Dilute Aerosols Research Articles

Analysis of electrodynamic fluidization

Electrodynamic fluidization is a technique to generate suspensions of electrically conducting particles using electric forces to overcome their weight. An analysis of electrodynamic fluidization is presented for a monodisperse aerosol of non-coalescing particles of infinite electrical conductivity and negligible inertia suspended in a gas in the gap between two horizontal plate electrodes. A DC voltage is applied between the electrodes that charges the particles initially deposited on the lower electrode and leads to a vertical electric force that lifts the particles and pushes them upwards across the gap. The direction of this force reverses when the particles reach the upper electrode, pushing them downwards until they fall onto the lower electrode and repeat the cycle. Stationary distributions of particles are computed for given values of the applied voltage and the number of suspended particles per unit electrode area. Interparticle collisions play a role when the second of these parameters is of the order of the inverse of the particle cross-section or larger. The electric field induced by the charge of the particles opposes the field due to the applied voltage at the lower electrode and thus sets an upper bound to the number of particles that can be suspended for a given voltage. This bound is attained in the normal operation of a fluidization device, in which there is an excess of particles deposited at the lower electrode, and is computed as a function of the applied voltage. The predictions are compared to experimental results in the literature. A linear stability analysis for dilute aerosols with negligible collision effects shows that the stationary solution becomes unstable when the deposition threshold is approached with a number of suspended particles per unit electrode area larger than a certain critical value. A hydrodynamic instability appears near the lower electrode, where the electric force on a localized accumulation of charged particles leads to an upward gas flow that helps carrying the particles away from the electrode and increases the amplitude of the initial particle accumulation. The instability gives rise to electrohydrodynamic plumes whose dynamics involves collisions, mergers and generation of new plumes.

Numerical investigation of dilute aerosol particle transport and deposition in oscillating multi-cylinder obstructions

The transport and deposition of aerosol particles through a fibrous filter is encountered in many natural and industrial processes. As the filtration performance for a stationary filter has been extensively studied in the literature, the present work focuses on the effect of fiber oscillation in a filter where the fibers are allowed to vibrate periodically. The transport and deposition of dilute aerosol particles in such a system is simulated using an efficient numerical model, where an iterative immersed-boundary lattice Boltzmann method is applied to solve the background flow with finite-size moving fibers, and the motion of aerosol particles is then tracked by a one-way coupling Lagrangian approach. In the present scheme, the no-slip boundary condition at the fiber surface can be exactly enforced with an iterative approach and the numerical stability is improved by adopting the MRT collision model. After the model validation in the two special cases of flow over an oscillating fiber in a quiescent fluid and particle capture by a stationary fiber, the filtration performance of an oscillating multi-fiber filter is investigated to study the effects of fiber number, arrangement and vibration mode. It is found that the oscillating motion of fiber has significant influence on the filtration performance. For a single fiber, with larger oscillation amplitude, the distribution ranges of the release position and impact angle of captured particles both increase. On the other hand, a larger fiber oscillation frequency tends to reduce the width of release position but increase the width of impact angle of deposited particles. Furthermore, the collection efficiency is found to be linearly related to the oscillation amplitude or frequency. For multiple fibers, the collection efficiency always increases with larger fiber number, but it is a non-monotonic function of the arrangement parameters, i.e., the longitudinal and transverse spacings, and the vibration parameters such as the amplitude, frequency and vibration mode. It is interesting to find that the in-phase mode can usually lead to excellent collection efficiency.

Design and pharmaceutical applications of a low-flow-rate single-nozzle impactor

A new low-flow-rate (0.5L/min) single-nozzle impactor for the concentration of dilute aerosol particles with selected pharmaceutical applications is described in this paper. The impactor can be configured up to 11 stages with a wide range of cutoff diameters from 0.6μm to 21.1μm, enabling convenient sampling of inhalable drug particles from inhalation devices and drug production processes. Its unique single-nozzle design and removable impaction plate allow direct sample transfer for subsequent compositional, morphological, solid-state, and other analysis. Agreement between the measured size distribution of fluticasone propionate particles actuated from commercial pMDI Flixotide® 250 Evohaler® and reported data in the literature verified that the impactor stages have accurate cutoff diameters as designed. The multi-stage configuration of the impactor allows rapid separation of polydisperse aerosol particles into different size classes for further characterization. Overlapping of the Raman spectra of the double-component powders from the Seretide® 250 pMDI collected using two different methods demonstrated the applicability of the impactor for a representative sampling of multi-component aerosol particles for bulk composition analysis. A time-dependent and size-dependent stability study was conducted consuming only a single sample canister with 80mg of amorphous indomethacin particles suspended in HFA-134a. It was found that amorphous indomethacin particles converted to the γ crystalline polymorph upon storage at 45°C and that the crystallization rate is strongly size dependent. With its highly effective aerosol collection capability and accurate cutoff diameters for aerosol classification, the impactor will have various applications in the pharmaceutical industry.

Instability of aerosol boundary accelerated in a direction perpendicular to its plane

Theory of Rayleigh–Taylor instability (RTI) is extended for the case of a dilute aerosol system. Equations of interpenetrating continua are investigated on stability to small perturbations by the method of normal modes. In contrast with homogeneous, equations of two-phase, two-velocity medium possess three types of disturbances: an entropy-vorticity one along with two acoustic ones. The difference in the amplitude constant signs of these types is able to affect the disturbance shape and phase convection inside mixing layer. The acoustic branch of solution is responsible for mechanism of instability, while the entropy-vorticity one determines damping mechanism. Instability of a plane accelerating surface, which separates aerosol and homogeneous incompressible fluid, is then investigated. Dispersion relation of the boundary-value problem for perturbations has two aperiodic unstable roots; one of them coincides with the classic R–T root in the particle absence limit, another is able to compete with only at huge accelerations. Instability is governed by modified Shields and Atwood numbers. As applications, systems with different aerosol densities are considered: atmospheric dust, mist around atomizing drop and explosively dispersed powder. Conclusion is done that a suspension somewhat destabilizes the system at small particle volume concentrations, while at a denser content it has essential damping influence on RTI.

Modelling of two-phase, transient airflow and particles distribution in the indoor environment by Large Eddy Simulation

ABSTRACTPrediction of particles distribution in the smaller-scale atmospheric environment, such as the indoor atmosphere, is of major importance for the comfort and the well-being of its occupants. The objective of this study is to investigate the airflow and particles transport, as well as the particles concentration evolution indoors, using Computational Fluid Dynamics (CFD) techniques. A three-dimensional, Euler–Euler two-phase flow model for the investigation of the indoor aerosol is developed, within a CFD general-purpose computer program (PHOENICS), and is validated against experimental measurements from the literature, for an ordinary case of indoor dilute aerosol. Turbulent flow is simulated by Large Eddy Simulation (LES) and the results are compared with those obtained applying the Reynolds-averaged Navier–Stokes (RANS) equations together with the ReNormalisation Group (RNG) k–ϵ model. Τwo-way coupling between the two phases is modelled by means of appropriate interphase interactions. This study focused on particles of one size group (mean aerodynamic diameter of 10 μm) but the numerical method described can equally well be applied for a broader size range. It is concluded that for the very dilute aerosols considered here, simpler models (such as single-phase and drift flux) do as well in predicting the important parameters of the flow, as the more complex ones.

Geometrical Optics of Dense Aerosols: Forming Dense Plasma Slabs

Assembling a freestanding, sharp-edged slab of homogeneous material that is much denser than gas, but much more rarefied than a solid, is an outstanding technological challenge. The solution may lie in focusing a dense aerosol to assume this geometry. However, whereas the geometrical optics of dilute aerosols is a well-developed field, the dense aerosol limit is mostly unexplored. Yet controlling the geometrical optics of dense aerosols is necessary in preparing such a material slab. Focusing dense aerosols is shown here to be possible, but the finite particle density reduces the effective Stokes number of the flow, a critical result for controlled focusing.

Coagulation of highly concentrated aerosols

A number of material synthesis processes such as flame, plasma and laser ablation have been developed for production of films and powders at low pressure and high temperature. At these conditions particle growth typically takes place by coagulation in the free molecule and transition regimes. As economic manufacturing of these materials favors operation at high particle concentrations, classic coagulation theory may not be sufficient to describe the ensuing aerosol dynamics, especially if fractal-like particles are formed. The coagulation rate of highly concentrated, polydisperse aerosols is investigated here from the free molecule to the continuum regime by solving the corresponding Langevin dynamics (LD) equations. The LD simulations are validated by monitoring the attainment of the self-preserving size distribution (SPSD) for dilute particle volume fractions, φ s, below 0.1%. High particle concentrations in the free molecule regime lead to deviations of the aerosol dynamics from the kinetic theory of gases especially during instantaneous coalescence (completely inelastic particle–particle collisions) resulting in slower coagulation rates and slightly narrower SPSDs than in conventional dilute aerosols. In the transition regime, the coagulation rate of highly concentrated aerosols is progressively higher than that for dilute aerosols as growing particles enter the continuum regime where coagulation rates are 2–30 times higher than that of classic Smoluchowski theory. At high particle concentrations ( φ s>1%), a SPSD is approached ( σ g,n=1.42) that does not exhibit the characteristic minimum at the transition regime of dilute aerosols. A relationship is developed for the aerosol coagulation rate of highly concentrated aerosols from the free molecule to continuum regime.

EYE SAFE POLARIZATION DIVERSITY LIDAR FOR AEROSOL STUDIES: CONCEPT DESIGN AND PRELIMINARY APPLICATIONS

The concept design and preliminary applications of a new Eye Safe Polarization Diversity Lidar (ESPDL) instrument are described. The lidar operates with polarization diversity in the laser emission and polarization discrimination in the receiver at 1574 nm for tropospheric aerosol studies in the Arctic atmosphere. This instrument was originally designed to operate in the eye-safe wavelength range with a one-channel receiver and 20 dB linear polarization accuracy in the laser emission, and assembled in a compact optical bench. The instrument was upgraded for polarization diversity laser emission, i.e polarization selectivity better than 30 dB, and for linear polarization reception discrimination, i.e. polarization discrimination better than 50 dB. Geophysical lidar applications under the scope of this instrument with an overall instrumental polarimetric accuracy better than 0.1% focus on the identification of very dilute suspended aerosols in the troposphere, ice in-cloud initiation and aerosol/water interfaces, complex aerosols, sub-visible high altitude clouds and environmental issues and dynamic processes in the Arctic such as ice fog, forest fire, and a multilayered stably stratified Arctic boundary layer. In this article we describe the instrument design concept and the electronic synchronization necessary to achieve the maximum instrumental polarization accuracy. We report a preliminary case study of differential polarization analysis.

Analysis of dilute aerosol flow and noise generation in an acoustic transducer

This combined experimental and computational study is an extension of prior efforts by the present authors to provide a physical explanation of the ability of a particle detection technique based on the acoustic signature of airborne particles. As a particle-laden flow accelerates through a converging–diverging tube consisting of a capillary (called the acoustic transducer), the suspended particles cannot follow the rapidly changing flow velocity. Consequently, vortices are generated in the wake of the particle, which excite the air column inside the transducer tube producing noise. Experimental results show how the frequency content of the acoustic signatures relates to the fundamental frequency of the transducer's air column. The acoustic transducer is able to detect micron-sized particles and the sound intensity is a function of flow rate but not of particle size. The ability of the transducer to determine particle concentration (as low as a few ppl) is also experimentally shown and compared to data obtained from a commercially available aerodynamic particle sizer. A computational model is developed which incorporates detailed treatment of air compressibility as well as the variability in Reynolds number and particle drag coefficient. Computational results help determine the location of the acoustic disturbance, which is at the entrance of the capillary section of the transducer where the flow speed is nearly sonic and the acceleration is the greatest throughout the flow.

The influence of entrainment on acoustically induced interactions between aerosol particles—an experimental study

An experimental study of particle interactions acoustically induced in monodisperse aerosols is presented in this paper. It is mainly devoted to analyse the influence of the acoustic entrainment experienced by the single particles on their attraction processes and, in particular, on the hydrodynamic mechanisms that govern them. Glass micro-spheres immersed in air as very dilute aerosols were subjected to homogeneous plane standing waves at frequencies ranging from 200 Hz up to 5 kHz . At these variable acoustic conditions the particle experiences different acoustic entrainment coefficients, varying from q p=0.86 down to 0.05 and covering almost its complete range of variability. A collection of experimental particle interactions were filmed, from which the particle behaviours were analysed. Our results experimentally confirm for the first time the presence of the acoustic wake effect (AWE) as a dominant mechanism of attraction at every entrainment ratio. In addition, the mutual radiation pressure is observed as a repulsion mechanism acting on nearby particles (separated at few diameter distances). It produces a disturbing effect on the particle attractions due to the AWE. Its influence becomes negligible for entrainment coefficients below q p=0.1.

The effect of weak Shear-induced motion on brownian coagulation of aerosol particles

The coagulation rate of a dilute polydisperse aerosol dispersion of particles is considered for small Peclet number, which provides a measure of the ratio of the relative shear-induced motion to Brownian motion between two rigid spherical aerosol particles. The asymptotic form of the relative velocity of two unequal particles immersed in a simple shear flow when they are far apart is obtained. Using a singular perturbation method, a two term expansion for the dimensionless coagulation rate (Nusselt number) as function of the Peclet number is developed. In the limit of the radius of one of the two spheres becoming small, the result agrees with the dimensionless mass transfer rate to an aerosol particle at small Peclet number.

266 Maintenance, bacterial contamination and performance of jet nebulizers used by out patients

Jet nebulizers are frequently used for asthma treatment in children. Aim of study To evaluate technical maintenance, cleaning procedures, bacterial contamination and performance of nebulizers used by out patients. Methods We selected 39 families where an asthmatic child was on maintenance treatment with a nebulizer. User aspects of nebulizer therapy were evaluated with a questionnaire. Bacterial samples were taken from the liquid container of the nebulizer, the NaCl 0.9% solutions used as diluent, and from the aerosol. We measured the operating pressure generated by the compressor with a manometer. Particle-size of the aerosol was measured by laser diffraction. Results None of the nebulizers were periodically maintained. The cleaning procedure was insufficient in 38% of the users. One or more strains of pathogenic micro-organisms were cultured from 18/39 reservoirs, 16/39 NaCl 0.9% solutions, and 5/10 aerosols. Operating pressure was lower than specified by the manufacturer in 50% of the nebulizers. There was a straight negative linear correlation between age of the nebuliser and the respirable output. Conclusion Periodically technical maintenance of nebulizers and evaluation of user aspects is needed. Only single dose vials of NaCl 0.9% should be used to dilute aerosol medication.

Particle Mixing and Diffusion in the Turbulent Wake of a Single Cylinder

Particle mixing and turbulent diffusion from a dilute monodisperse aerosol stream into the turbulent wake of a single cylinder was studied over a velocity range of 6.1, 12.2, and 18.3 m/sec. Dioctyl phthalate droplets with diameters of 4, 8, and 15 μm were used resulting in effective Stokes numbers ranging from 0.18 to 5.11. Particle velocity and concentration data in the aerosol stream around the cylinder was taken with a laser doppler anemometer. The velocity and concentration data were analyzed empirically, examining diffusivities via the particles' rms velocity, and also using a model from a similarity solution to the equation of motion, assuming a constant diffusivity in the wake. The experimental data showed good agreement between the models and the air and particle momentum data. The concentration data contained more scatter, but was consistent with the air and particle momentum data. The air momentum Peclet numbers agreed with literature values for cylinder wakes. Diffusivity ratios, em/emp, and e...

Dosimetry of Localised Accumulations of Cigarette Smoke and Radon Progeny at Bifurcations

The work focuses upon deposition and clearance processes affecting cigarette smoke particles and radon progeny within surrogate airway models, replica casts and the human lung. As shall be demonstrated, 'cloud motion' for mainstream cigarette smoke can produce locations of enhanced deposition not experienced with dilute aerosols composed of like-sized particles. These sites of concentrated deposits occur at airway bifurcations, especially at the inclusive carinal ridges.

The Role of Platelet-Activating Factor in the Pulmonary Response to Inhaled Bacterial Endotoxin

Quantitative morphometric analyses were carried out on animals subjected to aerosols of bacterial endotoxin (LPS) to further define the role of platelet-activating factor (PAF) in the development of pulmonary injury. Hamsters were exposed to either saline aerosol or dilute aerosols of LPS (4 micrograms/m3) for standard lengths of time. Within each aerosol exposure group, animals were further subdivided into groups receiving either the PAF receptor binding antagonist, RP 48740, or saline injections. LPS inhalation resulted in decreased fixed lung volume, increased sequestration of polymorphonuclear leukocytes and platelets in pulmonary capillaries, increased type I epithelial and endothelial cellular volumes, increased cellular interstitium, and increased endothelial pinocytotic vesicles. Treatment with RP 48740 either attenuated or abolished the ability of inhaled LPS to induce these structural alterations. The PAF antagonist also inhibited LPS-induced increases in pulmonary capillary permeability. It is concluded that PAF is one of the major injury-promoting mediators released upon inhalation exposure to environmentally realistic concentrations of bacterial endotoxin. A major but not exclusive target of this mediator is the pulmonary vascular endothelium.

Finite‐amplitude wave propagation through a two‐phase system using coupled generalized Burgers' equations

The propagation of finite‐amplitude acoustic waves through a system of fluid particles in a fluid matrix (aerosols or emulsions) has been examined theoretically by using the continuum volume‐averaged balance equations and linear constitutive equations for a two‐phase system. Utilizing the technique developed by Lighthill for clean perfect gases, and by Davidson [G. A. Davidson, J. Sound Vib. 38, 475–495 (1975)], who employed the equations of Marble to dilute aerosol media, coupled Burgers' equations for each phase were obtained and analyzed by a perturbation method. The behavior of the higher harmonics of an initial sinusoidal plane wave was determined, and compared to previous work. The present results are applicable to moderately concentrated particulate systems. Also, the use of two‐phase equations obtains the effects of viscosity of the media, and also phoresis, such as thermophoresis, on the waveforms.

A Proportional Method for the Dilution of Submicron Hygroscopic Aerosols

A method for the dilution of hygroscopic aerosols up to 1000 times has been developed. Since the devised system could dilute aerosols by the filtered air in which the particles were dispersed, the particle size was not changed by evaporation or condensation of water. It also was necessary to eliminate the pulsation and heat evolution of diaphragm pumps in order to achieve the exact dilution ratios and maintain constant relative humidity in dilution air. The validity of this method was corroborated with sulfuric acid particles in submicron size. Moreover, DOP particles (0.12 µm–1.52 µm) and polystyrene spheres (0.330 µm–2.02 µm) can be diluted evenly with regard to their particle sizes by this diluter.

A Proportional Method for the Dilution of Submicron Hygroscopic Aerosols

A method for the dilution of hygroscopic aerosols up to 1000 times has been developed. Since the devised system could dilute aerosols by the filtered air in which the particles were dispersed, the particle size was not changed by evaporation or condensation of water. It also was necessary to eliminate the pulsation and heat evolution of diaphragm pumps in order to achieve the exact dilution ratios and maintain constant relative humidity in dilution air. The validity of this method was corroborated with sulfuric acid particles in submicron size. Moreover, DOP particles (0.12 µm–1.52 µm) and polystyrene spheres (0.330 µm–2.02 µm) can be diluted evenly with regard to their particle sizes by this diluter.

Removal Mechanisms for Calf Barn Aerosol Particles

ABSTRACT Aventilation-dilution model has been adapted to determine total and respirable aerosol concentrations in an enclosed calf barn using experimentally determined aerosol generation rates. Respirable aerosol removal mechanisms such as ventilation rate and a supplemental air cleaning device may be evaluated using the model. In a simulated calf barn where 5.8 m^ airspace per calf was allowed, increasing ventilation rates to dilute aerosol concentration was impractical, especially in cold weather. During winter, using a recommended ventilation rate of 0.0045 m^/s per calf, an air cleaner with a removal efficiency of at least 80% and an air recirculation rate of 0.0255 mVs per calf significantly reduced respirable aerosol concentrations.

Particle capture on fibers in strong electric fields: I. Experimental studies of the effects of fiber charge, fiber configuration, and dendrite structure

Fibers exposed to electric fields acquire charge which, along with polarization of the incident field, affects the way particles are captured on the fibers. Several fiber-electrode arrangements were used to investigate the influence of the charge acquired by single polyester fibers on the capture of monodisperse polystyrene particles from a dilute aerosol. The charge on a typical fiber was found to increase its capture efficiency by nearly an order of magnitude over that for a neutral fiber polarized by the same field. Another factor which was investigated was the degree to which dendrite growth affects the overall capture efficiency. According to the results obtained here the effect is not significant when the dendrites are formed by capture processes taking place in the presence of a strong external field.