Efficiency of inertial deposition of aerosol particles in fibrous filters with regard to particle rebounds from fibers

The inertial deposition of submicron aerosol nanoparticles onto fibers during gas filtration through fine-fiber filters is considered. It is shown that there is critical filtration velocity U* below which the energy loss upon collisions has no influence on the filtration efficiency. Above this critical velocity, the filtration efficiency depends on the mechanism of the inelastic energy loss and can be noticeably lower than the result of its estimation with no allowance for the particle rebound. For a rather dense fibrous medium, when not all particles that have rebounded from a fiber have time to attain the flow velocity before the next collision with another fiber, the filtration efficiency depends on the velocity distribution of the rebounding particles. It is shown that, in this case, the filtration efficiency must increase with the packing density of a filter.     

Inertial Deposition of Aerosol Particles on Fiberous Filters

The inertial deposition of aerosol particles on model filters at low and intermediate Stokes numbers and low Reynolds numbers was studied. It was shown that the efficiency of the inertial deposition of submicron particles is markedly affected by retarded van der Waals forces and the gas slip in the vicinity of thin fibers.     

Nanoaerosol Filtration with Fine-Fiber Polydisperse Filters

The self-consistent theory of nanoaerosol filtration has been considered at small Peclet numbers. It has been shown that, at Pe < 1, it is necessary to consider the deposition of particles simultaneously on thePe < entire ensemble of fibers. Only at can the filtration efficiency be found from the collection efficiencies calculated separately for each fiber. The self-consistent theory has been used to estimate the efficiencies of filtration with polydisperse fibrous filters. The penetrations of particles through filters with different variances of the functions of fiber length distribution over fiber radii have been compared at a constant packing density or a constant total length of all fibers. It has been shown that, at a constant packing density, a rise in the variance leads to a decrease in the filtration efficiency, while, at a constant total fiber length, the efficiency is almost independent of the width of the distribution function.     

The effect of diffusion on nanoaerosol filtration through porous materials with allowance for desorption processes

The effects of the finite residence time of aerosol particles in the bound state and their detachment due to thermal fluctuations on the filtration efficiency of porous and fibrous materials have been investigated with allowance for longitudinal diffusion in a flow. It has been shown that the desorption of particles affects the filtration efficiency even at times shorter than residence time τ_d of the particles in the bound state, while, at t ? τ_d, filtration stops. Allowance for the diffusion of aerosol particles in the flow leads to a decrease in the filtration efficiency as compared with the calculations performed without taking into account the longitudinal diffusion.     

Filtration of nanoaerosols through porous materials with allowance for desorption processes

The effects of finite residence time of aerosol particles in a bound state and their detachment due to thermal fluctuations on the filtration efficiency of porous and fibrous filters have been studied. It has been shown that, when desorption processes are taken into account, nanoparticle filtration efficiency decreases with time already at times that are short compared with the residence time of particles in the bound state.     

气溶胶粒子通过填充柱的保留时间分布测定

采用亚微米单分散聚苯乙烯球形硬气溶胶粒子和脉冲进样技术,测定了气溶胶粒子通过无规则石英砂填充柱的保留时间分布,从保留时间分布曲线得到了气溶胶粒子在填充柱中的平均保留时间和穿透率.研究了平均保留时间和穿透率与流体流速、填充柱的长度、填料粒度和气溶胶粒子大小之间的关系.研究发现,流速越大,保留时间分布曲线越尖锐,流速越小,保留时间分布曲线越平坦;气溶胶粒子的穿透率随着柱长的增加而降低,随流速、气溶胶粒子粒径和石英砂颗粒大小的减小而减小;平均保留时间随柱长增加而增大,随流速增大而减小,随气溶胶粒子粒径减小而减小,而与石英砂颗粒大小几乎无关.     

The Influence of Fluctuations on the Efficiency of Aerosol Particle Collection with Fibrous Filters

The influence of fluctuations on the filtration efficiency was studied. In the case of arbitrary random macroscopic inhomogeneities in the packing density and fluctuations in the filter surface form, the expressions were derived for the pressure drop and the penetration of aerosol particle, expressed via the correlation functions of fluctuating parameters. For the high degrees of cleaning, the probabilistic approach was developed, which takes into account the discreteness of the number of aerosol particles deposited on filter fibers.     

Inertial deposition of aerosol particles from laminar flows in fibrous filters

The deposition of aerosol particles onto filter fibers under the effect of inertial forces is studied in a wide range of Stokes numbers (St) at Reynolds numbers close to unity (Re ∼ 1). Coefficients η of the capture of inertial particles with finite sizes in model filters composed of parallel rows of identical parallel fibers located normal to the direction of a flow are determined based on the numerical solution of the Navier-Stokes and particle motion equations. It is shown that, at Re < 1 and a constant particle-to-fiber radius ratio, R = r _p/a, number St uniquely characterizes capture coefficients η for particles with different densities, while, at Re ≥ 1, the capture coefficient depends on both St and Re. At constant R and St values, the larger Re the higher the capture coefficient. The influence of the structure of the model filter on pressure drop Δp and η is investigated. A nonuniform arrangement of fibers in rows is shown to increase the Δp/U ratio at lower Re values and to make the η -St dependence more pronounced than that for systems of uniformly ordered fibers. The results of calculations agree with the experimental data.     

Emission spectroscopic monitoring of particle composition,size and transport in laser ablation inductively coupled plasma spectrometry

The spectroscopic line emissions of copper and zinc from atomized particles generated by orthogonal pre-pulse laser ablation of brass and transported from the ablation cell through tubes into an ICP have been simultaneously measured end-on with fast photomultipliers. It was shown that simultaneous line monitoring of major elements provides not only information on the aerosol transport in laser ablation ICP-spectrometry, but also on the ratios of small to single larger particles and their respective elemental compositions and, therefore, on possible elemental fractionation problems. Furthermore, the spectroscopic information can be easily exploited for proper adjustment of the laser fluence in order to minimize the production of large particles, to improve the transport efficiency and to reduce the noise of analytical signals in laser ablation ICP-spectrometry. The present particular experiment on orthogonal pre-pulse laser ablation of brass confirms the recent finding that such kind of double-pulse arrangement produces predominantly ultra-fine particles. Individual brass particles with diameters ≳ 250 nm could be analyzed. They showed large Zn depletions as expected. Finally, strong accumulations of aerosol particles were found in the ablation cell used even at low laser pulse frequencies.     

Visualization of microscale particle focusing in diluted and whole blood using particle trajectory analysis

Inertial microfluidics has demonstrated the potential to provide a rich range of capabilities to manipulate biological fluids and particles to address various challenges in biomedical science and clinical medicine. Various microchannel geometries have been used to study the inertial focusing behavior of particles suspended in simple buffer solutions or in highly diluted blood. One aspect of inertial focusing that has not been studied is how particles suspended in whole or minimally diluted blood respond to inertial forces in microchannels. The utility of imaging techniques (i.e., high-speed bright-field imaging and long exposure fluorescence (streak) imaging) primarily used to observe particle focusing in microchannels is limited in complex fluids such as whole blood due to interference from the large numbers of red blood cells (RBCs). In this study, we used particle trajectory analysis (PTA) to observe the inertial focusing behavior of polystyrene beads, white blood cells, and PC-3 prostate cancer cells in physiological saline and blood. Identification of in-focus (fluorescently labeled) particles was achieved at mean particle velocities of up to 1.85 m s(-1). Quantitative measurements of in-focus particles were used to construct intensity maps of particle frequency in the channel cross-section and scatter plots of particle centroid coordinates vs. particle diameter. PC-3 cells spiked into whole blood (HCT = 45%) demonstrated a novel focusing mode not observed in physiological saline or diluted blood. PTA can be used as an experimental frame of reference for understanding the physical basis of inertial lift forces in whole blood and discover inertial focusing modes that can be used to enable particle separation in whole blood.     

Depth-resolved speciation of nitrogen compounds in environmental solids

Dynamic SIMS has been applied to the analysis of inorganic and organic nitrogen compounds. A depth-resolved speciation of nitrogen, based on the intensities of small ionic clusters, has been developed. An evaluation method is outlined to distinguish inorganic and organic nitrogen compounds. Furthermore, several organic compound families can be recognised including detailed nitrogen speciation. Because this method is suitable even for characterisation of complex mixtures, urban outdoor aerosol particles have been analysed with Dynamic SIMS.     

Removal of nanoaerosol during the bubbling of the salt melt of beryllium and lithium fluorides for the preparation of reactor radioisotopes

The parameters of aerosol particles formed in the course of the spontaneous thermal condensation of vapors and bubbling a 66LiF-34BeF₂ (mol %) eutectic salt mixture with helium have been studied. For this purpose, a vertical bubbling mode at T ≈ 900 K and an ampule device for obtaining reactor radioisotopes for medical applications were used. The rate of the bulk removal and the chemical composition of aerosols were measured. The size distribution of the aerosol particles was bimodal, and the mass concentration of the particles exceeded by far the maximum permissible concentration (MPC). The characteristics of regenerated nickel multilayer nanofilters for ultrahigh filtration of aerosols from the salt liquid melt were analyzed.     

Use of electrostatic preconcentration at high salt concentrations

Apparatus is described for the electrostatic preconcentration of aerosol particles with a collector trough electrode from samples containing large amounts of matrix salts. A cooling and filtration cycle is used to precipitate and separate the matrix salts, and prevent saturation of the collector solution. The system has been applied to the preconcentration of sea-water species before analysis by flame atomic-absorption spectroscopy.     

Defining critical flux in submerged membranes: Influence of length-distributed flux

Flux can vary along the fiber length in submerged hollow-fiber membranes depending upon the axial gradients of both pressure and foulant layer build-up. However, the measurement of flux is necessarily length-averaged because it is determined by the flow rate exiting the end of the fiber. The length-averaged flux below which no foulant accumulates in a specified filtration time is defined as the critical flux. Critical flux is shown in this work to be a relative rather than absolute value. It depends on the fiber length, observation time, aeration rate and the compressibility of the particles. Fouling will occur in full-scale if the critical flux test is established in tests with fibers that are much shorter than in full-scale and/or with a filtration time that is shorter than in full-scale.     

On molecular chirality within naturally occurring secondary organic aerosol particles from the central Amazon Basin

In this perspectives article, we reflect upon the existence of chirality in atmospheric aerosol particles. We then show that organic particles collected at a field site in the central Amazon Basin under pristine background conditions during the wet and dry seasons consist of chiral secondary organic material. We show how the chiral response from the aerosol particles can be imaged directly without the need for sample dissolution, solvent extraction, or sample preconcentration. By comparing the chiral-response images with optical images, we show that chiral responses always originate from particles on the filter, but not all aerosol particles produce chiral signals. The intensity of the chiral signal produced by the size resolved particles strongly indicates the presence of chiral secondary organic material in the particle. Finally, we discuss the implications of our findings on chiral atmospheric aerosol particles in terms of climate-related properties and source apportionment.     

Gravitational Deposition of Aerosol Particles on a Fibrous Filter with Account for the Effect of Van der Waals Force

The deposition of inertialess aerosol particles with a finite radius is investigated on fibrous filters under the effect of gravitational and molecular forces. It is shown that particles are deposited on fibers from an upgoing slow flow as a result of the action of van der Waals forces that balance the gravitational particle displacement from the flow lines near the fibers     

Depth-resolved investigation of the element and compound inventory of aerosol particles from outdoor air

Plasma-based SNMS has been employed to perform a depth-resolved elemental analysis of aerosol particles collected in a suburban area near Karlsruhe. Two classes of particles could be distinguished. Both of them exhibited a significant depth structure. Submicron particles have been found with a 15 nm deep ammonium sulphate surface layer on carbon rich cores. The other class consisted of coarse particles of several m size which even showed a double layer system enclosing geogenic cores. For depth-scaling a new internal calibration procedure has been tested and found to be suitable for the complex environmental material. Dynamic SIMS measurements have been used to investigate especially the carbon compounds of the particles by evaluation of small cluster ion signal patterns.     

On dry granular bed filtration of aerosols induced by irradiation

An analytical solution of the aerosol filtration process and determination of pressure drop on the granular bed have been presented. Results of simulated calculations and measurements show a good agreement. The elaborated analytical procedure applied for the postirradiation aerosol filtration enables an adjustment of granular filter parameters in certain conditions of the filtration process.     

Calibration of Polarization-Sensitive and Dual-Angle Laser Light Scattering Methods Using Standard Latex Particles

A polarization-sensitive laser light scattering (PSLLS) method and a dual-angle laser light scattering (DALLS) method have been studied for in situ measurement of submicrometer hydrosol and aerosol particles. By using standard monodisperse polystyrene latex particles suspended in water and air as test particles, calibration of systems built based on the above methods have been performed. The effects of light scattered by agglomerated aerosol particles (multiplets) were corrected by considering the fraction of multiplets as determined with an aerosol measurement technique using a differential mobility analyzer. The change in the measured intensities of scattered light with particle diameter was then determined by calculations based on Mie theory. It was shown that the PSLLS system can determine particle diameters as small as approximately 60 nm for the test hydrosol particles and approximately 100 nm for test aerosol particles, respectively. The DALLS system can determine smaller diameters than the PSLLS system for test particles with no light absorption. The change in scattered light intensities with particle diameter was also investigated by theoretical calculations with various refractive indexes and scattering angles. The PSLLS and DALLS systems promise to become routine measurement tools for absorbing and nonabsorbing particles, respectively. Copyright 2001 Academic Press.     

气溶胶单粒子化学成分的实时测量

自行设计了国内第一台可实时测量气溶胶单粒子化学成分的气溶胶飞行时间质谱仪,并用该仪器对两类燃烧过程产生的气溶胶粒子进行了实时测量。实验结果展示了本仪器能够同时实现实时在线测量和单粒子检测,从而克服了传统的离线检测方法如色谱法、化学方法的局限性。