of positive corona precharger and collecting electrode sections. Attention was focused on studying the indoor air pollution deposition and reentrainment into six size ranges from 0.3 to >10?μm. Tests were performed in an office room (200?m³) for airflow velocities from 1.4 to 8?m/s. The effect of airflow velocity on the collection efficiency of the ESP was investigated both experimentally and analytically to study reentrainment phenomena in a turbulent flow. A stationary two-dimensional analytical model was carried out by modeling the particle transport. The boundary conditions for charged particles on collecting and repelling electrodes were determined by physical considerations, including chaotic and drift motions, the reflection of charged particles from a surface, and the reentrainment of charged particles. A decrease in the experimental collection efficiency for large particle diameters (≥0.5?μm), as compared to the theoretical prediction, was interpreted as the reentrainment of particles. The size-resolved dust reentrainment fluxes from the collecting electrode were evaluated in two limiting cases, considering that either the reentrained particles are not charged or that they are charged as the particles in the deposition flux. Dimensional analysis is applied to these results, introducing the wall friction velocity as a universal parameter that determines the flow character. In general, the particles with diameters <5?μm and >5?μm exhibit different reentrainment behavior.

Copyright © 2018 American Association for Aerosol Research     

Filtration Characteristics of a Miniature Electrostatic Precipitator

This study investigates the filtration characteristics of a miniature dual saw-like electrodes electrostatic precipitator (ESP). Parameters such as particle size, rate of airflow through the ESP, voltage of charge electrode, and discharge polarity were considered to study their influence on aerosol penetration through the ESP. Polydisperse and monodisperse particles with sizes ranging from 30 nm to 10 w m were used as the challenge aerosols. Experimental results indicated that the aerosol penetration through the ESP decreased (from 96% to 15% for 0.3 w m) as the voltage of the discharge electrode increased (from + 4 kV to +8 kV) at a flow rate of 30 L/min. At a fixed electrode voltage (+8 kV), aerosol penetration increased from 15% to 69% for 0.3 w m particles as the flow rate increased from 30 to 120 L/min. The most penetrating particle size was in the range of 0.25 w m to 0.5 w m depending on the discharge voltage and the flow rate. In general, the most penetrating particle size of the ESP decreased with decreasing discharge voltage or with increasing flow rate. At the same voltage level but opposite polarity, the aerosol penetration through the ESP with negative corona was lower than that with positive corona. The difference in aerosol penetration was a factor of about 2 between the negative and positive coronas for 0.3 w m particles, and this difference was found to be independent of discharge voltage. Regarding energy conservation, use of a negative-polarity ESP was more economical if the same efficiency was required. However, the ozone generated by the ESP with negative polarity was about five times greater than that generated with positive polarity. Therefore when using an ESP as an indoor air cleaner, the search for an optimum balance between ozone production and aerosol collection efficiency should be considered.     

Development of an Electrostatic Partitioner for Highly Efficient Partitioning of Gas and Particles with Minimal Effect on the Gas Phase

Particle-free air is required in a wide range of scientific and technical applications such as prefiltering for gas analyzers or for the artifact correction of particle mass concentration measurements. Most common processes for the removal of particles from a gas stream are filtration and electrostatic precipitation. However, both mechanism introduce changes to the thermodynamic conditions and/or the composition of the gas, which might be detrimental to the downstream measurement of gas or particle concentrations. For highly efficient separation of gas and particles with no changes to the thermodynamic conditions and substantially no changes to the gas phase, a coaxial gas particle partitioner (GPP, patent pending) has been developed and initially tested. The GPP utilizes corona charging to electrically charge the particles and a strong electric field in a separate unit to take them out of the sample flow when switched on. The vicinity of the corona wire gets continuously flushed by means of a wash flow, which is spatially separated from the sample flow, to prevent gases formed by the corona, such as ozone and oxides of nitrogen, from reaching the sample aerosol flow. Internally, the total aerosol flow is split into a sample flow and an excess gas flow. The splitting of the flow ensures that evaporation or release of particles deposited on the outer wall does not affect the sample flow but only the excess gas flow. The flow splitter is designed such that the particle number size distribution in the sample flow is identical to the ambient distribution when the GPP is switched off. When switched on, the sample flow is particle free. The initial investigations have shown that the GPP is able to separate gas and particles with a higher efficiency than another investigated common point-to-plane electrostatic precipitator (ESP). Particle losses inside the GPP are lower than in the ESP. The response time of GPP and ESP are similar.     

Wind Tunnel Modeling of Small Particle Deposition

An inexpensive microsphere dispersion, deposition, and sampling system is described. This system was used to examine the transport and deposition of small particles (? 1 μm diameter) across the aerodynamic boundary layer which developed over a prototype deposition surface (smooth, flat, acrylic plate). Unit density, polystyrene latex microspheres (0.8 and 1.1 μm diameter) were deposited onto both oil-coated and dry, upper and lower surfaces. Observed deposition velocities were significantly larger than those reported in the literature, possibly because low relative humidities increased electrostatic charge on the experimental plate. The experimental results were greater than those predicted by a turbulent flow particle deposition model.     

Evaluation of a Perpendicular Inlet for Airborne Sampling of Interstitial Submicron Black-Carbon Aerosol

The majority of airborne aerosol measurements employ forward-facing inlets with near-isokinetic sampling; these inlets have known artifacts when sampling in clouds such that data taken in cloud must typically be discarded. Here we report first results from a perpendicular inlet for sampling interstitial submicron black-carbon (BC) containing aerosol. The inlet, consisting of a flat plate to stabilize flow prior to perpendicular sampling, was evaluated using a single particle soot photometer (SP2) aboard the NASA WB-57F aircraft during the Midlatitude Airborne Cirrus Properties Experiment (MACPEX) of 2011. The new inlet rejects large particles and is free of aerosol artifacts when sampling in ice clouds while allowing sampling of submicron BC-containing aerosol with the same unit efficiency as a validated isokinetic inlet, thus allowing for airborne sampling of interstitial BC aerosol.

Copyright 2013 American Association for Aerosol Research     

Depositional arrangement of non-spherical atmospheric particles on impaction plate of a multi-nozzle impactor

Abstract

Aerosols and dust particles as a main component of atmospheric composition are of different shapes and sizes and affect the human health. Over the recent decades, the sampling, analysis and characterization of aerosol and dust particles have been a significant challenge. Finding a relationship between the location of particle deposition on impaction plate and its size and shape is very important for mineralogical and geochemical analysis. Hence, in this investigation, a common multi-nozzle impactor was taken and the arrangement of collected particles with different shapes and diameters on impaction plate was analyzed. Because of the highly priced geochemical and mineralogical analysis of atmospheric particles collected by the impactor, the results of this study can be used as a preliminary classifier for analyzing the accumulated atmospheric particles. In this study, a multi-nozzle impactor was three-dimensionally simulated. The simulation was carried out by applying Eulerian-Lagrangian approach. The experimental tests were also accomplished for sampling of the atmospheric particles. As the results of this study, the collection efficiency curve for the atmospheric particles with different shape factors was numerically obtained. As the most important result of this study, the location of particles deposited with diameters 2.5?µm and 5?µm and with shape factor of 1, 0.5 and 0.3 on impaction plate was numerically calculated. Due to these results, on one hand the central/outer parts of primary deposits mostly contain relatively coarse/fine-sized particles with high sphericity. On the other hand, the linear/low-cumulative deposits between adjacent jets mostly contain relatively fine/coarse-sized particles with low sphericity and angular shapes. Three-dimensional simulation results matched well with experimental sampling data.

Copyright © 2019 American Association for Aerosol Research     

Particle deposition velocity onto a face-up flat surface in a laminar parallel flow considering Brownian diffusion and gravitational settling

The Gaussian Diffusion Sphere Model (GDSM) was developed and improved to reflect the effects of gravitational settling as well as Brownian diffusion of aerosol particles on deposition velocity onto a face-up flat surface in a laminar parallel flow. The model improvement also includes the applicability of the GDSM to a flat surface of any shape with finite dimensions. When deposition velocity for a face-up circular flat plate of 45 cm diameter, representing e.g. a semiconductor wafer in a laminar parallel flow, was calculated by the GDSM and compared with that by the theory of Liu and Ahn (1987). Particle deposition on semiconductor wafers. Aerosol Science and Technology, 6, 215–224, the agreement was good for the tested particle sizes ranging 0.003–1 μm and free stream velocities ranging 5–500 cm/s. Based on this result, deposition velocities onto the face-up square flat plates with different orientations in a laminar parallel flow, simulating e.g. photomasks, were predicted.     

Aerosol Deposition in Sampling Probes

Deposition of aerosol particles on the inner walls of sampling probes is of concern in many aerosol sampling applications. Only inertial and gravitational effects have been considered in previous studies of the aerosol deposition; however, the lift force on particles is also of significance. In this investigation, experiments have been conducted to construct a database for aerosol deposition in Willeke-type sampling probes. An empirical correlation has been made between wall losses and the depositional forces of drag, gravity, inertia, and particle lift through the use of dimensionless parameters. Inclusion of the lift effect in this correlation not only helps to better predict particle behavior in the sampling inlet, but it also provides a basis for understanding of the intrinsic deposition phenomenon. The correlation has a geometric standard deviation of 1.13 and a 0.93 correlation coefficient relative to the experimental data.     

A method to deposit a known number of polystyrene latex particles on a flat surface

Abstract

This study introduces a method to deposit polystyrene latex (PSL) particles on a silicon wafer in a manner that allows their number to be predicted with a high degree of accuracy. A laminar flow growth tube is used to condense supersaturated water vapor on seed aerosol particles that are water-insoluble. After condensation is complete the droplets are accelerated through a nozzle to form an aerosol jet, and the number of droplets in this jet is counted optically. The droplets are then deposited on a flat surface by inertial impaction. The particle number on the surface is predicted by multiplying the droplet number by an experimentally evaluated conversion coefficient of 0.991?±?0.011 (k?=?2). Uncertainty analysis showed with a 95% confidence interval that the particle number on a flat surface is ±?2.0%. The primary application of this method is to make a particle number standard (PNS) wafer whose intended use is to evaluate the counting efficiencies of wafer surface scanners, and this study demonstrates the fabrication of such PNS wafers. A motorized XY-stage moves the surface horizontally to deposit PSL particles along desired paths over a half-inch wafer. The particle number was varied over seven levels ranging from 10 to 10,000. The particle diameter was varied at four levels: 0.814, 0.18, 0.102, and 0.046?µm. In all PNS wafers, the number of deposited particles was counted using optical microscopes. The observed particle numbers were all within the 95% confidence interval of the predicted value.

Copyright © 2019 American Association for Aerosol Research     

A Numerical Study of the Performance of an Aerosol Sampler with a Curved,Blunt, Multi-Orificed Inlet

The purpose of this study was to numerically simulate the performance of an aerosol sampler with a curved, blunt, multi-orificed inlet in order to understand the sampling characteristics of the first prototype of the button personal inhalable aerosol sampler ("button sampler"). Because the button sampler inlet design is too complicated to apply a three-dimensional model, an axisymmetric two-dimensional model was created to be similar in geometry and to simulate the major features of the airflow through the sampler when facing the wind. Particle trajectories were calculated in a variety of wind velocities and were categorized into 5 groups based on their interactions with the curved surface of the sampling plane. Empirical sampling efficiencies of the button sampler for 3 particle sizes were used to adjust the calculated sampling efficiencies in an attempt to improve the accuracy of the two-dimensional axisymmetric model in accounting for interactions between particles and the surface of the inlet of the button sampler. Sampling efficiencies for other particle sizes were then predicted. The results showed that sampling efficiency decreased with increasing particle size up to approximately 40 w m and then remained virtually unchanged at about 35% up to 100 w m. Although the efficiencies were lower than the American Conference of Governmental Industrial Hygienists' (ACGIH) inhalability curve for larger particles, the pattern of the predicted sampling efficiency was quite similar to the ACGIH inhalability curve. Sampling efficiencies for liquid aerosol particles larger than 15 w m were predicted to be noticeably lower than those for solid particles. The results also showed that the multi-orificed curved surface played an important role in establishing a pressure drop with desired flow alignment inside the sampler, thus greatly reducing the wind effect and significantly improving the uniformity of particle deposition on the filter. The less uniform deposition found at high wind velocity can be improved by increasing the sampling flow rate.     

THE EFFECT OF THERMOPHORESIS ON PARTICLE DEPOSITION FROM A MIXED CONVECTION FLOW ONTO A VERTICAL FLAT PLATE

A theoretical study of the effect of thermophoresis on aerosol particle deposition onto a vertical flat plate is proposed. The flow was modeled as a two-dimensional, incompressible and steady-state laminar flow driven by a combination of forced convection and natural convection. The particle deposition mechanisms were coupled using convection, Brownian diffusion and thermophoresis. Similarity analysis with the box method and block-elimination was used to determine the velocity and temperature fields. Particle deposition velocities were solved using the finite difference method or the approach of numerical integration. The effects of thermophoresis and convection were predicted to be particularly important for submicron particles moving toward a cold surface or blowing away from a hot surface at a given temperature gradient. The results also showed that the particle deposition rates were strongly influenced by thermophoresis and buoyancy force, particularly for opposing flow and hot wall surfaces.     

Miniature Pipe Bundle Heat Exchanger for Thermophoretic Deposition of Ultrafine Soot Aerosol Particles at High Flow Velocities

The deposition of submicrometer soot aerosol particles in a miniature pipe bundle heat exchanger system has been investigated under conditions characteristic for combustion exhaust from diesel engines and oil or biomass burning processes. The system has been characterized for a wide range of aerosol inlet temperatures (390–510 K) and flow velocities (1–4 m s^?1), and particle deposition efficiencies up to 45% have been achieved over an effective deposition length of 27 cm. Thermophoresis was the dominant deposition mechanism, and its decoupling from isothermal deposition was consistent with the assumption of independently acting processes. The measured deposition efficiencies can be described by simple linear parameterizations based on an approximation formula for thermophoretic plate precipitators. The results of this study support the development of modified heat exchanger systems with enhanced capability for filterless removal of combustion aerosol particles.     

Sampling Artifacts from Conductive Silicone Tubing

We report evidence that carbon impregnated conductive silicone tubing used in aerosol sampling systems can introduce two types of experimental artifacts: (1) silicon tubing dynamically absorbs carbon dioxide gas, requiring greater than 5 minutes to reach equilibrium and (2) silicone tubing emits organic contaminants containing siloxane that are adsorbed onto particles traveling through it and onto downstream quartz fiber filters. The consequence can be substantial for engine exhaust measurements as both artifacts directly impact calculations of particulate mass-based emission indices. The emission of contaminants from the silicone tubing can result in overestimation of organic particle mass concentrations based on real-time aerosol mass spectrometry and the off-line thermal analysis of quartz filters. The adsorption of siloxane contaminants can affect the surface properties of aerosol particles; we observed a marked reduction in the water-affinity of soot particles passed through conductive silicone tubing. These combined observations suggest that the silicone tubing artifacts may have wide consequence for the aerosol community and the tubing should, therefore, be used with caution. Contamination associated with the use of silicone tubing was observed at ambient temperature and, in some cases, was enhanced by mild heating (<70°C) or pre-exposure to a solvent (methanol). Further evaluation is warranted to quantify systematically how the contamination responds to variations in system temperature, physicochemical particle properties, exposure to solvent, sample contact time, tubing age, and sample flow rates.     

电除尘器飞灰粒径表征及细颗粒降温团聚

基于50000m³/h实烧烟气中试系统,采用Mastersizer 2000E激光粒度分析仪和电子低压冲击仪（ELPI）,首次对电除尘器飞灰几何粒径和空气动力学粒径进行全面表征。结果表明,电除尘器入口及各电场的飞灰几何粒度分布均呈双峰分布特征,各电场峰值依次右移,但末级旋转电极电场≤ 1μm的颗粒占比略有升高,电除尘器入口及第1～5电场飞灰几何中位径分别为6.607μm、17.378μm、2.884μm、2.577μm、2.460μm、2.480μm;温度降低,电除尘器入口飞灰几何粒度分布的双峰均右移,颗粒团聚现象明显,80℃、90℃、110℃、130℃、150℃时电除尘器入口飞灰几何中位径分别为13.183μm、10.500μm、10.171μm、6.607μm、7.586μm,从130℃降至90℃,电除尘器入口几何粒径≤ 1μm、≤ 2.5μm、≤ 10μm的飞灰占比分别减少了19.8%、19.2%、12.6%;不同温度时,电除尘器对空气动力学粒径0.03～10μm段颗粒的个数浓度、质量浓度均有较高脱除效率,均在75%以上,最高可达99.9%;温度降低,电除尘器进出口空气动力学粒径不同粒径段颗粒个数浓度和质量浓度均有不同程度降低,从130℃降至90℃、80℃,对应电除尘器入口PM_2.5团聚效率分别为46.76%、60.08%,对应电除尘器出口PM₁₀减排分别为59.80%、91.08%,PM_2.5减排分别为45.94%、76.22%,PM1减排分别为40.40%、62.12%。     

Evaluation of a High Volume Aerosol Concentrator

A virtual impactor sampler, which is designed to concentrate aerosols from a 1000 L/min ambient air sample into a 1 L/min exhaust airflow stream, was tested with near monodisperse aerosols in aerosol wind tunnels to characterize sampling performance. New methodology is introduced to correct results for the presence of doublet and satellite aerosol particles that can be present in the particle size distribution from a vibrating jet atomizer. Aerosol penetration from the free stream near the sampler inlet to the outlet of the device has a peak value of 78% at a particle size of 3.9 w m AD. Sampling effectiveness, which is the mean penetration over the size range of 2.5 to 10 w m AD, is 48%. There are 4 virtual impaction stages in the sampler, and examination of the regional losses shows that most of the aerosol deposition occurs on surfaces of the last 2 stages. The ideal power expenditure of the sampler (excluding electrical and frictional losses in the motor and bearing losses in the blower) is 58 watts as compared to the actual power consumption of 320 watts.     

A simple criterion for particle-wall adhesion in a wire-plate electrostatic precipitator

A simple criterion is presented to determine whether or not charged particles will adhere to the collecting plate in a wire-plate electrostatic precipitator (ESP). Based on the energy balance, it was determined that when a charged particle collides with the collecting plate, the charged particle will adhere to the collecting plate if its kinetic energy is less than the sum of its electrostatic energy and the surface energy between the charged particle and the collecting plate; otherwise, the charged particle will rebound off the collecting plate and continue to be transported and charged until it is captured or exits the collection zone of the ESP. With this criterion, a large number of particles with both high and low resistivity were tracked numerically in the ESP of Riehle (Proceedings of the Fourth International Conference on Electrostatic Precipitation, 1990, p. 136) and their grade efficiencies were computed. For particles with low resistivity, the present criterion is very useful for understanding the collisions between the charged particles and the collecting plate in the ESP.     

Particle Deposition in a Cast of Human Oral Airways

Oral and nasal airways are entryways to the respiratory tract. Most people breathe through the nasal airway during rest or light exercise, then switch to oral/nasal breathing during heavy exercise or work. Resistance through the oral airways is much lower than through the nasal airways, so fewer aerosol particles are deposited in the oral airways. Aerosol drugs are usually delivered by inhalation to the lung via the oral route for that reason. Oral deposition data from humans are limited, and those available show great intersubject variability. The purpose of this study was to investigate the effects of particle size and breathing rate on the deposition pattern in a human oral airway cast with a defined geometry. The airway replica included the oral cavity, pharynx, larynx, trachea, and 3 generations of bronchi. The oral portion of the cast was molded from a dental impression of the oral cavity in a human volunteer, while the other airway portions of the cast were made from a cadaver. Nine different sizes of polystyrene latex fluorescent particles in the size range of 0.93-30 mu m were used in the study. Regional deposition was measured at a constant inspiratory flow rate of 15, 30, and 60 L min^-1. Deposition in the oral airway appeared to increase with an increasing flow rate and particle diameter. Deposition at the highest flow rate of 60 L min^-1 was close to 90% for particles >20 mu m. Particles> about 10 mu m deposited mainly in the oral cavity. Deposition efficiency has been found to be a unique function of the Stokes number, suggesting that impaction is the dominant deposition mecha nism. Oral deposition can be approximated by a theoretical deposition model of inertial impaction in a 180 degrees curved tube, assuming perfect mixing in a turbulent flow. Our model suggests that the minimum dimension near the larynx and the average cross-sectional area are important parameters for oral airway deposition; however, additional data from the oral airway replica are needed to ascertain whether these are indeed the critical dimensions. Information from the present study will add to our knowledge of the deposition mechanism, the correlation of particle deposition with airway geometry, and eventually the best way to deliver aerosol drugs.     

Variations in the Size and Chemical Composition of Nitrate-Containing Particles in Riverside,CA

High time resolution measurements of nitrate-containing particles were made in Riverside, CA using an automated particle nitrate monitor and an aerosol time-of-flight mass spectrometer. The automated particle nitrate monitor provides quantitative data on the concentration of total particle-bound nitrate with a temporal resolution of 10 min. The aerosol time-of-flight mass spectrometer provides continuous data on aerodynamic size and single particle chemical composition. Data sets acquired with the two instruments are compared for a two-day intensive sampling period in August 1997 as part of the 1997 Southern California Ozone Study-North American Research Strategy for Tropospheric Ozone (SCOS97-NARSTO). Temporal variations in the number of nitrate-containing particles observed by the mass spectrometry system track (R² 0.73) the nitrate mass concentrations measured by the automated particle nitrate monitor. Both systems detected four periods of elevated nitrate concentrations of several hours duration. For these periods, the nitrate mass concentrations as measured by the automated particle nitrate monitor were similar, ranging from 11 to 19 mu g m3. However, the particle size and single particle composition of nitrate-containing particles as measured by the aerosol time-of-flight mass spectrometer were distinctly different. Specifically, the nitrate maxima observed in the midmorning hours were characterized by supermicrometer nitrate particles associated with either ammonium and organic species or sodium. The afternoon maxima were characterized by submicrometer ammonium nitrate particles, most of which contained organic material.     

Physical factors determining particle deposition in the human respiratory tract

Total deposition in the human respiratory tract of uncharged, spherical aerosol particles depends on particle diameter, particle density, period of a breathing cycle and respiratory volumetric flow rate. Total deposition of particles larger than 0.5 μm in diameter increases at mouth-breathing with increasing values of these physical factors with the exception that total deposition of particles in the size range between 0.5 and 1 μm is independent of flow rate. These physical factors can be linked by a deposition parameter so that their complex effects on deposition can be described by an unique relationship.