Solid Particles In Air Research Articles

Extending the CFD-DEM coupling algorithm to accurately predict the particle separation in a two-phase air–solid particle flow through an aerofall AG mill

Extending the CFD-DEM coupling algorithm to accurately predict the particle separation in a two-phase air–solid particle flow through an aerofall AG mill

Characteristics of aerosol optical depth dynamics and their causes over typical cities along the 21st Century Maritime Silk Road.

Atmospheric aerosols, i.e., suspension of liquid and/or solid particles in air, have serious impacts on human health. Exploring the variation and patterns of regional atmospheric aerosols is of great significance to monitor and evaluate atmosphere quality, especially in urban areas with large population. Here, with nine typical pivotal cities along the 21st Century Maritime Silk Road through Southeast Asia, South Asia to West Asia as case studies, based on MCD19A2 550 nm AOD products, combined meteorological factors, land use data, and nighttime light data, we analyzed the spatio-temporal distribution, variation features, influencing and/or driving factors of aerosols in developed urban areas over Asia. The results showed that the descending sequence of the annual aerosol optical depth (AOD) of the nine cities was Karachi, Doha, Chittagong, Bangkok, Colombo, Ho Chi Minh, Singapore, Gwadar, and Yangon during 2013-2018. Due to the influence of regional climate system and atmospheric aerosol types, the time series of annual, seasonal, and monthly AODs were significantly different. The high values of AODs in most cities were mainly located in the urban center or rapid socio-economic (e.g., industrial and agricultural) development regions. The effects of different meteorological factors on the AODs varied in different cities. The rainfall, relative humidity, and wind speed had great impacts on AODs in Ho Chi Minh, Bangkok, Singapore, and Yangon. Temperature, relative humidity, and wind speed had close correlations with AODs in Chittagong, Colombo, Karachi, and Gwadar of South Asia and Doha in West Asia. The urban area's AOD was influenced by the combined and synergistic effects of socio-economy, urbanization, and meteorological factors, with that in Karachi being the most significant.

Contributions to development of an inertial separation system for suspended solid particles in air

The separation of particles with big inertial mass transported in the air, is most often done with the help of cyclones, due to their resistance to wear at contact with abrasive particles. However, pneumatic conveying of heavy and abrasive granular material mixed with fines, are problematic even for cyclone hardened inlets. The paper studies a new method of dividing the main airflow in two airstreams, one further divided in more smaller streams to achieve the laminar regime, by using a corrugated shaped inlet. Thus generating an improvement in impact resistance, overall cyclone grade efficiency and decreasing pressure drop.

Acoustophoretic separation of airborne millimeter-size particles by a Fresnel lens

We numerically demonstrate acoustophoretic separation of spherical solid particles in air by means of an acoustic Fresnel lens. Beside gravitational and drag forces, freely-falling millimeter-size particles experience large acoustic radiation forces around the focus of the lens, where interplay of forces lead to differentiation of particle trajectories with respect to either size or material properties. Due to the strong acoustic field at the focus, radiation force can divert particles with source intensities significantly smaller than those required for acoustic levitation in a standing field. When the lens is designed to have a focal length of 100 mm at 25 kHz, finite-element method simulations reveal a sharp focus with a full-width at half-maximum of 0.5 wavelenghts and a field enhancement of 18 dB. Through numerical calculation of forces and simulation of particle trajectories, we demonstrate size-based separation of acrylic particles at a source sound pressure level of 153 dB such that particles with diameters larger than 0.5 mm are admitted into the central hole, whereas smaller particles are rejected. Besides, efficient separation of particles with similar acoustic properties such as polyethylene, polystyrene and acrylic particles of the same size is also demonstrated.

Streamer branching on clusters of solid particles in air and air bubbles in liquids

We present the results from a two-dimensional computational investigation of the intersection of a streamer with solid particles and density fluctuations and gas bubbles filled with air and immersed in liquids. We consider the evolution of a streamer propagating along the vector of the applied external electric field. The clusters of particles in air or bubbles in liquids have a symmetric form with branches elongated either in horizontal or vertical direction. The orientation of the cluster determines the branching patterns. We show that clusters having the prevailing horizontal branches facilitate streamer branching in liquids, while clusters in gases with vertical branches promote the splitting or reinitiating of a discharge filament. The phenomenon is mainly due to different polarization patterns of vertical or horizontal clusters in air and liquids and the ratio of dielectric permittivity of medium/particle or medium/bubble.

Modeling of solid–gas flow fields via a correlation study

A correlation study for modeling of solid–gas flow fields is presented with an emphasis to the extents of single phase gas flow through stationary vertical packed beds of solid particles and dilute phase flow of suspended solid particles in horizontal channels. The experimental data in terms of local static pressure gradients dP/dx in flow direction are processed. The referred test cases are (i) horizontal air flow with fully suspended fine solid particles of the size range 75.5μm≤dp≤275μm for the magnitudes of superficial air velocity; U»minimum fluidization velocity; Umf, (ii ) air flow through a vertical fixed packed bed of solid particles with dp=6000μm for U«Umf. The visual observations at U=Umf through the vertical beds of solid particles with dp range; 150μm≤dp≤2750μm are also referred.A non-dimensional mode parameter P* is defined using well-known permeability parameter Pf, mixture density ρm and a time parameter t* as an original contribution. The proposed relationships of P* with air flow Froude Number, Fr and solid particle Froude number Frs determine the limits of the flow fields. The magnitude of P* is 1150 for stationary packed solid particles with Fr≤2. Meanwhile P*≤2 corresponds to the dilute phase flow of solid particles in air for Frs≤60. The proposed equations can also be used for the determination of solids velocity Us, U and Umf in the cited limits of Frs, Fr with an acceptable error margin of <±24%.

Agglomerate fragmentation in high-intensity acoustic standing wave fields

The potential of high intensity ultrasound to cause fragmentation of agglomerated solid particles in air is investigated. The dispersion of single functionalized particulate agglomerates was achieved by application of a standing acoustic wave field. The particle trajectories indicate that the levitated agglomerates initially move in a sinusoidal pattern through the standing acoustic field due to their irregular shape. The frequency of the particle movement is orders of magnitude lower than the base frequency of the applied wave field and thus the agglomerates experience alternating external and internal stresses while moving through the field. Specific energy dependent agglomerate breakage probabilities are investigated by variation of the primary particle size, the degree of particle surface functionalization of the primary particles and the humidity of the surrounding gas atmosphere.

Acoustic separation of submicron solid particles in air

The use of ultrasound to continuously separate submicron particles suspended in air is investigated in a rectangular channel with adjustable height. An electrostatic transducer is used to generate a standing wave in the 50–80kHz frequency range and the particles experience forces within the acoustic field causing them to concentrate at the pressure nodes. To assess the effect of several key design parameters on the separation efficiency, a simple method based on light scattering is implemented to provide information on the particle concentrations as a function of position in the channel. The images acquired are processed to yield a separation efficiency metric that is used to evaluate the effect of acoustic, flow and geometrical parameters. The results show that, in qualitative agreement with theoretical models, the maximum separation efficiency increases with the pressure amplitude of the sound wave. The separation efficiency is also linearly proportional to the standing wave frequency, when it is varied between 50–80kHz. On the other hand, the effect of the average fluid velocity is less pronounced than expected, suggesting that in our channel separation is not limited by the interaction length between the acoustic field and the suspended particles. The effect of the parallelism of the reflector relative to the transducer is also investigated.

Imaging particles in solid/air flows using an optical tomography system based on complementary metal oxide semiconductor area image sensors

Particle characterization has become important in the pharmaceutical, food, paint and chemical industries, among others. For example, particle size and shape can affect the taste, texture, and appearance of food. Particle characterization has become more significant for on-line production control because it influences product quality. Moreover, the images of particles generated from on-line measurements can better characterize the process itself. This paper discusses the development of an optical tomography system based on a complementary metal oxide semiconductor (CMOS) area image sensor, which is used for imaging particles in solid/air flows. The system consists of a lighting system, a measurement system and a data acquisition system. The CMOS area image sensor has a pixel size of 5.2μm×5.2μm, which provides a resolution of 1280×1024 pixels. A two-projection system is adopted to enable image reconstruction; in this system, two CMOS area image sensors are configured around a pipe, and a light source is placed on the opposite side of the pipe. When the light illuminates through the pipe, the shadows of any particles in the pipe will be cast onto the CMOS area image sensors. Four experiments on solid particles in air under different conditions are performed. The output of the CMOS area image sensor is processed to reconstruct the tomographic image. The results indicate that CMOS based optical tomography is capable of imaging particles in solid/air flows.

Separation of small solid particles based on ultrasonic rotary driving

In this work, we proposed and developed an ultrasonic actuator which can be used to separate mixed small solid particles in air. The device consists of an ultrasonic sandwich-type transducer and an aluminum plate which is extended from an end plate of the transducer. The aluminum plate is asymmetrically excited by the transducer, to vibrate flexurally. The transducer operates in the thickness mode and has a resonance frequency of 91.7kHz. Particle mixtures used in the experiments are obtained by the combination of any two particles among shrimp eggs, glass balls and salt particles. The experiments showed that the glass balls could be separated from a mixture of glass balls and shrimp eggs, and salt particles separated from a mixture of salt particles and shrimp eggs. The device used in the method has no mechanical rotary part and the separation can be quickly completed.

Optical trapping and Raman spectroscopy of solid particles.

The heterogeneous interactions of gas molecules on solid particles are crucial in many areas of science, engineering and technology. Such interactions play a critical role in atmospheric chemistry and in heterogeneous catalysis, a key technology in the energy and chemical industries. Investigating heterogeneous interactions upon single levitated particles can provide significant insight into these important processes. Various methodologies exist for levitating micron sized particles including: optical, electrical and acoustic techniques. Prior to this study, the optical levitation of solid micron scale particles has proved difficult to achieve over timescales relevant to the above applications. In this work, a new vertically configured counter propagating dual beam optical trap was optimized to levitate a range of solid particles in air. Silica (SiO2), α-alumina (Al2O3), titania (TiO2) and polystyrene were stably trapped with a high trapping efficiency (Q = 0.42). The longest stable trapping experiment was conducted continuously for 24 hours, and there are no obvious constraints on trapping time beyond this period. Therefore, the methodology described in this paper should be of major benefit to various research communities. The strength of the new technique is demonstrated by the simultaneous levitation and spectroscopic interrogation of silica particles by Raman spectroscopy. In particular, the adsorption of water upon silica was investigated under controlled relative humidity environments. Furthermore, the collision and coagulation behaviour of silica particles with microdroplets of sulphuric acid was followed using both optical imaging and Raman spectroscopy.

Environmental exposure assessment of engineered nanoparticles: Why REACH needs adjustment

Engineered nanomaterials (ENMs) possess novel properties making them attractive for application in a wide spectrum of fields. These novel properties are not accounted for in the environmental risk assessment methods that the Registration, Evaluation, Authorization, and Restriction of Chemicals (REACH) proposes in their guidance on environmental exposure estimation, although ENMs are already applied in a variety of consumer and industrial products. It is thus necessary to evaluate the guidance document REACH provides on environmental exposure estimation on its applicability to ENMs. This is most urgently the case for engineered nanoparticles (ENPs), as the novel properties are most often only applicable to them. The environmental fate of ENPs was reviewed and compared to the environmental fate of chemicals according to the REACH guidance. Major deviations between the fate of ENPs and predicted fate by REACH were found. They were related to at least 1 of 3 major assumptions made in REACH guidance: 1) in REACH, environmental alteration processes are all thought of as removal processes, whereas alterations of ENPs in the environment may greatly affect their properties, environmental effects, and behavior, 2) in REACH, chemicals are supposed to dissolve instantaneously and completely on release into the environment, whereas ENPs should be treated as nondissolved nanosized solids, and 3) in REACH, partitioning of dissolved chemicals to solid particles in air, water, and soil is estimated with thermodynamic equilibrium coefficients, but in the case of ENPs thermodynamic equilibrium between "dispersed" and "attached" states is generally not expected. The environmental exposure assessment of REACH therefore needs adjustment to cover the specific environmental fate of ENPs. Incorporation of the specific environmental fate processes of ENPs into the environmental risk assessment framework of REACH requires a pragmatic approach.

Determining the unique refractive index properties of solid polystyrene aerosol using broadband Mie scattering from optically trapped beads

A method is described to measure the refractive index dispersion with wavelength of optically trapped solid particles in air. Knowledge of the refraction properties of solid particles is critical for the study of aerosol; both in the laboratory and in the atmosphere for climate studies. Single micron-sized polystyrene beads were optically trapped in air using a vertically aligned counter-propagating configuration of focussed laser beams. Each bead was illuminated using white light from a broadband light emitting diode (LED) and elastic scattering within the bead was collected onto a spectrograph. The resulting Mie spectra were analysed to accurately determine polystyrene bead radii to ±0.4 nm and values of the refractive index to ±0.0005 over a wavelength range of 480-700 nm. We demonstrate that optical trapping combined with elastic scattering can be used to both accurately size polystyrene beads suspended in air and determine their wavelength dependent refractive index. The refractive index dispersions are in close agreement with reported values for polystyrene beads in aqueous dispersion. Our results also demonstrate a variation in the refractive index of polystyrene, from bead to bead, in a commercial sample. The measured variation highlights that care must be taken when using polystyrene beads as a calibration aerosol.

High-Temperature Erosion Resistance of FeBSiNb Amorphous Coatings Deposited by Arc Spraying for Boiler Applications

Erosive high-temperature wear in boilers is one of the main causes of downtime and one of the principal engineering problems in these installations. This article discusses the use of FeBSiNb amorphous coatings synthesized by arc spraying to improve elevated-temperature erosion resistance for boiler applications. The influence of test temperature, velocity, and impact angle on material wastage was revealed using air solid particle erosion rig. The experimental results showed that moderate degradation of the coating was predominant at lower impact velocity and impact angles, while severe damage arose for higher velocities and impact angles. The erosion behavior of the coating was sensitive to test temperature. The erosion rates of the coating decreased as a function of environment temperature. The relationship between microstructure and erosion resistance of the coating was also analyzed in details. The FeBSiNb coating had excellent elevated-temperature erosion resistance at temperatures at least up to 600 °C during service.

The Progress in Electron Microscopy Studies of Particulate Matters to Be Used as a Standard Monitoring Method for Air Dust Pollution

The present article reviews studies on air solid particles carried out with the use of electron microscopy. Particle analysis combining scanning and transmission electron microscopy (SEM and TEM) can be used to derive size-resolved information of the composition, mixing state, morphology, and complex refractive index of atmospheric aerosol particles. It seems that electron microscopy is more widely used in atmospheric particulate matter analysis, but the usage of this method is sometimes problematic. First of all, there is no standard methodology adapted for dust analysis, and it is difficult to compare the results coming from different laboratories. Nevertheless, it was shown that this method has potential to be used in the future as a standard monitoring procedure of air solid particles.

Erosion properties of Fe based amorphous/nanocrystalline coatings prepared by wire arc spraying process

Fe–Cr–B–Si–Mn–Nb–Y and Fe–B–Si–Nb–Cr amorphous/nanocrystalline coatings were fabricated by wire arc spray process. The microstructure and phase compositions of the coatings were characterised by means of SEM, EDAX and XRD. The formation mechanism of amorphous phase and nanocrystalline phase was discussed. The elevated temperature erosion behaviour of the coatings was also investigated using an air solid particle erosion rig. The microstructures of the coatings consist of amorphous phase and α-(Fe,Cr) nanocrystalline phase. Both coatings are fully dense with low porosity. The coatings exhibit better erosion resistance at lower impact angles. With increasing erosion temperature, the erosion rates of the coatings decrease. The Fe–B–Si–Nb–Cr coating has better elevated temperature erosion resistance than that of the Fe–Cr–B–Si–Mn–Nb–Y coating.

Tribology of thermal sprayed WC–Co coatings

This paper looks at the tribology of thermal sprayed WC–Co based coatings and covers the high energy air–sand erosion resistance and slurry jet impingement erosion performance, dry and wet sliding tribology of thermal spray WC–Co based coatings as well as the abrasion and abrasion–corrosion of these coatings. The tribological and tribo-corrosion performance of the coatings will be related to their mechanical and corrosion properties as well as deposition parameters, microstructure and actual composition. For example, the anisotropic microstructure of thermally sprayed WC–Co–Cr coatings, in particular the low fracture toughness in a direction parallel to the substrate, has been observed to affect the nature of crack formation under 200 μJ air–solid particle erosion conditions. Voids and occasionally other microstructural features (i.e., cobalt lakes, splat boundaries, interfacial inclusions) in the coating act as crack initiation sites. The erosion rate was dominated by cracks within 5 μm of the surface and was relatively insensitive to total length of cracks, showing a near-surface damage front controls the erosion rate and this region is coincident with the region of maximum shear stress induced by erodent impacts. Optimisation of the deposition parameters of HVOF 86WC–10Co–4Cr coatings show an improvement in erosion resistance of more than 50% over the conventional D-gun applied coating of identical nominal composition. The variation in the slurry erosion performance of the thermally sprayed coatings is also linked to directional fracture toughness and crack propagation paths which are influenced by the presence of pores, inhomogeneous carbide distributions and substrate grit blast remnants. The influence of slurry jet angle is more pronounced under 0.4 μJ energy conditions where maximum erosion occurred at 90° and the minimum at 30° in contrast to 7 μJ slurry erosion rates which were independent of jet angle. This reflects the lower levels of fluctuating stresses imparted to the coating during low energy slurry impacts leading to the impact angle having a greater effect on sub critical crack growth rate than for higher energy conditions. The abrasion resistance of these coatings was found comparable to sintered cermets of the same composition. The synergistic effects between micro and macro abrasion and corrosion for detonation gun (D-gun) sprayed WC–10Co–4Cr coatings are shown to be significant and depend on the environment. The size effect of the abradant relative to the microstructure and splat size is important as well as the propensity for the various phases to passivate to control corrosion levels. Comparisons between exposed and freshly polished coating surfaces in strong NaOH solutions (pH 11) show that significantly lower wear rates were seen for the exposed surface due to a negative wear–corrosion synergy due to selective phase removal and the effects of localised passivation. Dry and wet sliding wear resistance of these coatings is shown to be high (wear rates of 10−16–10−18 m3/Nm) with modest coefficient of friction levels between 0.2 and 0.5. The presence of oxides on the binder phases appears to influence the friction and wear levels. Wear appears to be by carbide ejection and/or by tribo-chemical processes.

Analytical prediction of pressure loss through a sudden-expansion in two-phase pneumatic conveying lines

Estimation of separation or minor pressure losses for pipe fittings of a pneumatic conveying system at design stage is critical as much as determination of frictional pressure losses through it. The flow in many pneumatic conveying systems is a two-phase flow; it is so complex and difficult to be investigated by experimental techniques. The static pressure recovery and the minor loss coefficient through an axis-symmetric, circular cross-section, sudden-expansion fitting of a horizontal pneumatic conveying line with air–solid particle flow are analytically studied. The theoretical models proposed in the literature are scarce, and do not confirm the experimental studies. The well-known homogeneous and separated flow models proposed in the literature are initially applied to the case by means of mass and momentum conservation laws. The predictions of both the models on the pressure recovery were compared with the experimental and the numerical data in the literature and a bad agreement was observed between them; therefore, a new original analytical model is proposed by the present study. The new model is called as the slip flow model, which takes into account the slip velocity between gas and solid phases evaluated by coupling the well-known separated flow model with the empirical slip ratio predictions in the literature. The predictions of the proposed slip flow model on both the pressure recovery and minor loss coefficient are found in good agreement with the corresponding data in the literature.

Erosion behavior of EEDS cermet coatings

This paper investigates the erosion performance of electro-thermal explosion directional spraying (EEDS) cermet WC/Co coatings using an air solid particle erosion rig. The influences of the different parameters such as impact angle, impingement velocity, environment temperature, particle diameter, on the erosion property of the coatings were studied. The eroded surfaces were examined by scanning electron microscopy (SEM) equipped with energy dispersive spectrometry (EDS) and the erosion mechanisms were discussed. The structure and bond characters of the coatings were also determined by transmission electron microscopy (TEM), X-ray diffraction (XRD) and SEM. The results indicate that the EEDS WC/Co coatings are characterized by fine grain structure, good metallurgical bond and brittle erosion character. The erosion rates of the coatings decrease with temperature increasing and increase with impact angle and impingement velocity increasing. At elevated temperature, the oxidation happens on the coatings surface, which affects the erosion behavior.

Extraction of biologic particles by pumping effect in a π-shaped ultrasonic actuator

This paper presents a new method of extracting biologic particles from a mixture of particles. The method is based on the pumping effect in a π-shaped ultrasonic actuator, which has a gap between its two vibrating metal plates. An adhesive tape is placed at a proper position in the gap. Due to the pumping effect which is induced by the sound field in the gap, the particles with smaller mass and radius in the mixture can be pumped up to reach the adhesive tape; while the ones with larger mass cannot. Therefore, the particles with smaller mass and radius can be extracted from the mixture. A theoretical model which can well explain the operation principle and experimental phenomena is developed. By the experimental results and the theoretical analyses based on the model, the validity of the method in extracting small particles from a mixture of solid particles in air is confirmed, and the effects of the actuator’s vibration, adhesive tape height, contents of the mixture and viscosity of fluid on the extraction are clarified. Also, it is theoretically predicted that the method will work under the microgravity condition in air.