Performance of Electrically Augmented Fibrous Filters,Measured with Monodisperse Aerosols

A study of the performance of filters in an external electric field has been carried out in controlled conditions using monodisperse aerosols with a charge distribution close to a Boltzmann. The effects of electric field strength, particle diameter, fiber diameter, and filtration velocity have been measured. The results are in basic agreement with existing theory for the capture of particles by polarization forces and indicate that this process is dominant, although other filtration processes are also significant, particularly when the electric field is low. Reasonable correlation between single fiber efficiency and the dimensionless parameter for capture by polarization forces is observed. The agreement is improved by the inclusion of the hydrodynamic factor into the calculation, but this does not remove all discrepancy. If filters are exposed to an electric field for a protracted period, a small persistent residual polarization of the fibers is observed. Loading the filter with aerosol in the laboratory has little effect on its performance, though eventually the aerosol captured in the vicinity of the live electrode causes shorting out.     

Computer Simulation of Monodisperse Aerosol Collection in Fibrous Filters

A computer program has been developed to simulate the filtration process in fibrous filters collecting monodisperse aerosol particles. The model filter is represented by an array of parallel cylindrical fibers and the Kuwabara flow field is employed to determine the particles trajectories inside the filter. The simulation model is based on the Monte-Carlo (self-driven) principle, and a sequence of uniform pairs of pseudorandom numbers is generated representing the initial locations of the approaching particles. The estimation of the initial collection efficiency through the simulation model that considers the deposited particles and the presence of dendrites is in good agreement with published experimental data. The development of the quasi three-dimensional simulation model offers a detailed information about the transient progression of the deposition process. The transient behavior of the pressure drop across the collector system (the fiber and the collected particles) and the morphology of deposit are presented and the results are in good agreement with experimental data.     

Analytic Solutions to Diffusional Deposition of Polydisperse Aerosols in Fibrous Filters

Deposition of polydisperse aerosols by Brownian diffusion was studied analytically using the penetration efficiency of monodisperse aerosols combined with the correlations among the moments of lognormal distribution functions. The analytic solutions, so obtained were validated using the exact solutions, which were applied to recalculate the filtration efficiencies of the existing experimental data for various filtration conditions. It was found that the collection efficiency of a fibrous filter should be corrected with respect to the position in the filter, if the particles are polydisperse. By considering the effect of the polydispersity of particle size, the analytic solutions showed good agreement with existing experimental data. It is believed that the present work makes it possible to determine the filtration efficiency of polydisperse aerosols in fibrous filters and to estimate errors associated with the degree of polydispersity of the particles quickly and accurately for the diffusion dominant regime.     

Structural Property Effect of Nanoparticle Agglomerates on Particle Penetration through Fibrous Filter

Most filtration studies have been conducted with spherical particles; however, many aerosol particles are agglomerates of small primary spheres. Filtration efficiency tests were conducted with silver NP agglomerates, with the agglomerate structure controlled by altering the temperature of a sintering furnace. The mobility diameter and mass of the silver NP agglomerates were measured using a differential mobility analyzer together with an aerosol particle mass analyzer. From these measurements, it was found that the fractal-like dimension, D _fm, varied from 2.07 to 2.95 as the sintering temperatures was increased from ambient to 600°C. The agglomerates were essentially fully coalesced at 600°C allowing direct comparison of the filtration behavior of the agglomerate to that of a sphere with the same mobility diameter. Other agglomerate properties measured include the primary diameter, the agglomerate length and aspect ratio, and the dynamic shape factor.

Agglomerate filtration modeling with no adjustable parameters has been investigated in terms of diffusion, impaction, and interception. The model results agree qualitatively with the experimental results in the particle size range of 50 to 300 nm. The results indicated that the larger interception length of agglomerates is responsible for the smaller penetration through a fibrous filter in comparison to spherical particles with the same mobility diameters.     

Filtration Efficiency of Non-Uniform Fibrous Filters

Although theoretical models of the filtration efficiency of fibrous filters are typically based on a single type of fiber in an ordered array, many actual fibrous filters comprises fibers that are inherently randomly distributed. It is desirable to be able to estimate the filtration efficiency of such non-uniform fibrous filters from their composition arrangement and the filtration property of individual fibers. Toward that end, we approximate the filter system as a series of cells comprising individual fibers with random distribution, deviating from the homogeneity assumption in the classical models. With better characterization of the filter structure based on the Voronoi diagram, we theoretically revisit filtration efficiency by the top-down (TD) approach and the bottom-up approach. The proposed models are compared with the existing experimental and numerical results under different fiber volume fractions, indicating the high accuracy of the TD model for the filtration of submicron aerosol particles. The influence of the degree of randomness of fiber distribution on filtration efficiency is also quantified.

Copyright 2015 American Association for Aerosol Research     

Numerical Investigation of the Capture of Polydisperse Particles by Charged Droplets

The capture of particles by charged droplets was simulated by considering the electrostatic interactions of droplet-droplet and droplet-particle. The results indicate that the electrostatic repulsion between droplets leads to a dynamic accumulation mode of particles. However, the droplet spacing has an insignificant effect on the capture efficiency when the electrostatic deposition predominates. The increase of droplet charge remarkably improves the capture efficiency, in which the capture of fine particles accounts for the largest proportion. Compared to the droplet charge, the droplet size shows a limited improvement in the capture efficiency. Reducing the droplet velocity prolongs the capture time instead of enhancing the capture capacity per unit time, thereby improving capture efficiency.     

Structural and Percolation Parameters of Porous Monodisperse Fibrous Systems

The structure of permeable or composite monodisperse fibrous systems is described in terms of average coordination number, specific surface area, specific contact surface area, and average radius of pores or matrix channels, which are calculated from porosity, average fiber size, and average interfiber contact size. Expressions are proposed for the apparent and threshold densities of monodisperse fibrous systems as functions of the relative fiber length.     

高纯、单分散二氧化硅纳米粒子的制备

本文以单晶硅粉作为硅源,氨水作为催化剂,制备了二氧化硅纳米粒子.研究了单晶硅粉制备二氧化硅纳米粒子时反应条件对反应产率和粒径的影响,并通过单因素实验法和正交设计法获得了最佳实验条件.通过激光粒度分析仪、电感耦合等离子体-质谱(ICP-MS)、扫描电子显微镜(SEM)等方法对纳米二氧化硅粒子的纯度、形貌、粒径进行了表征,得到了高纯、单分散的二氧化硅纳米粒子.     

Theoretical Study of Aerosol Filtration by Fibrous Filters

A theoretical analysis of filtration mechanisms has been made for fibrous filters in the region of maximum penetration. The theory is based on a boundary layer approach using the Kuwabara flow field to account for the interference effects of neighboring fibers. An improved expression for the diffusion and interception filtration efficiencies has been derived that compares well with the existing theories. A comparison of the developed theory with experimental data also confirms the validity of the present work.

     

Theoretical Modeling of Filtration by Nonuniform Fibrous Filters

Modeling commercial filter media using classical theories results in incorrect filter pressure drop and efficiencies and this is attributed to media inhomogeneity. The use of an arbitrary inhomogeneity factor is seen to be inadequate in accounting for the effect of operating conditions on the performance of inhomogeneous filter media. A simplistic theoretical modeling approach is described here to account for the variations in filter packing densities and to estimate their effect on the media particle capture characteristics. The theoretical modeling results are used in obtaining an equivalent filter packing density distribution from media local efficiency measurements. Considering this variation in the theoretical models is seen to result in better prediction of media performance over a wide range of operating conditions. The observed deviation of experimental efficiencies from the theory at smaller Peclet numbers can be explained from the results of the theoretical analysis.     

单分散纳米二氧化硅的制备与表征

利用Stober溶胶-凝胶法,合成了大小约为80～150nm的单分散纳米二氧化硅微球,并且探讨了温度、氨水浓度以及TEOS浓度等因素对合成二氧化硅的影响.利用XRD、 IR 、SEM、 TEM 等手段对样品进行了表征.结果表明:水解温度的升高加速二氧化硅颗粒在溶液中的熟化引起团聚;氨水浓度的增加使得成核速度增加,水解速度加剧,二氧化硅的粒径增大,团聚程度也增大.TEOS的量的增加也导致二氧化硅粒径稍有增加,团聚程度加深.     

Filtration of Multi-Walled Carbon Nanotube Aerosol by Fibrous Filters

Filtration efficiency of multi-walled carbon nanotube (MWCNT) aerosol by fibrous filter was evaluated experimentally. Mono-mobility test aerosols with electrical mobility diameter of 100, 200, and 300 nm were generated by the atomization of MWCNT aqueous suspension followed by mobility classification with a differential mobility analyzer (DMA). By analyzing the shape of classified aerosol particles under a scanning electron microscope, it was found that the DMA-classified 300 nm particles were fibrous in shape and had uniform diameter of about 60 nm and length of 2.1 micrometer. On the other hand, 100 nm and 200 nm particles contained a fairly large amount of multiply charged fibrous particles with a larger diameter. These test aerosols were challenged to a medium performance fibrous filter at various filtration velocities. As a result, fibrous particles were captured by fibrous filter at a higher collection efficiency than the spherical particles with the same mobility. By analyzing the single fiber capturing efficiency, interception incorporating the rotation of fibrous particles is found to be the dominant capturing mechanism for the fibrous particles in the studied size range.     

纤维深层过滤器的最新发展及应用

本文综述了国内外纤维深层过滤器的最新发展，重点介绍了几种新型纤维深层过滤器的结构特点及工业应用。     

Experimental Investigation of Local Efficiency Variation in Fibrous Filters

Classical filtration models consider filter media as homogeneous and use an arbitrary inhomogeneity factor to account for the differences between theory and experiments. The variations in media-packing density have been experimentally observed and seen to be significant. There is minimal experimental information on the effect of these variations on filter performance. In this paper, local efficiency variations in commercial fibrous filters have been obtained for varying operating conditions. A filter scanner was developed to measure local filter efficiencies, and the features of the scanner are described in detail. The variations in local efficiencies are seen to have a Gaussian distribution, and the operating conditions and media behavior influence the variability in the local efficiency data. Theoretical modeling can be used to obtain two-dimensional packing density distribution data from the local efficiency measurements.     

Chemical Synthesis of Monodisperse Magnetic Nanoparticles for Sensitive Cancer Detection

This short tutorial review highlights the advance in high temperature solution phase chemical synthesis of monodisperse magnetic nanoparticles (MNPs), especially iron oxide NPs, as contrast enhancement agents for cancer detection by magnetic resonance imaging (MRI). It introduces briefly the unique nanomagnetism of MNPs required for MRI. It then summarizes some typical methods used to prepare monodisperse Fe₃O₄ and ferrite MFe₂O₄ MNPs from high temperature organic phase reaction with controlled magnetic properties. It further outlines the chemistry used to make these MNPs biocompatible and target-specific. Finally it presents two examples to demonstrate the MNP control achieved from chemical synthesis for sensitive detection of cancer.     

Study on Composition Distribution and Ferromagnetism of Monodisperse FePt Nanoparticles

Monodisperse FePt nanoparticles with size of 4.5 and 6.0 nm were prepared by simultaneous reduction of platinum acetylacetonate and thermal decomposition of iron pentacarbonyl in benzylether. The crystallography structure, size, and composition of the FePt nanoparticles were examined by X-ray diffraction and transmission electron microscopy. Energy dispersive X-ray spectrometry measurements of individual particles indicate a broad compositional distribution in both the 4.5 and 6 nm FePt nanoparticles. The effects of compositional distribution on the phase-transition and magnetic properties of the FePt nanoparticles were investigated.     

Experimental Study on the Loading Characteristics of Needlefelt Filters with Micrometer-Sized Monodisperse Aerosols

In this work, three types of needlefelt filters, made of Polyester (PE), Ryton Sulfar (RS), and Polyaramid (PA), were tested to in- vestigate the aerosol loading characteristics of fabric filters when challenged with micrometer-sized monodisperse potassium sodium tartrate (PST) particles. A fibrous filter with packing density of 9%, thickness of 0.38 mm, and fiber diameter of 5.1 θ m was included for comparison. A vibrating orifice monodisperse aerosol generator was used to produce three different sizes (5, 10, and 20 θ m) of PST particles for aerosol loading experiment. An ultrasonic atomizing nozzle and a TSI constant output nebulizer were used to generate polydisperse PST particles for the aerosol penetration test. The aerosol penetration of submicrometer-sized particles through the filters was measured by using a Scanning Mobility Particle Sizer. An Aerodynamic Particle Sizer was used to measure the penetration fraction of aerosol particles larger than 0.8 θ m. The pressure drop across the filter was monitored by using pressure transducers, which were calibrated against an inclined manometer. Airflows of 5, 10 , 20, and 30 cm/s were used to study the flow dependency. The aerosol penetration results showed that the particles larger than 3 θ m did not penetrate the clean fabric filters tested in the present study. The loading curves (plots of pressure drop against sampling time) displayed three regions: an initial region of fast increase, a transition region, and a final linear region after the dust formation point. After the formation point of the dust cake, both fabric and fibrous filters shared the same slope (of the loading curves). The slope of different regions of the loading curves was determined by many factors, such as size of challenge aerosol, face found to be critical to the performance of the fabric filters. In order lower porosity, which caused an extra rise in pressure drop across velocity, surface treatment, and the compressibility of the dust cake forming on the filter. The method of final surface treatment was to avoid the unnecessary rise in air resistance, the melting clumps formed during final surface treatment should be as thin and narrow as possible, just enough to support the filter bag cleaning. From the standpoint of filter quality and energy consumption, the low filtration velocity has to be adopted whenever possible, because high filtration velocity not only led to lower filter quality (in particular for submicrometer-sized particles) but also created dust cake of lower porosity, which caused an extra rise in pressure drop across thet dust cake.     

单分散纳米二氧化硅微球表面化学修饰与表征

通过溶胶-凝胶法制备纳米二氧化硅微球,以乙醇作为分散介质,用硅烷偶联剂采用一步法对纳米二氧化硅进行了表面化学修饰。通过X-射线光电子能谱（XPS）、透射电子显微镜（TEM）和傅立叶红外光谱仪（FT-IR）等手段对其改性前后效果进行了分析。研究发现修饰后的纳米二氧化硅微球的疏水性增强;硅烷偶联剂与纳米二氧化硅表面羟基发生了化学反应。     

单分散超顺磁性Fe_3O_4纳米粒子的合成及表征

利用高温裂解法制备了油酸修饰的Fe3O4磁性纳米粒子,采用TEM、VSM、XRD以及FT-IR对粒子的结构和性能进行了表征和分析。结果表明:Fe3O4纳米粒子粒径为2nm,呈单分散性,结晶性能良好,室温下为超顺磁性,饱和磁化强度为11.73emu/g,可应用于磁流体、磁性分离等领域。