摘要:

The problem of absorption and scattering of electromagnetic radiation by particles can be solved analytically for only the simplest cases, but established numerical methods can allow a straightforward extension to particles with arbitrary inhomogeneities, shapes, and nonlinear response. In this paper a recently developed finite element technique for the scattering and absorption by a homogeneous sphere is extended to the problem of a coated dielectric sphere of arbitrary size parameter. Numerical results showing good agreement with analytical solutions for the size parameter of 5.93 and various core radii are presented. Results obtained suggest that finite element methods have promise for analytically intractable scattering/absorption problems and show that the Debye amplitude formulation of the problem offers some advantages in a numerical scheme.

ISSN:0098-6445    

DOI:10.1080/00986449608936538

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Experiments are carried out using special filters with known structural characteristics to study the stationary phase of filtration of liquid aerosols by fibrous filters in the diffusion, interception and inertial regimes. In the domains of diffusion and interception, a comparison made with published models has enabled the selection of two relationships correctly describing these mechanisms both in the slip and continuum regimes. On the other hand, in the inertial domain, an empirical correlation has been established for the single fiber impaction efficiency for viscous flows. All our results have been validated by measurements on an industrial filter.

ISSN:0098-6445    

DOI:10.1080/00986449608936539

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Particles from several types of sources in central Taiwan were sampled and analyzed to obtain representative source profiles for use in receptor modeling. The sources studied include field soil, paved road dust, unpaved road dust, coastal sand, river sand, and aerosols generated by burning of agricultural wastes. The agricultural wastes studied include rice straw, corn stalk, and sugarcane leaves. All the samples were analyzed for 20 elements by an X-ray fluorescence spectrometer. The mass percentages of elemental carbon (EC) and organic carbon (OC) in particle samples were determined by an elemental analyzer. For particles obtained from resupension of soils and dusts, the mass percentage of the fine fraction is in the range of 40 to 61 %. The abundant elements in the coarse fraction are Si, Al, Fe, K., and Ca. In the coarse fraction, only paved road dust contains OC. The abundant species in the fine fraction are OC, Si, Fe, Al, K, and Ca. Particles generated by combustion of agricultural wastes are mainly in the fine fraction, with the mass percentage ranging from 87 to 98%. The abundant species in the fine fraction are identical for all three types of agricultural wastes, but different in the coarse fraction.

ISSN:0098-6445    

DOI:10.1080/00986449608936540

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Measured production rates of tropospheric ultrafine particles (˜3nm diameter) are reported for the first time and are shown to be orders of magnitude greater than nucleation rates predicted by the binary theory of homogeneous nucleation for sulfuric acid and water. Furthermore, the functional dependence of observed particle formation rates on sulfuric acid vapor concentrations is much weaker than predicted by binary theory. We present arguments to show that these discrepancies might be due to the participation of a species such as ammonia which could stabilize subcritical clusters, thereby enhancing nucleation rates. The data suggest that atmospheric nucleation may occur by a collision-limited process, rather than by a condensation/evaporation controlled process as is assumed in the classical theory.

ISSN:0098-6445    

DOI:10.1080/00986449608936541

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The anterior adult human nasal passage contains hairs which act as a fibrous filter for inhaled particles. Particles which are collected by these hairs are likely removed by physical means and are unable to produce toxic effects in the respiratory tract. Measurements have been made of the anterior nasal passage and nasal hair of an adult Caucasian subject, and calculations have been carried out using fibrous filter theory to determine the deposition efficiency for unit density spherical particles of diameter ranging from 1 nm-40 μm at three steady inspiratory flow rates

ISSN:0098-6445    

DOI:10.1080/00986449608936542

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

This paper describes an on-line instrument, capable of measuring the size and chemical composition of single, aerosol particles. Possible applications include monitoring aerosol reactors and studying atmospheric chemistry. The main conclusion is that a working prototype has been built and tested. It uses a three stage vacuum system to generate an aerosol beam with a low divergence angle and a high transmittance. The pressure is reduced sufficiently to allow the application of a time-of-flight mass analyzer. The aerosol beam is probed in the analysis section by the focused beam of a low-power helium-neon laser. Every particle crossing the laser beam scatters light, which is detected by two photomultiplier tubes, mounted at angles of 45 and 90°. The signal is stored when both detectors produce a pulse simultaneously, and this event triggers the chemical analysis cycle. A pulsed Nd: YAG laser vaporizes the particle and generates ions, which are next analyzed by a time-of-flight mass spectrometer. In this way combined information on the size and the composition of the particle is obtained.

ISSN:0098-6445    

DOI:10.1080/00986449608936543

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Due to their finite inertia, particles carried in a gas follow trajectories that differ from fluid streamlines. They may thus be size-segregrated, though only with the modest resolving power allowed by the dependence of their trajectories on the fluid streamline Ψin which they are initially seeded. This limitation is greatly reduced in the present work via aerodynamic focusing, a phenomenon discovered by Israel and Friedlander and briefly reviewed here. McM urry and his colleagues have recently shown that passage through a succession of coaxial converging nozzles brings virtually into the axis of symmetry (Ψ= 0) all particles within a relatively wide size range. Considerable gains in instrument resolution may thus be achieved by inserting a series of focusing nozzles right upstream of an aerosol size spectrometer. This general principle is demonstrated here for the case of variable-pressure impactors, which are ideally suited for coupling to the focusing lenses. The resolution improvements found are impressive, particularly in the viscous and compressible regimes.

ISSN:0098-6445    

DOI:10.1080/00986449608936544

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The aim of this work is a theoretical analysis of the deposition of charged aerosol particles in a fibrous electret filter under nonsteady-state conditions. In this case loading of the filter's collectors with deposits causes two oppositeeffects: growth of the dendritic structures on the collectors’ surfaces results in an improvement of the deposition efficiency due to inertial impaction, sedimentation and Brownian motion, whilst it simultaneously leads to a decrease of the efficiency corresponding to the electrostatic attraction as a result of weakening of electric field around an electret fiber becoming more and more loaded with the deposits, having the opposite electric charge. At the same time, the second of the most important parameters of the fiber (in addition to its efficiency of the dust removal) -i.e. the pressure drop, increases continuously with the filter loading. Moreover, it should be remembered, that loading of the filter is -in a principle-non-uniform along it. Hence, the variation of the filter quality factor, QF, in time may be different for various operating conditions. The simplified model formulated, which takes into account the local changes of the gas flow pattern and electrostatic field intensity around a loaded collector, allows to study the course of the process, and it may be useful for optimization of gas-cleaning filters that make use of electrostatic interactions. The description of the mathematical model formulated is as complete as passible to make it easily adaptable for all specific situations. Some results of numerical simulations obtained are discussed.

ISSN:0098-6445    

DOI:10.1080/00986449608936545

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Ultrafine (“nano”-) particles produced from highly supersaturated vapors or liquids are usually aggregated, often containing thousands of small 'primary’ particles bound together in tenuous structures characterized bymass fractal dimensionsless than 3. Such aggregates have large initial surface area but aremetastablewith respect to more compact configurations. Availablerestructuringmechanisms include surface energy driven coalescence, which, in the case of viscous flow at high gas temperatures, is ultimately able to obliterate all evidence of the original (“primary”) particles. We here exploit the notion that, provided an aggregate is sufficiently large, it can be treated like a spatially non-uniform porous medium, undergoing finite-rate surface energy driven viscous flow sintering leading to final collapse to a single dense sphere. For this purpose, after aDƒ≌s const stage of sintering [associated with a corresponding increase in mean apparent primary particle ('grain’) size], we use an extension of the sintering rate models of Mackenzie and Shuttleworth (1949) and Scherer (1977), treating the material of the restructuring aggregate to be a Newtonian viscous fluid. We predict and report here the time-dependent increase in fractal dimension,Dƒ, and associated decreases in: aggregate outer (maximum) radius, mobility radius, and changes in accessible surface area with dimension-less time [real time in multiples of the characteristic sintering time, μ (R1)t=0/σ cr, where u is the material's viscosity (Rl)t=0is the effective initial grain radius and a the material surface tension]. In these units, we find that thetotal required coalescence timedoes not increase with N as sensitively asN1/3an important observation for processes involving very large aggregates. With validation and the indicated extensions, our pseudo-continuum methods are efficient enough to be used for estimating the morphological- and transport property-evolution of entirepopulationsof restructuring aggregates, perhaps characterized by some non-separable probability density functionpdf(N,Dƒ,R1,) locally, in non-isothermal combustion-synthesis reactors.

ISSN:0098-6445    

DOI:10.1080/00986449608936546

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

We developed a method of on-line measurement of the magnetic moment of aerosol particles. For this purpose we determine the deposition of the particles in a filter consisting of magnetic nickel screens. Deposition is governed by the dipole force magnetic particles experience near the magnetized wires of the screens. The number concentration at the in-and outlet of the filter is measured by optical particle counters. The so called single fiber efficiency is derived directly from the measured data as a function of particle size, particle magnetic moment and fiber magnetization. The relationship between this dimensionless value and the magnelic moment is obtained by a computer simulation using trajectory analysis. The method is tested with equally sized iron/iron oxide particles in the size range from 40 to 170nm.

ISSN:0098-6445    

DOI:10.1080/00986449608936547

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor