Simulation of Particle Size-Selective Air Samplers Placed in a Polydisperse Aerosol

The results of a numerical simulation of several air sampling instruments are presented. They are assumed to sample the same aerosol, with a log-normal particle-size distribution. Four instruments were studied: the 10-mm nylon cyclone, the MRE 113A gravimetric sampler, the CPM 3, and the CIP 10. The experimental data of particle collection efficiency were reduced by a model for each instrument. The model used combines two cumulative log-normal distribution functions, in order to have a good degree of flexibility necessary for representing the data of some devices that exhibit a maximum in efficiency (CPM 3, CIP 10). The concentrations “measured” by several air samplers were compared with each other; the differences were analyzed as functions of the aerosol parameters: mass median aerodynamic diameter and σ_g. The results that were obtained and those calculated from standard collection efficiencies, defining the conventional alveolar fraction of the aerosol, were also taken into account. This simulation method can be extended to any type of instrument and aerosol, and enables the prediction of the maximal deviations that could be observed between different instruments, or between one instrument and some reference standards.     

Thermophoretic Motion of a Cylindrical Particle with Chemical Reactions

The thermophoresis of a circular cylindrical particle bearing a chemical reaction in a gas prescribed with a uniform temperature gradient in the direction perpendicular to its axis is analyzed. The Knudsen number is assumed to be moderately small so that the fluid motion is in the slip-flow regime with effects of temperature jump, thermal creep, frictional slip, and thermal stress slip at the particle-gas interface. The appropriate governing equations of heat conduction/generation and fluid motion are solved analytically and the thermophoretic velocity of the particle is obtained in closed forms. The thermophoretic velocity is a linear function of the thermal stress slip coefficient whose effect increases with an increase in the Knudsen number. When the composition-dependent factor of the chemical reaction within the particle does not depend on position, the thermophoretic velocity is diminished as the reaction is endothermic and augmented as the reaction is exothermic. When this factor is a function of position, the particle velocity can deflect from the direction of the imposed temperature gradient. For specified system characteristics, the effect of the chemical reaction on the thermophoretic velocity of a circular cylindrical particle is significantly greater than that of a spherical particle due to its smaller specific surface area.

Copyright 2014 American Association for Aerosol Research     

Thermophoretic Deposition Efficiency for a Nonuniform Inlet Particle Concentration

The calculation of the cumulative thermophoretic efficiency of particles in tube flow with nonuniform inlet particle concentration is considered. Using previous results, a simple but exact expression is found for the cumulative deposition efficiency. An illustrative calculation is performed for a specific initial particle distribution with one parameter. The total deposition efficiency is computed as a function of this parameter and the results are compared to the uniform inlet concentration case. It is observed that the deposition efficiency is higher when the initial particle concentration is greater near the tube wall.     

Technical Note: Performance of a Personal Electrostatic Precipitator Particle Sampler

Currently, photocatalytic oxidation (PCO) is considered to be an effective process for removing and destroying low-level pollutants, which makes it a strong candidate for indoor air quality applications. Our study evaluated the titanium dioxide (TiO 2 ) filter media in controlling bioaerosols in a laboratory test chamber. The influences of microorganism species, relative humidity, and face velocity on the germicidal effectiveness of a TiO 2 -coated filter with 365 nm 8 W and 36 W blacklight irradiation were investigated. A Collison nebulizer generated Escherichia coli ( E. coli ), Bacillus subtilis ( B. subtilis ) endospores, yeast cells of Candida famata ( C . famata ) var. flareri , and spores of Penicillium citrinum ( P. citrinum ). The PCO control effectiveness was determined as the ratio, N S / N 0 , where N S and N 0 were the culturable concentrations collected by an Andersen one-stage sampler downstream of TiO 2 -coated filters with and without blacklight irradiation, respectively. Our results demonstrated that there were no significant differences in microorganism penetrations for TiO 2 -coated filters with and without blacklight irradiation. It was recommended that TiO 2 filter media used in this study did not perform as a good germicidal capability for airborne microorganisms.     

Thermophoretic Motion of a Spherical Aerosol Particle in a Cylindrical Pore

The thermophoretic motion of a spherical aerosol particle in a cylindrical pore, with the ambient temperature gradient imposed parallel with the pore direction, is investigated. Both particle and pore surfaces can have frictional and thermal slip, and discontinuity in temperature fields across both surfaces is allowed. The relevant boundary value problem is solved with a truncated-domain boundary collocation method. It is found that the thermo-osmotic flow of the surrounding fluid caused by the thermal slippage of the pore wall plays a dominant role in determining the thermophoretic motion of the particle. This thermo-osmotic flow is directed toward the hotter region and thus leads to a toward-hot-region thermophoretic motion of the particle, which is opposite to the usual toward-cold-region particle thermophoretic motion. The effect of this thermo-osmotic flow toward particle thermophoretic motion is quite different for the particle in a closed cavity situation. For the particle in a closed cavity scenario, the thermo-osmotic flow is toward the hotter region along the cavity wall, but has to circulate back in the middle of the cavity and thus helps to push the particle toward colder region.     

Development of an Electrostatic Precipitator for Off-Line Particle Analysis

Due to the lake of in-situ aerosol particle analysis systems, aerosol samples are taken and analyzed off-line. For detailed analysis of particle properties such as shape, morphology, and composition, off-line operating analytical tools like light microscopes, scanning electron microscopes (SEM), total reflection x-ray fluorescence (TXRF), and so on are used. The analysis must be performed on a representative sample of particles homogeneously deposited on a flat sample plate. This avoids sample preparation steps which may change the sample. In this paper we describe the design, construction, and evaluation of a continuous sampling device that deposits gasborne particles on an analytically suitable sample plate. The collection efficiency and the deposition pattern were optimized using a numerical model and experiments. It turned out that representative samples appropriate for further analysis can be taken in the particle size range from 0.03 mu m < Dp < 10 mu m. Additionally, the sampling efficiency was investigated for particles smaller than 0.03 mu m using electrical and non-electrical deposition mechanisms like diffusion and thermophoresis. The investigations performed demonstrate that the designed electrostatic precipitator (ESP) is a very useful tool for homogeneous particle deposition on analytically suitable flat sample plates and can be used as a back-up filter. Further, the ESP especially can be used in combination with a differential mobility analyzer (DMA) if detailed investigations of a narrow particle size range of a polydisperse aerosol are required.     

A Differentially Pumped Particle Inlet for Sampling of Atmospheric Aerosols into a Time-of-Flight Mass Spectrometer: Optical Characterization of the Particle Beam

Two methods of characterizing the particle beam generated with a differentially pumped particle inlet are presented. Both methods are based on optical scattering of a laser beam by the particle beam. The first method images a time integrated scatter signal from the entire particle beam onto a charge coupled device (CCD), and an Abel inversion is performed on the image data to arrive at the radial particle density distribution in the beam. The second method, based on counting individual (particle) scatter pulses, yields the radial particle density directly. Initial results of the performance of the particle inlet are reported for particles with diameters between 40 and 800 nm. Under optimal working conditions, particle beams were generated with a full angle divergence on the order of 1-2 mrad. The width, measured 285 mm downstream from the exit of the particle inlet, was 250mu m, half width at half maximum (HWHM).     

Numerical Modeling of Particle Dynamics in a Cylindrical Chamber Containing a Rotating Disk

Numerical modeling was performed to study the submicron particle dynamics in a confined flow field containing a rotating disk, temperature gradient, and various inlet gas flow rates. The Lagrangian model was employed to compute particle trajectories under the temperature gradient, disk rotation speed, and inlet gas flow rate effects. The trajectories of particles with diameters of 1 μm, 0.1 μm, and 0.01 μm were examined in this study. When the inlet gas temperature was lower than that of the disk, particle-free zones were created due to upward thermophoretic force for 1 μm and 0.1 μm particles. Disk rotation was found to depress the size of the particle-free zone. Particle deposition onto the disk for 0.01 μm particles was possible because of the Brownian motion effect. A detailed evaluation of the particle-free zone size as a function of the temperature gradient, disk rotation speed, and inlet gas flow rate was performed. When the inlet gas temperature was higher than the disk temperature, particle deposition onto the disk was enhanced due to the downward thermophoretic force for 1 μm and 0.1 μm particles. Disk rotation was found to increase the deposition rate. For 0.01 μm particles, Brownian motion was more important than thermophoretic force in controlling particle behavior. The particle deposition rates as a function of the temperature gradient, disk rotation speed, and inlet gas flow rate were performed.     

A Computational Fluid Dynamics Study of Particle Penetration through an Omni-Directional Aerosol Inlet

Computational fluid dynamics (CFD) was used to study aerosol penetration through the entrance section of a bell-shaped omni-directional ambient aerosol sampling inlet. The entrance section did not include either an insect screen or a large-particle pre-separator. Simulations of the flow field were carried out for wind speeds of 2, 8, and 24 km/h and a fixed exhaust flow rate of 100 L/min; and, particle tracking was performed for 2 to 20 μ m aerodynamic diameter particles. Penetration calculated from CFD simulations was in excellent agreement with experimental results from previous studies with the root mean square relative error between simulation and experimental data being 3.8%. CFD results showed that the most significant regional particle deposition occurred on the upwind side of a curved flow passage between two concentric axisymmetric shells of the inlet housing and that deposition at the leading edges of the shells and within the exhaust tube was far less significant. At a wind speed of 2 km/h, penetration was affected by gravitational settling, e.g., penetration of 20 μ m particles was 71.9% when gravity was included and 80.4% without gravity. At higher wind speeds gravity had little effect. An empirical equation was developed to relate aerosol penetration to a Stokes number, a gravitational settling parameter, and a velocity ratio. Good fits of the correlation curves to experimental data and numerical results were obtained.     

Cost-Efficiency Analysis of a Model Wire-Plate Electrostatic Precipitator via DNS Based Eulerian Particle Transport Approach

The object of this work is to outline a methodology that can improve the current procedures used to size cost-optimized, efficient electrostatic precipitators (ESP). Focusing on a model wire-plate ESP initially, we develop a two-dimensional Eulerian, advection-diffusion type model for particle transport with distributed parameters. The Eulerian model is assessed against the accurate Lagrangian particle tracking database obtained for a model ESP using the parameter-free, highly accurate direct numerical simulation database obtained in previous work (Soldati 2000; Soldati and Banerjee 1998). Results show that the simplified Eulerian model can have good performances, provided that the functional form of the required transport parameters (i.e., turbulent dispersion coefficient, electromigration velocity, and convection velocity) are properly defined. Next, the cost function for a model ESP is defined and the influence of several design parameters on cost and collection efficiency is examined to identify guidelines to increase the collection efficiency at the lowest cost. Considering the cost associated with variation of precipitator length and width, wire-to-plate distance, and voltage applied to the wires, results show that the most cost-effective way to increase the collection efficiency of a wire-plate ESP is to decrease the wire-to-wire distance. Furthermore, the reasons for cost effectiveness of wider-spacing ESPs are demonstrated from a theoretical viewpoint, thus confirming the experimental observations of Navarrete et al. (1997).     

Size-Selective Oxidation of Aldehydes with Zeolite Encapsulated Gold Nanoparticles

Here, we report a synthesis and catalytic study of hybrid materials comprised of 1?C3 nm sinter-stable Au nanoparticles in MFI-type zeolites. An optional post-treatment in aqua regia effectively remove Au from the external surfaces. The size-selective aerobic aldehyde oxidation verifies that the active Au is accessible only through the zeolite micropores.     

我国电除尘灰的粒度分布特征

本文针对我国燃煤电厂电除尘器收集的粉煤灰的粒度分布特征,质量等级和影响粒度分布的因素作了阐述。     

Electrohydrodynamic Precipitator Flow with a Barbed Plate Discharge Electrode

The nonuniform corona discharge in the wire-plate electrostatic precipitator results in a rotational electric body force which is a source of large-scale secondary flows and turbulence within the flow channel. The electrically induced flow causes large increases in diffusivities detrimental to the particle collection process. Since the electrode geometry and the structure of the corona discharge define the magnitude and character of the electric body force, it is theoretically possible to design a discharge electrode which minimizes electrohydrodynamic flow disturbances. As a first step in this direction, a novel planar electrode design in which electrical discharges are configured to reduce the inhomogeneities of the electric body force is experimentally studied in a negative polarity laboratory electrostatic precipitator. Hot-film anemometer measurements of the electrohydrodynamic turbulent velocity field downstream of the plate electrode are compared to those of a conventional wire-plate precipitator. Results confirm that electrode geometry has a significant role in turbulence production. Although there is some evidence that secondary flows are reduced in the planar geometry, spectral analysis of the flow downstream of the electrodes indicate that the barbed plate design increases turbulence intensity as much as 50% without reducing eddy size. Continued experimentation is necessary to fully assess the possible benefits of such a design.     

Comparison of Two Aerodynamic Lenses as an Inlet for a Single Particle Laser Ablation Mass Spectrometer

By means of a newly designed portable aerosol mass spectrometer SPLAT (Single Particle Laser Ablation Time-of-flight mass spectrometer) for the analysis of single atmospheric aerosol particles we investigated the system performance in dependency on two different aerodynamic lenses (Liu and Schreiner type) capable of focusing particles with diameters ranging from 80 nm to 800 nm and 300 nm to 3000 nm, respectively. By using the pressure regulated Schreiner lens, the instrument is independent of variations in atmospheric pressure which would lead to changing dynamical properties of the aerosol particles. Active pressure control inside the inlet system facilitates airborne measurements without complicated corrections. With the Liu setup no pressure regulation was used. Here the overall efficiency of our instrument was 7% while with the Schreiner setup 2% was achieved. The Liu lens setup is optimal for measuring submicron particles at low particle concentrations. To detect supermicron particles the Schreiner lens setup is favored. Together with these experiments we present key details of the SPLAT setup and its characterization. Our instrument is able to measure simultaneously the size and the chemical composition of individual aerosol particles larger than 300 nm in diameter. It uses forward scattered light of single aerosol particles at two positions to determine their vacuum aerodynamic diameter from the flight time between the two lasers. Chemical analysis of the particles is done by laser ablation mass spectrometry utilizing a bipolar time-of-flight mass spectrometer.     

Effect of Interlayer Thickness on Residual Thermal Stresses in a Ceramic-to-Metal Cylindrical Joint

It is well-known that large thermal stresses are caused in a ceramic-to-metal joint by thermal expansion mismatch. Barton's analysis for stresses in cylindrical rods (M. V. Bartoh, J. Appl. Mech. , 8, A97 (1941)) was reformulated in terms of stresses arising from thermal expansion mismatches in elastically homogeneous cylindrical joints, and qualitative agreement was shown between these results and those produced by finite element calculations. Here, we apply these results to homogeneous joints with interlayers having different thermal expansion coefficients, to examine the effect of interlayer thickness on diminishing thermal expansion mismatch stress.     

Size-Selective Homocoupling of Arylboronic Acids Mediated by a Copper-Based Metal–Organic-Framework

Copper-based metal–organic-frameworks with open metal sites have received increasing research interest as heterogeneous catalysts for various organic transformations. A copper-based metal organic framework (1) built with L-NO₂ ligand (L-NO₂?=?4,4′-dicarboxy-4″-nitrotriphenylamine) was selected for catalyzing aerobic homocoupling of arylboronic acid toward biaryl products given its structural robustness and 1-D channels lined with rich open metal sites. The experimental results show that MOF (1) exhibits pronounced size selectivity over arylboronic acid molecules, which is only effective for short arylboronic acid molecules (e.g. phenylboronic acid, p-methylphenylboronic acid and p-fluorophenylboronic acid), giving the corresponding biaryl products in good yields. Moreover, MOF (1) also demonstrates a good recyclability which only shows a small decay in the catalytic performance after five repeated runs.

     

切流式三旋单管内固相颗粒运动规律的数值研究

采用商用软件Fluent 6.3,利用随机轨道模型对切流式三旋单管内固相颗粒运动轨迹进行了数值模拟,结果表明颗粒运动轨迹随机性较大,对气流的跟随性较好,受湍流脉动的影响较大。颗粒在单管内的逃逸与捕集运动规律的统计结果表明:不大于5μm的颗粒主要在排气管底部和分离筒体发生逃逸,随着粒径的增加,颗粒的分离效率增加,在排气管底部和分离筒体上部逃逸比重增加,而在分离筒体中底部颗粒的逃逸比重减少。总结来说,排气管短路流、分离筒体内旋流夹带逃逸是影响切流式三旋分离性能的主要原因,可以从这些方面进行三旋分离性能的优化与改进。     

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.     

一种新型入口结构的下行管的流体力学性能

提出了一种新型下行管入口结构. 这种结构设有雾化空间,能够提供适宜的床层密度,剂油初始接触采用逆流、错流方式. 在下行管入口处,颗粒浓度呈管中心高、边壁低的分布;随着颗粒向下运动,中心区颗粒浓度逐渐降低,边壁区颗粒浓度升高. 在下行管完全发展段,颗粒浓度径向分布形式不再随轴向位置发生变化.     

Packing of Cylindrical Particles with a Length Distribution

The packing of cylindrical particles with log-normal and modified power-law length distributions has been experimentally studied. The results indicate that the packing density is heavily dependent on the parameters in the two distributions. However, this dependence cannot be predicted by the direct analogy to that for the packing of spherical particles. It is postulated that the packing of nonspherical particles be governed by two factors: the shape effect and the size effect, which respectively correspond to the unmixing and mixing states of a particle mixture and are quantified from the specific volumes of the two states. Analysis of the results suggests that the shape effect is dominant for the packing of cylindrical particles with a wide length distribution.