Influence of Particle Shape on Filtration Processes

The influence of particle shape on filtration processes was investigated. Two types of particles, including spherical polystyrene latex (PSL) and iron oxide, and perfect cubes of magnesium oxide, were examined. It was found that the removal efficiency of spherical particles on fibrous filters is very similar for corresponding sizes within the range of 50–300 nm, regardless of the fact that the densities of PSL and iron oxide differ by a factor of five. On the other hand, the removal efficiency of magnesium oxide cubic particles was measured, and found to be much lower than the removal efficiency for the aerodynamically similar spheres. Such disparity was ascribed to the different nature of the motion of the spherical and cubic particles along the fiber surface, following the initial collision. After touching the fiber surface and before coming to rest, the spherical particles could either slide or roll compared to the cubic ones, which could either slide or tumble. During tumbling, the area of contact between the particle and the fiber changes significantly, thus affecting the bounce probability, whilst for the spheres, the area of contact remains the same for any point of the particle trajectory. The extra probability of particle bounce by the cubes was derived from the experimental data. The particle kinetic energy was proposed to be responsible for the difference in removal efficiency of particles with alternative shapes, if all other process parameters remain the same. The increase in kinetic energy is shown to favor the increase of the bounce probability.     

Filter efficiency as a function of nanoparticle velocity and shape

The filtration efficiency of a conventional fibrous filter was investigated with particular emphasis on the removal of particles with different shapes. A previous study has shown that particles of spherical shape are removed from the gas carrier with efficiencies which are higher when compared to cubic particles of the same aerodynamic size. In this project, to challenge our previously made explanation, spherical PSL and cubic MgO particles were tested along with particles of sodium chloride (NaCl) of intermediate shape (cubic particles with rounded edges) at a range of filtration velocities from 5 to 20 cm/s. It was found that particles of NaCl are removed with efficiencies lower then those for PSL particles but higher than the efficiencies for cubic particles of MgO, at the lowest filtration velocity when inertial effects are negligible. The rounded NaCl particles, depending on the geometry of the contact, could either land on the rounded corner and hence roll, land on a sharp edge and hence tumble, or slide. This range of options alters the probability of detachment of the particle. The difference between the filter efficiencies for cubic MgO particles and intermediate shaped NaCl particles is decreasing with the increase in velocity. With increasing velocity, the filtration efficiency of the cubic MgO particles, exceeds the filtration efficiency for the intermediate shaped NaCl particles, due to the dominating inertial effects of the denser, and hence heavier, MgO particles. This paper shows the results of these experiments and, we hope, will ignite the interest of the aerosol community towards further theoretical analysis of the phenomenon.     

Particle Bounce During Filtration of Particles on Wet and Dry Filters

This paper experimentally examines the bounce and immediate re-entrainment of liquid and solid monodisperse aerosols under a stable filtration regime (precake formation) by wet and dry fibrous filters. PSL and DEHS were the solid and liquid aerosols, respectively, used in four monodisperse sizes of 0.52, 0.83, 1.50, and 3.00 w m. Three different fibrous filters were used to filter the aerosol streams, and the efficiency of the filtration process for each aerosol type under dry and wet regimes was measured. It was found that the solid particles generally exhibited a lower fractional filtration efficiency than liquid particles, although this difference decreased in the smaller size fractions. The difference between solid and liquid efficiencies was found to be greatest in the 1.5 w m size range. As particle sizes of liquid/solid aerosols and filtration parameters were similar, this difference is most likely to be due to the effect of particle bounce and or immediate re-entrainment occurring inside the filter, with the greater efficiency of filtration of the liquid particles being due to their greater capacity to plastically/elastically deform in order to absorb the impact forces. However, for the wet filtration regime (each fibre of the filter was coated by a film of water), no significant difference in filtration efficiency was detectable between solid and liquid aerosols. Therefore, the conclusion can be drawn that the either the bounce effect of the particles is inhibited by the liquid film, or the filtration conditions in the wet filter are so different that the aerosol properties are less significant with respect to capture.     

Unstable Neck Formation during Initial-Stage Sintering

By observing pairs of single-crystal cubic-faceted small particles of MgO in contact along faces, edges, and corners, it was possible to determine the effects of contact geometry on the stability of neck regions formed during the initial stages of sintering. Changes in the neck morphology were monitored during the sintering process by continuous TEM/ video observations of particles heated in situ between 1100° and 1260°C. Particles in face-to-face contact were found to coalesce in the classic manner to form a single particle, Particles in contact along either cube corners or edges, however, developed unstable necks that initially grew, then shrunk, and eventually broke so that such particle pairs effectively "de-sintered."     

DEM simulations of spherical particle–particle collisions

The randomness, diversity, and complexity of the high-speed particle crushing process bring great difficulties to the theoretical analysis of powder engineering. In this paper, the discrete element method is used to simulate the collision of spherical particles, which provides a reference for studying the process and mechanism of crushing between particles under impact load. The Hertz–Mindlin with bonded contact model is used as the particle–particle contact model. The central collisions of particles with different diameter ratios under different high-speed motions and the eccentric collisions with different eccentricities are discussed. The results show that the bond damage increases with the increase of relative velocity in both centre impact and eccentric impact. In centre collisions, particles of smaller objects are more fragmented than particles of larger objects. For smaller target particles, the larger the diameter ratio is, the more particle elements are detached from the target particles, and the greater the bond breakage rate. For larger target particles, the larger the diameter ratio is, the less the particle element falls off and the smaller the bond breakage rate. This provides guidance for the collision and crushing of particles with different particle size ratios and different eccentricities during high-speed motion in engineering applications in the future.     

Characterisation of hydrodynamics induced by air injection related to membrane fouling behaviour

The study focuses on fine bubble and spherical cap bubble injection in case of outside/in fibres immersed in a tank. The objectives are to quantify liquid circulations and shear stresses along the membrane and to understand their effects on the fouling resistance. Thus, both filtration experiments and hydrodynamic characterisation were performed in the same aeration conditions. Only particle cake deposit was studied as fouling mode and the hydrodynamics was characterized experimentally by 2-phase flow particle image velocimetry (PIV) and numerically with the CFD code FLUENT. Results presented in this paper are limited to tight hollow-fibres to well understand hydrodynamics without fibre motion. One important result is that mean values of wall shear stress are very low (maximum 0.25 Pa) eliminating the wall shear stress as a mechanism able to explain filtration performances for a non-confined aeration without fibre displacement. In addition, an analysis has been conducted in terms of horizontal liquid flow toward the membrane. The quantification of this flow allowed to conclude that i) for a given local bubble flow rate, there is no influence of the kind of aeration on this horizontal flow rate; and ii) the filtration performances can be correlated with this horizontal flow. The reason might be an over concentration of particles near the membrane surface induced by this flow.     

Influence of Particle Characteristics on Filter Ripening

Abstract

To investigate the dependence of filter ripening on particle size and surface charge, multiple experiments were conducted under different particle destabilization conditions including pH control, alum, and polymer destabilization. Laboratory‐scale filtration experiments were performed at a filtration velocity of 5 m/h using spherical glass beads with mean diameter of 0.55 mm as collectors. Particle suspensions with a broad size distribution and a 1.7 µm mean particle size were filtered through a 10 cm depth filter column. Better initial solids removal was confirmed under favorable particle and collector conditions (i.e., under smaller surface charge), but better initial particle removal does not necessarily mean better overall particle removal efficiency. It was shown that changes of the particle size distribution (PSD) in the effluent can significantly influence overall particle removal efficiency. Chemical parameters such as zeta potential can be important during the initial stage of filtration, but their importance can decrease over time depending on the specific chemical conditions. The influent PSD and the removal of certain size particles during the initial stage of filtration can significantly influence ripening, which in turn, can influence the overall particle removal efficiency.     

Flotation behaviour of fine particles with respect to contact angle

The flotation behaviour of methylated quartz particles of different size, but within the size range from 0.2 to 50 μm, and varying contact angle, was probed in a mechanical flotation cell. Results suggest that particles of a given size need to possess a minimum critical contact angle (θ_crit) for flotation to occur. This behaviour is shown not to be solely dependent on fine particles having lower collision efficiency with bubbles, but rather due to a combination of low collision efficiency and particles not having enough kinetic energy at collision with bubbles to form the three phase line of contact and initiate the attachment process. In the particle size range investigated, the critical contact angle increases with a decrease in particle size.     

撞击流中颗粒旋转特性

搭建了研究撞击流中颗粒旋转特性的气固两相撞击流实验台,使用高速摄像机拍摄一个截面为0.15 m×0.08 m的撞击区域内固体颗粒的运动。利用所搭建的实验台设置了单喷口和双喷口两种实验方式来研究颗粒旋转影响因素,得出撞击流内颗粒的旋转特性。结果表明:固体颗粒在气相中运动过程一直伴随着其自身的旋转;气相场对颗粒转速的影响较小,可忽略不计;相同实验条件下,颗粒直径越小其转速越大;颗粒以及气相速度越大,则固体颗粒在碰撞后的转速越大,当加速气相速度为25 m·s^-1,氧化铝陶瓷直径为0.003 m时,颗粒碰撞前后转速差平均值可达280 r·s^-1;颗粒间碰撞过程中,颗粒相对运动偏置角度对转速变化影响很小。     

Aerosol measurement by laser doppler spectroscopy—I. Theory and experimental results for aerosols homogeneous

The basic theory, experimental techniques and results are presented describing a technique for sizing aerosol particles in situ using laser Doppler spectroscopy. Unlike conventional light scattering procedures which use average intensity information, this technique utilizes the Doppler shifted frequency of the scattered light produced by the Brownian motion of the aerosol particles to determine particle diffusion coefficients and size. Experiments were carried out using monodisperse dibutylpthalate aerosols and monodisperse polystyrene latex spheres, in concentrations ranging from 10³ to 10⁶ particles per cubic centimeter. Measured particle sizes were within 10 per cent of the size predicted by conventional light scattering methods for the DBP particles and the reported sizes of the PSL particles. Based on these results it is concluded that laser Doppler spectroscopy can be utilized to accurately measure aerosol particle size in situ.     

A High Resolution Study of the Effect of Morphology On the Mass Spectra of Single PSL Particles with Na-containing Layers and Nodules

The interpretation and quantification of measurements of particle composition by laser ablation based single particle mass spectrometry is complex. Among the most difficult systems to quantify are internally mixed particles containing alkali metals and organics. The alkali atoms in such particles tend to suppress the formation of other ions sometimes to below the detection limit. Here we present a study of the behavior of single particle mass spectral peak intensities as a function of the amount of the sodium containing compounds deposited on the surface of 240 nm polystyrene latex (PSL) spheres. We generate three morphologically distinct and well defined coating types: uniform layers, cubic nodules and rounded nodules, and measure the individual particle mass spectra as a function of the vacuum aerodynamic diameter with nanometer resolution. The data show that the probability of detecting the PSL spheres depends on the amount of the alkali metal on the PSL sphere surface, its morphological distribution and the ablation laser power. The data suggest that PSL spheres with localized Na-containing nodules are easier to detect than those which are completely encapsulated. We show, for example, that at low laser power, PSL particles that are completely encapsulated with Na-containing compounds, whose weight fraction is close to 50%, cannot be detected, while 35% of PSL spheres with same amount of coating can be detected if coating is localized in nodules on a fraction of the particle surface.     

Possible Explanations of Transient Neck Formation between Pairs of (100) Faceted Particles

Rankin and Boatner have observed that (100) faceted MgO particles in contact along either corners or edges developed, on heating, necks that initially grew, but then shrank and broke. General thermodynamic models are provided that predict this transient neck formation for any cubic particles that share only a small fraction of an edge and for particles that share an edge that is less than 0.6 times as long as the orthogonal edges. A third model, which assumes that reaction of MgO particles with their carbon substrate removes MgO from the MgO–carbon interface at a constant rate, explains the observation that the neck shrinkage rate greatly exceeds the neck growth rate. Application of the theory to explaining the seeming prevalence of rounded surfaces in sintering powders is described.     

Sieving of aerosol particles with metal screens

Metal screens with uniform micrometer-sized opening were employed to sieve aerosol particles by suppressing the adhesion of particles smaller than the openings. The collection efficiencies of monodispersed polystyrene latex (PSL) particles were experimentally determined using the metal screens with 1.2, 1.8, 2.5, and 4.2 μm openings at various filtration velocities. The particles smaller than the mesh opening adhered on the metal screen at a low filtration velocity, but the bounce-off of particles on the mesh surface suppressed the adhesion at a high velocity. As a result, we found that the adhesion of PSL particles larger than 0.3 μm mostly suppressed at a filtration velocity higher than 10 m s^?1 and therefore we can sieve aerosol particles according to the opening size of metal screens. We also found that the particle number concentration could be determined by measuring the increase in pressure drop since the clogging of metal screen openings takes place by the individual particles.

© 2016 American Association for Aerosol Research     

Distinguishing Between Spherical and Nonspherical Particles by Measuring the Variability in Azimuthal Light Scattering

Azimuthal variabilities in scattering of monochromatic, circularly polarized light by individual spherical and nonspherical particles were measured using the DAWN-A (Wyatt et al. Appl. Opt. 27:2405–2421, 1988) differential light scattering detector. Measured aerosols included polystyrene latex spheres (PSL), quartz, and sodium chloride particles of 0.576, 0.741, 0.966, and 1.250 μm diameter. Signals from eight detectors at different azimuthal angles at a polar angle of 55° showed that variabilities for nonspherical particles significantly exceeded values for the spherical PSL. The probability that a quartz or sodium chloride particle would be incorrectly identified as a sphere are less than about 5% for all sizes investigated.     

Numerical simulation on mixing kinetics of slender particles in a rotary dryer

The mixing processes of slender particles in a rotary dryer fitted with lifters were simulated in three dimensions. Particle motion was modeled by the Discrete Element Method (DEM) and a three dimensional collision model for slender particles was developed. Contact force, friction force and gravitational force acting on an individual slender particle were considered when establishing mathematics models. The influences of rotational velocity on the mixing of slender particles were discussed and compared with those of spherical particles under identical operating conditions. It was found that the mixing characteristics of slender particles and spherical particles all followed a constant rate until a completely mixed state was encountered. But there were still certain differences between these two kinds of particles. The influences of the lifters with different shapes were further discussed for slender particles. Selected stimulation results were obtained and would provide consults for the further study of slender particles.     

An Early Stage of Aerosol Particle Transport in Flows Past Periodic Arrays of Clear Staggered Obstructions: A Computational Study

Transport of aerosol particles is a fundamental phenomenon in many environmental and industrial applications. Among the several computational fluid dynamical schemes used to study this problem, the lattice Boltzmann methods (LBMs) have shown great promise. Using a 2-D LBM model coupled with a Lagrangian formalism, this study investigates an early stage of particle–surface collisions in a free-stream flow over a semi-infinite array of staggered obstructions at operating conditions of woven-wire screens. After an initial validation of the model, the particle–surface collision efficiency with different diameters is then examined as functions of the number of staggered obstructions, obstruction morphology, and separation distance between two center points of obstructions. Particle motion mechanisms include drag, lift, and Brownian forces. Enhanced collision efficiency of particles to obstructions due to the presence of multiple staggered cylindrical obstructions is identified and highlighted. Based on these insights, our work also explores the possibility that collision efficiency of particles to obstructions can be enhanced by a change of obstruction morphology. Finally, the results highlight the range where particle–surface collision efficiency is sensitive to the longitudinal spacing between two cylindrical obstructions.

Copyright 2014 American Association for Aerosol Research     

Migration and Deposition of Submicron Particles in Crossflow Microfiltration

Abstract

The migration and deposition of submicron particles in laminar crossflow microfiltration is simulated by integrating the Langevin equation. The effects of operating conditions on the particle trajectories are discussed. It is found that the Brownian motion of particles plays an important role in particle migration under a smaller crossflow velocity of suspension or a smaller filtration rate. Based on the simulated trajectories of particles, the transported flux of particles arriving at the membrane surface can be estimated. The particle flux increases with an increase of filtration rate and with a decrease of particle diameter; however, the effect of crossflow velocity on the particle flux is not obvious. The forces exerted on particles are analyzed to estimate the probability of particle deposition on the membrane surface. The probability of particle deposition increases with an increase of filtration rate, with a decrease of crossflow velocity, with a decrease of particle diameter, or with an increase of zeta potential on the particle surfaces. The simulated results of packing structures of particles on the membrane surface at the initial stage of filtration show that a looser packing can be found under a larger crossflow velocity, a smaller filtration rate, or a smaller diameter of filtered particles. Crossflow micro-filtration experiments are carried out to demonstrate the reliability of the proposed theory. The deviation between the predicted and experimental data of filtration rate at the initial period of filtration is less than 10% when the Reynolds number of the suspension flow ranges from 100 to 500.     

润湿剂促进燃油细颗粒捕集的实验研究

在反应室中研究了雾化液滴与燃油细颗粒的相互作用机制。提出了一种润湿剂对细颗粒捕集促进的新方法,用于研究颗粒与液滴作用前后颗粒数浓度和粒径分布变化特性及细颗粒的捕集特性。采用电称低压冲击器（ELPI）在线测试分析颗粒数浓度和粒径分布特性,用SEM和EDS分析了颗粒的形态和元素组分对捕集机理的影响。结果表明：颗粒的物化特性与液滴对颗粒的捕集作用密切相关;燃油排放颗粒主要为球形含炭疏水颗粒;雾化液滴对其的捕集效率较低;添加润湿剂有利于提高液滴对燃油细颗粒的捕集效率,添加Silanol w22比用纯水液滴对颗粒的捕集效率提高了15%;不同润湿剂对捕集效率的促进作用差别较大。研究结果可以用于对不同润湿剂除尘效果进行测试和筛选。     

逆向-同向流气浮工艺构建与运行特性

针对传统溶气气浮（DAF）工艺中气泡对絮体颗粒捕集、黏附效率低,泡絮体黏附不稳定等问题,基于泡絮碰撞黏附理论,研发了集逆向流与同向流于一体的DAF工艺（CCDAF）。该工艺溶气水分两次投加,接触室分为碰撞接触室和黏附接触室。试验结果表明,CCDAF工艺显著提高了泡絮黏附效率和泡絮体稳定性,对浊度、藻类平均去除率达到96.4%、96.50%,出水颗粒物以2~7 μm粒径为主。工艺主要去除大分子、疏水性有机物,COD_Mn、UV₂₅₄、DOC平均去除率分别达到37.6%、46.3%和32.11%,CCDAF比同向流及逆向流DAF除污染效能更加显著。泡絮黏附机理分析表明,CCDAF工艺逆向流碰撞区主导作用机制为碰撞黏附及共聚作用,同向流接触区为碰撞黏附及网捕、包卷和架桥作用。     

The Collision of Nonspherical Aerosol Particles with Small Evaporating Water Drops

Studies in the collision efficiencies of aerosol particles with water drops have generally assumed that the particles are spherical, even though ambient aerosol particles may tend to be nonspherical. The present theoretical study estimates the effects of nonspherical particle rotation in shear flow and shape dependency of the thermophoretic force in the computation of collision efficiencies of small evaporating water droplets. The results obtained show considerable differences between collision efficiencies of spherical and nonspherical particles of equivalent Stokes radii.