查看更多>>摘要：Nephelometers are regularly used for air quality monitoring through scattering measurements of aerosols in the atmosphere. Less attention has been paid to the potential of estimating soot morphological parameters from nephelometer measurements in combination with scattering theory. In this work, we perform a fundamental laboratory study where an Ecotech Aurora 3000 nephelometer is used for measuring the scattering properties of soot with different characteristics. The nephelometer monitors the forward and backward scattering at three wavelengths (450 nm, 525 nm, and 635 nm) to retrieve information on the ratio of forward and backward scattering intensities, as well as the scattering wavelength dependence. An inverse scattering method, based on the Rayleigh-Debye-Gans theory, for determination of the equivalent fractal dimension (D-f) and radius of gyration (R-g) of soot particles from the experimental scattering data was developed. It is shown that the inverse method can estimate R-g and D-f when R-g < 185 nm and from the wavelength dependence, information on the relative amount of organic soot could be obtained. For validation, estimated parameters are compared with the morphological parameters of soot sampled from the mini-CAST soot generator and evaluated using transmission electron microscopy (TEM). Uncertainties and limitations of the procedure are discussed.
查看更多>>摘要：Aerosol particle scavenging by droplets is commonly seen in workplaces as a process of air depollution, but it is used in an empirical way. Although many studies have been conducted on this process, there is still a lack of data concerning its efficiency in the field of occupational health. Thus, an experimental study is presented in this paper, where a mist is sprayed in counter-current to an ascending aerosol of alumina using commercial nozzles. The influence of various parameters on the spectral and total collection efficiency is studied. It is put in evidence that the collection of particles above the micron takes place mainly on the first tens of centimeters. Although an increase in the liquid flow rate and the positioning height lead to a better collection efficiency, the droplet diameter seems to be a predominant parameter to take into account. From the results, recommendations are thus proposed to optimize the aerosol collected by droplets. It is also shown that a simple impaction model can be used to predict the scavenging of aerosol particles, despite the strong assumptions.
Plack, AlexanderBierwirth, MalteWeber, Alfred P.Gunkelmann, Nina...
12页
查看更多>>摘要：The collision of nanoparticles with surfaces plays an important role in many particle processing technologies. The mechanical properties after collision are described by the atomistic structure of the particles and the surface. By means of molecular dynamics simulations and experiments we evaluate the coefficient of restitution and the critical velocity for the collision of gold nanoparticles with a non-rigid gold substrate. We use different interatomic potentials including a Lennard-Jones potential, a hybrid Lennard-Jones-EAM potential and a ReaxFF potential and compare our simulation results with experiments. Our results show that the ReaxFF potential can most adequately reproduce the experimentally determined critical rebound velocity. The coefficient of restitution is in good agreement with the experiments. Additionally ReaxFF delivers insights into the sticking probability and shows material transfer from particle to substrate and vice versa.
Bertomeu, Pablo FerrerMcIntyre, CharlotteRennie, CatherineGouder, Kevin...
13页
查看更多>>摘要：In view of the ongoing COVID-19 pandemic and its effects on global health, understanding and accurately modelling the propagation of human biological aerosols has become crucial. Worldwide, health professionals have been one of the most affected demographics, representing approximately 20% of all cases in Spain, 10% in Italy and 4% in China and US. Methods to contain and remove potentially infected aerosols during Aerosol Generating Procedures (AGPs) near source offer advantages in reducing the contamination of protective clothing and the surrounding theatre equipment and space. In this work we describe the application of computational fluid dynamics in assessing the performance of a prototype extraction hood as a means to contain a high speed aerosol jet. Whilst the particular prototype device is intended to be used during tracheotomies, which are increasingly common in the wake of COVID-19, the underlying physics can be adapted to design similar machines for other AGPs. Computational modelling aspect of this study was largely carried out by Barcelona Supercomputing Center using the high performance computational mechanics code Alya. Based on the high fidelity LES coupled with Lagrangian frameworks the results demonstrate high containment efficiency of generated particles is feasible with achievable air extraction rates.
查看更多>>摘要：The filtration performance of three types of commercially available face masks (hygienic, surgical, and FFP2) has been evaluated for aerosol particle size in the range 10-150 nm at a fixed face velocity of 9.5 cm/s. Two parameters have been varied in this study: the time of use of the mask, and the electrical charging state of the particles. Mask aging has been carried out by wearing it for a prescribed period of time. Four different charging states of NaCl particles generated by evaporation-condensation have been examined: positively charged particles, negatively charged particles, uncharged particles, and a mixture of the three former types (the latter referred to as "mixed aerosol"). Aerosol charging was carried out with a low activity radioactive source so that most of the charged particles carried a single charge of either sign in all instances. The charging state of the aerosol exerts a considerable effect on filtration efficiency for the three types of masks. Highest filtration efficiencies are attained for positively and negatively charged particles, but polarity seems to play a role: some masks capture more efficiently particles of a given polarity, although differences in the capture efficiencies of positively and negatively charged particles are insignificant in comparison with those observed between charged and uncharged particles. Uncharged particles give the lowest efficiency and the mixed aerosol leads to filtration efficiencies between those of charged and uncharged particles. The time of use of the mask has also a great influence on its filtration performance: while hygienic masks give poor results from the very beginning, one of the two FFP2 masks assayed has shown a good performance even after 24 h of use, and the surgical mask also performed well after 8 h. The relative importance of mechanical (diffusion and or interception) and electrostatic particle deposition, as a function of particle diameter and aging time, has been estimated and discussed. The electrostatic mechanism generally dominates for small particles and short aging times. Pressure drop across the mask has also been measured: the largest pressure drop was observed for the FFP2 and the surgical masks. The time of use of the masks did not affect the pressure drop. Lack of a good fit between the mask and the face of the wearer drastically reduces the actual filtration efficiency of the mask because part of the incoming aerosol bypasses the filtering medium. A few additional efficiency measurements were done after cutting off a small surface area of the mask sample.
查看更多>>摘要：In this paper, a comprehensive predictive model is established for the synergetic removal of fine particulate matter (PM) and SO3 in the ultralow cold-side electrostatic precipitator (ESP). Based on the population balance model framework, the current model incorporates SO3 condensation and coagulation in the low temperature economizer (LTE), as well as the quasi-1D dynamics of particle charging, migration, resuspension and deposition in the multi-staged ESP. Simplifications are made so that the resulting model can serve as a convenient software tool aiding the design and operation evaluation of the ultralow cold-side ESP. The model is validated against measurements of two coal-fired units with varied LTE outlet temperatures. In all cases, the predicted PM and SO3 emissions escaping the ESP agree well with the test data, and the relative errors are less than 17%. Quantitatively, running LTE removes similar to 60% of ultrafine PM0.1, and 80-95% of SO3 condense onto the preexisting fly ash in the LTE, resulting in d(p)(-2)-dependence of sulfur content in the size-segregated ash particles. Notably, the emitted PM1, PM10 and SO3 concentrations are largely reduced with lower LTE outlet temperatures; however, this promoting effect becomes marginal if further reducing the LTE outlet temperature under 100 degrees C. Finally, we conclude from a parametric analysis that the most important factor in the ultralow cold-side ESP is the flow residence time in the ESP, followed by the effects of intensified coagulation and elevated ash permittivity.
查看更多>>摘要：A critical factor affecting the accuracy of Computational Fluid Dynamic (CFD) simulations and the time required to conduct them is construction of the computational mesh. This study aimed to evaluate the relatively new polyhedral mesh style for simulating aerosol deposition in the upper conducting airways compared with established meshing techniques and experimental data. Hexahedral and polyhedral mesh solutions were compared in two benchmark geometries: 1) a 90 degrees -bend with flow characteristics similar to the extrathoracic airways of an adolescent child, and 2) a double bifurcation representing bifurcations B3-B5 in an adult. Both 4-block and 5-block hexahedral meshes were used in the 90 degrees -bend to capture the potential of fully-structured hexahedral meshes. In the 90 degrees -bend, polyhedral elements matched polydisperse in vitro deposition data with 20% relative error (RE; averaged across the particle sizes considered), which is an improvement on the accuracy of the 4-block hexahedral mesh (35% RE) and is similar to the accuracy of the 5-block hexahedral mesh (19% RE). In the double bifurcation, deposition fraction relative differences evaluated between polyhedral and hexahedral meshes ranged from 0.3% to 28.6% for the different particle sizes assessed, which is an order of magnitude improvement compared with previous studies that considered hexahedral vs. hybrid tetrahedral-prism meshes for the same flow field. Solution convergence time with polyhedral elements was found to be 50%-140% higher than with hexahedral meshes of comparable size. While application dependent, the increase in simulation time observed with polyhedral meshes will likely be outweighed by the ease and convenience of polyhedral mesh construction. It was concluded that the polyhedral mesh style, with sufficient resolution especially near the walls, is an excellent alternative to the highly regarded hexahedral mesh style for predicting upper airway aerosol transport and deposition, providing a powerful new tool for the assessment of respiratory aerosol dosimetry.
查看更多>>摘要：Research interest in eliminating airborne aerosol particles has been growing for decades. Aerosol particles with aerodynamic diameters between 0.2 and 1 mu m are difficult to remove by traditional spray systems. To improve the aerosol scavenging efficiency, a new method is proposed that employs the addition of water mist. Water mist is supposed to aggregate with aerosol particles to form large-sized coagulated aerosol-mist particle clusters, which can subsequently be removed by spray droplets with a higher efficiency. To investigate the effect of the vessel's size scale on the aerosol collection efficiency, aerosol spray scavenging experiments without and with water mist were conducted in both the UTARTS facility and TOSQAN facility. The cylindrical vessels of the two facilities have the same internal diameter, but the vessel of the TOSQAN facility has a larger height. Experimental results show that water mist has the potential to improve aerosol scavenging efficiency in both facilities but to a higher degree inside the TOSQAN facility. Numerical simulations of aerosol removal by spray droplets without mist were conducted, and different scaling criteria for the geometry of the vessel and gaseous entrainment rate were discussed to explain the experimental results. With water mist, the coagulated aerosol-mist particles have a stronger inertia and a larger Stokes number; these particles are easier to capture by using spray droplets and a water film. The contribution of the spray injection height, spray spread volume, and water film to aerosol removal in cases with mist increased to a larger degree than that of the gaseous entrainment rate, finally resulting in a higher improvement ratio of aerosol removal efficiency in the TOSQAN facility.
查看更多>>摘要：Computational modeling of multispecies aerosols generated from nucleating supersaturated vapors is a challenging task because of the complexity of the involved thermodynamic phenomena, non-existing validated and/or first principles-based models for the nucleation and condensation/evaporation processes, necessity for high computational effort, and, finally, lack of simulation data and software for validation. Here, we present our contribution towards tackling at least some of these challenges by developing AeroSolved, a publicly available open source computational fluid dynamics code for simulation of multispecies evolving aerosols in an Eulerian framework. We present a consistent modeling approach to nucleation and condensation using a multispecies extension of the classical nucleation theory and a multispecies Stefan flow model for particle condensation, respectively. The internally mixed assumption is used, i.e., the species concentration partitioning is particle size independent and uniform across local particle size distribution. Instantaneous temperature equlibration is also assumed between the phases. Applied assumptions were tested for aerosol flows with mean diameter particle size in the range of micrometers. Applications beyond tested regimes (e.g., nanometer or sub millimeter particle size ranges, thermodynamical) would need revisiting the assumptions and consequently modeling limitations with required inclusion of additional processes (e.g., Kelvin effect, Fuchs-Sutugin corrections or Marangoni flows influence). The developed computational models are tested in three separate scenarios simulating: uniform nucleation/condensation conditions, single-particle evaporation/condensation, and laminar flow diffusion chamber flows. Two distinct approaches are proposed to solve the population balance equation and calculate the particle size distribution. The moment method assumes a log-normal shape and fixed width of distribution, while the sectional method resolves the particle size distribution without constraints on its shape, thus being more accurate but also computationally expensive. These two methods are demonstrated as complementary tools for industrial real-case scenarios where complex aerosol flow is simulated in a simplified geometry of the capillary aerosol generator. The more accurate and detailed sectional method serves as a tuning tool for the less computationally demanding log-normal moment method, which is then practically used for parametric studies concerning system performance. The simulations presented here unravel details on particle formation and the sensitivity of the setup to thermodynamic conditions and pave the road towards engineering application of the developed methods.
Ozon, MatthewKulmala, MarkkuLehtinen, Kari E. J.Lehtipalo, Katrianne...
18页
查看更多>>摘要：Resolving aerosol dynamical processes in the sub-10 nm range is crucial for our understanding of the contribution of new particle formation to the global cloud condensation nuclei budget or air pollution. Accurate measurements of the particle size distribution in this size-range are challenging due to high diffusional losses and low charging and/or detection efficiencies. Several instruments have been developed in recent years in order to access the sub-10 nm particle size distribution; however, no single instrument can provide high counting statistics, low systematic uncertainties and high size-resolution at the same time. Here we compare several data inversion approaches that allow combining data from different sizing instruments during the inversion and provide python/Julia packages for free usage of the methods. We find that Tikhonov regularization using the L-curve method for optimal regularization parameter estimation gives very reliable results over a wide range of tested data sets and clearly improves standard inversion approaches. Kalman Filtering or regularization using a Poisson likelihood can be powerful tools, especially for well-defined chamber experiments or data from mobility spectrometers only, respectively. Nullspace optimization and non-linear iterative regression are clearly inferior compared to the aforementioned methods. We show that with regularization we can reconstruct the size-distribution measured by up to 4 different mobility particle size spectrometer systems and several particle counters for datasets from Hyytiala and Helsinki, Finland, revealing the sub-10 nm aerosol dynamics in more detail compared to a single instrument assessment.
NSTL
Elsevier