Fractal scaling of coated soot aggregates

Recent field observations have shown soot aggregates (SAs) to contain significant amounts of surface coatings of organic compounds which obfuscate their native fractal morphology and make them visually appear as “near-spherical.” Morphologies of these aggregates are currently parameterized using fractal dimension (D_f) values greater than the universal 1.8. This is done to account for the supposedly morphological restructuring of an aggregate to a more compact form upon condensation of organic materials. Using multiple-angle light scattering analysis, it has been experimentally shown that restructuring of SA morphology only takes place during the evaporation process, not condensation. Based on this seminal finding, here we formulate the correct parameterizations to describe the morphology of surface coated aggregates. We perform detailed three-dimensional morphological characterization of computer simulated coated aggregates that mimic atmospheric SAs and show that their D_f remains invariant at 1.8 with increasing coating mass by as much as 18 fold. We find coating to affect only the fractal prefactor k₀, an understudied parameter which controls the aggregate shape anisotropy and local packing fraction of monomers. Specifically, k₀ was observed to scale with the ratio of aggregate's total (coating + bare) mass M_total to bare mass M_bare as k₀ = 1.34*(M_total/M_bare)^0.56.

Copyright © 2017 American Association for Aerosol Research     

The effect of electric-field-induced alignment on the electrical mobility of fractal aggregates

We study the effects of electric field strength on the mobility of soot-like fractal aggregates (fractal dimension of 1.78). The probability distribution for the particle orientation is governed by the ratio of the interaction energy between the electric field and the induced dipole in the particle to the energy associated with Brownian forces in the surrounding medium. We use our extended Kirkwood–Riseman method to calculate the friction tensor for aggregates of up to 2000 spheres, with primary sphere sizes in the transition and near-free molecule regimes. Our results for electrical mobility versus field strength are in good agreement with published experimental data for soot, which show an increase in mobility on the order of 8% from random to aligned orientations. Our calculations show that particles become aligned at decreasing field strength as particle size increases because particle polarizability increases with volume. Large aggregates are at least partially aligned at field strengths below 1000 V/cm, though a small change in mobility means that alignment is not an issue in many practical applications. However, improved differential mobility analyzers would be required to take advantage of small changes in mobility to provide shape characterization.

Copyright © 2018 American Association for Aerosol Research     

Enhanced growth rates of nanodroplets in the free molecular regime: The role of long-range interactions

Recently, Pathak et al. (2013) Pathak, H., Mullick, K., Tanimura, S., and Wyslouzil, B. E. (2013). Nonisothermal Droplet Growth in the Free Molecular Regime. Aerosol Sci. Technol., 47:1310–1324.[Taylor &; Francis Online], [Web of Science ®] , [Google Scholar] conducted a series of non-isothermal D₂O nanodroplet growth studies in the free molecular regime. They found that under highly non-equilibrium conditions, the condensation (q_c) and evaporation coefficients (q_e) can differ from each other and from the expected value of 1. Here, we confirm these observations by analyzing comparable experiments using n-propanol. We show that the best agreement with the non-isothermal Hertz–Knudsen growth law corresponds to setting (q_c, q_e) = (1, 0.6) or (q_c, q_e) = (1.3, 1). The approach of retarded evaporation yields values close to those observed by Pathak et al. for D₂O, but is difficult to justify theoretically. Enhancing the condensation coefficient is consistent with long-range attractive interactions between the vapor molecules and droplets in the nanometer size range.

© 2016 American Association for Aerosol Research     

Dynamic characteristics for the normal impact process of micro-particles with a flat surface

A dynamic model has been developed to simulate the normal impact of an elastic-plastic adhesive sphere with a flat surface. The model combines the extended JKR theory considering both adhesion and plastic deformation with Newton's motion equation to describe the rebound behavior of the impacting particles. Theoretical expressions for velocity, contact time and restitution coefficient are obtained. The models were validated by comparison with the experimental results. Especially, a new empirical critical capture velocity expression was proposed which can be used to determine whether the particle will stick or bounce off the surface after the impact.

Copyright © 2018 American Association for Aerosol Research     

Effect of crystallization kinetics on the properties of spray dried microparticles

A droplet chain technique was used to study the influence of the crystallization process on the morphology of spray dried microparticles. A piezoceramic dispenser produced a chain of monodisperse solution droplets with an initial diameter in the range of 60–80 µm. Aqueous solutions of sodium nitrate were prepared in concentrations ranging from 5 mg/ml to 5?10^?5 mg/ml. The solution droplets were injected into a laminar flow with gas temperatures varying from 25 to 150°C, affecting the droplet temperature and the evaporation rate, accordingly. Dried particles with diameters between 0.3 and 18 µm were collected. The properties of the collected microparticles were studied and correlated with a particle formation model which predicted the onset of saturation and crystallization. The model accounted for the dependence of the diffusion coefficient of sodium nitrate in water on droplet viscosity. The viscosity trend for sodium nitrate solutions was determined by studying the relaxation time observed during coalescence of two aqueous sodium nitrate droplets levitated in optical tweezers. The combination of theoretical derivations and experimental results showed that longer time available for crystallization correlates with larger crystal size and higher degrees of crystallinity in the final microparticles.

© 2016 American Association for Aerosol Research     

Influence of gas velocity on the particle collection and reentrainment in an air-cleaning electrostatic precipitator

Particle deposition and reentrainment experiments were performed in a two-stage electrostatic precipitator (ESP), consisting of positive corona precharger and collecting electrode sections. Attention was focused on studying the indoor air pollution deposition and reentrainment into six size ranges from 0.3 to >10?μm. Tests were performed in an office room (200?m³) for airflow velocities from 1.4 to 8?m/s. The effect of airflow velocity on the collection efficiency of the ESP was investigated both experimentally and analytically to study reentrainment phenomena in a turbulent flow. A stationary two-dimensional analytical model was carried out by modeling the particle transport. The boundary conditions for charged particles on collecting and repelling electrodes were determined by physical considerations, including chaotic and drift motions, the reflection of charged particles from a surface, and the reentrainment of charged particles. A decrease in the experimental collection efficiency for large particle diameters (≥0.5?μm), as compared to the theoretical prediction, was interpreted as the reentrainment of particles. The size-resolved dust reentrainment fluxes from the collecting electrode were evaluated in two limiting cases, considering that either the reentrained particles are not charged or that they are charged as the particles in the deposition flux. Dimensional analysis is applied to these results, introducing the wall friction velocity as a universal parameter that determines the flow character. In general, the particles with diameters <5?μm and >5?μm exhibit different reentrainment behavior.

Copyright © 2018 American Association for Aerosol Research     

Examining the ability of empirical correlations to predict subject specific in vivo extrathoracic aerosol deposition during tidal breathing

The accuracy of five extrathoracic deposition equations was examined by comparing model predictions with in vivo deposition measurements of ^99mTc-DTPA radiolabeled 0.9% saline delivered via PARI LC Sprint nebulizers in 19 healthy human subjects. The average extrathoracic deposition fraction measured in vivo was 0.19 ± 0.10 (average ± standard deviation). Comparing to this average value, the extrathoracic deposition fraction predicted by Golshahi et al. equation was the most accurate (0.18 ± 0.08), followed by the model described by the ICRP (0.16 ± 0.03). However, prediction of subject-specific deposition proved more challenging; the Golshahi et al. model performed the best of the examined equations, yet showed only a small positive correlation between measured and predicted deposition in individual subjects with a Pearson correlation coefficient of 0.34. The difficulties in predicting subject-specific deposition likely result from geometric dissimilarity both within and between subjects, and may require more complicated modeling methods than algebraic equations of the kind examined in this study.

Copyright © 2017 American Association for Aerosol Research     

The effect of materials and obliquity of the impact on the critical velocity of rebound

The critical velocity of rebound was determined for spherical ammonium fluorescein particles in the size range of 0.44–7.3 μm. The method was based on measurements with a variable nozzle area impactor (VNAI) and numerical simulations. A comparison to previous results with spherical silver particles obtained with the same method showed that the critical velocity was approximately two orders of magnitude higher for ammonium fluorescein than for silver at the same size range. Among the hard test materials, including steel, aluminium, molybdenum, and Tedlar, the surface material had no significant effect on the critical velocity of rebound within the accuracy of the method. On the contrary, the critical velocity was observed to be highly dependent on the obliquity of the impact at the onset of rebound. While the ratio of the maximum tangential and normal velocities was defined as a measure for the obliquity, the critical velocity was found to be more than a magnitude smaller for very oblique impacts with the velocity ratio above 9 than for close-to-normal impacts with the velocity ratio below 1.5. The results of this study can be considered as a link between the recently published critical velocity results for nanoparticles and the older results for micron-sized particles.

Copyright © 2017 American Association for Aerosol Research     

Deposition of droplets from the trachea or bronchus in the respiratory tract during exhalation: A steady-state numerical investigation

Abstract

Respiratory droplets are bioaerosols that originate from the respiratory tract. Knowing their deposition characteristics during exhalation would facilitate the understanding of the source of large respiratory droplets and their importance in the spread of respiratory infectious diseases. In this study, computational fluid dynamics is used to simulate the motion and deposition of droplets released from either trachea or bronchi in a realistic reconstruction of the human respiratory tract. Influences of airflow structures and locations of droplet generation on droplet deposition are studied, and droplets with diameters between 1 and 50?µm are examined. The deposition of droplets is found to be influenced mainly by the droplet diameter and the flow rate of exhalation. The number of droplets released from the trachea or bronchi that can escape into the environment decreases as the flow rate increases. When the flow rate is low (10?L/min), the critical diameter of droplets generated in the lower respiratory system that can escape into the air is approximately 12?µm, but this diameter is approximately 5?µm when the flow rate is medium (30 to 60?L/min) or large (90?L/min). The larynx is the dominant site of deposition for droplets smaller than the critical diameter, while trachea and bronchus are more important locations that account for the deposition of larger droplets. This study indicates that the lower respiratory tract is an important source of fine droplets (<5?µm) in indoor environments, and larger droplets probably originate from the upper respiratory tract, which needs further investigation.

Copyright © 2020 American Association for Aerosol Research     

Effect of friction coefficient and density on mixing particles in the rolling regime

In this work, optical imaging is used to quantify the mixing dynamics of granular materials. Two different methods (GLCM and Multivariate RGB analysis) are combined to extract surface information and the time to achieve a specific degree of mixing. In particular, the effect of density, friction coefficient, surface quality and particle geometry was studied. The results show that the Froude number alone is not enough to completely characterize the rolling regime. In addition, the filling ratio must be in a specific range which depends on the material properties.     

Novel method for determining the dynamic friction coefficient of explosives

A new method for determining the dynamic friction coefficient of explosives is presented. The method combines the physical model of friction sensitivity with theoretical analysis and numerical calculations. Experimental measurements of the dynamic friction coefficient of steel indicate that the proposed method is effective, provides secure and reliable data, and can be used to calculate the dynamic friction coefficient between cyclonite (RDX) and steel.     

Mobility radius of fractal aggregates growing in the slip regime

A method is presented for the calculation of the mobility radius of fractal aggregates. The connection between the aggregate permeability and the monomer friction factor is derived, which makes it possible to convert the permeability in the continuum regime to that in the slip regime. The method elaborated here estimates the permeability of an aggregate treated either as impermeable sphere of the size equal to mobility radius or a permeable self-similar structure consisting of impermeable monomers. The internal permeability of a fractal aggregate growing in the slip regime is analyzed assuming that the aggregate consists of no more than twelve effective impermeable monomers, the number being a result of hydrodynamic considerations. The method makes it possible to estimate the aggregation number dependence of the mobility radius. A system of carbonaceous flame soot aerosol containing fractal aggregates with D=1.8 is analyzed. The mobility radius-aggregation number relation is found to be very close to that obtained experimentally. For large aggregation numbers this relation tends to that, which is valid for the dynamic radius.     

Analytical estimate of the heating regime for a recuperative glass-melting furnace

The results of an analytical evaluation of the heat transfer processes in the working space of the furnace are presented. Four atomizers located under the nozzle of the burner are used for heating. The calculations are performed using a three-dimensional heat transfer zonal model for three variants of the thermal load distribution between the atomizers, as well as for heating of the furnace with natural gas (non-luminous flames).     

Observations on the coefficient of friction of polypropylene film

Data are presented relating the time change of the coefficient of friction of cast polypropylene films to changes in film density and concentration of surface lubricant. It is shown that during aging the density of polypropylene increases, thus causing a decrease in the friction coefficient. It is also shown that although the lubricant added to the polymer will diffuse to the surface of copolymer films, no diffusion occurred in polypropylene films.     

Taylor-expansion moment method for agglomerate coagulation due to Brownian motion in the entire size regime

Through applying the Taylor-expansion technique to the particle general dynamic equation, the newly proposed Taylor-expansion moment method (TEMOM) is extended to solve agglomerate coagulation due to Brownian motion in the entire size regime. The TEMOM model disposed by Dahneke's solution (TEMOM–Dahneke) is proved to be more accurate than by harmonic mean solution (TEMOM–harmonic) through comparing their results with the reference sectional model (SM) for different fractal dimensions. In the transition regime, the TEMOM–Dahneke gives the more accurate results than the quadrature method of moments with three nodes (QMOM3). The mass fractal dimension is found to play an important role in determining the decay of agglomerate number and the spectrum of agglomerate size distribution, but the effect decreases with decreasing agglomerate Knudsen number. The self-preserving size distribution (SPSD) theory and linear decay law for agglomerate number are only applicable to be in the free molecular regime and continuum plus near-continuum regime, but not perfectly in the transition regime.