查看更多>>摘要：? 2022 Elsevier LtdA theoretical analysis of the formation of titanium dioxide (TiO2) nanoparticles from titanium tetraisopropoxide (TTIP) in premixed, jet-wall stagnation flames was performed to investigate the variation of the particle properties as a function of deposition radius. Two different TTIP loadings (280 and 560 ppm) were studied in two flames: a lean flame (equivalence ratio, ?=0.35) and a stoichiometric flame (?=1.0). First, the growth of particles was described using a spherical particle model that was fully coupled to the conservation equations of chemically reacting flow and solved in 2D using the finite volume method. Second, particle trajectories were extracted from the 2D simulations and post-processed using a hybrid particle-number/detailed particle model solved using a stochastic numerical method. In the 2D simulations, the particles were predicted to have mean diameters in the range 3–10 nm, which is consistent with, but slightly less than experimental values observed in the literature. Off-centreline particle trajectories experienced longer residence times at higher temperatures downstream of the flame front. Two particle size distribution (PSD) shapes were observed. In the lean flame, a bimodal PSD was observed due to the high rates of inception and surface growth. In contrast, the stoichiometric flame was dominated by coagulation and the particles quickly attained a self-preserving size distribution. The PSDs were found to be different beyond a deposition radius of approximately one and a half times the nozzle radius due to a small degree of aggregation; this may impact the synthesis of nanoparticles using jet-wall stagnation flames for novel applications. Suggestions are made for future work, not least including the need for the predicted radial behaviour to be tested experimentally.
查看更多>>摘要：? 2022 Elsevier LtdSocial distance will remain the key measure to contain COVID-19 before the global widespread vaccination coverage expected in 2024. Containing the virus outbreak in the office is prioritised to relieve socio-economic burdens caused by COVID-19 and potential pandemics in the future. However, “what is the transmissible distance of SARS-CoV-2” and “what are the appropriate ventilation rates in the office” have been under debate. Without quantitative evaluation of the infection risk, some studies challenged the current social distance policies of 1–2 m adopted by most countries and suggested that longer social distance rule is required as the maximum transmission distance of cough ejected droplets could reach 3–10 m. With the emergence of virus variants such as the Delta variant, the applicability of previous social distance rules are also in doubt. To address the above problem, this study conducted transient Computational Fluid Dynamics (CFD) simulations to evaluate the infection risks under calm and wind scenarios. The calculated Social Distance Index (SDI) indicates that lower humidity leads to a higher infection risk due to weaker evaporation. The infection risk in office was found more sensitive to social distance than ventilation rate. In standard ventilation conditions, social distance of 1.7 m–1.8 m is sufficient distances to reach low probability of infection (PI) target in a calm scenario when coughing is the dominant transmission route. However in the wind scenario (0.25 m/s indoor wind), distance of 2.8 m is required to contain the wild virus type and 3 m is insufficient to contain the spread of the Delta variant. The numerical methods developed in this study provide a framework to evaluate the COVID-19 infection risk in indoor environment. The predicted PI will be beneficial for governments and regulators to make appropriate social-distance and ventilation rules in the office.
查看更多>>摘要：? 2022 Elsevier LtdThis study focused on developing and testing a stationary electrostatic bioaerosol collector with a high concentration rate while preserving viable and culturable microorganisms. Using our earlier advances in the electrostatic collection of airborne microorganisms, we designed and optimized a Stationary Electrostatic Bioaerosol Sampler (SEBS), which incorporates our previously developed wire-to-wire charger and a newly-designed removable particle collector. The sample elution system was also redesigned and optimized to achieve a practical solution yielding a high sample concentration rate. The sampler's collector and its hydrophobic coating methods, collection voltages, and material of sample removal tubes were optimized through an iterative process. SEBS is a two-stage electrostatic sampler with a wire-to-wire charger and a stainless steel collection electrode coated using polydimethylsiloxane (PDMS) coating technique and ultraviolet/ozone surface treatment method. The entire collected sample is eluted into 0.2 or 1 mL of sterile phosphate-buffered saline (PBS) using a customized particle removal system made of glass. The developed SEBS was operated at 20 L/min; its physical and biological performance was compared to the Button filter sampler (SKC, Inc.) when sampling two airborne bacteria and two fungi in the laboratory. On average, SEBS showed actual physical collection efficiency of ～50% when samples were eluted into 0.2 mL; the efficiency increased to ～75% when 1 mL elution liquid was used. The average relative viability efficiency reached ～80%, suggesting that the sampling stress was reduced compared to the filter sampler. The relative culturability efficiency was ～60%. The use of 0.2 mL elution liquid resulted in a sample concentration rate of ～5 × 104 min-1, which will enable faster detection and determination of viable and culturable bioaerosols, especially when sampling in low concentration environments. Future studies will evaluate SEBS's performance and utility in field studies.
查看更多>>摘要：? 2022 Elsevier LtdThis paper provides insights into processes governing oil–mist filtration in coalescing filters. In particular, it resolves an apparent inconsistency between different published studies regarding the occurrence (or not) of internal gravity-induced flows. As a result of this, it also clarifies whether in industrially-relevant scenarios such as those pertaining to vertical filter cartridges, non-homogeneous vertical saturation patterns are triggered by these internal flows or are just a result of different oil loading rates. To address these issues, we use Eulerian–Lagrangian CFD simulations, which properly account for the effects of turbulent diffusion of liquid aerosol particles, to replicate an experimental setup available in the literature. In order to interpret results data, we introduce a new dimensionless number, termed SM, defined as the ratio between pressure gradient and gravitational forces, which provides a bulk characterization of the flow and allows to assess whether internal gravity-induced flows, in a given cartridge–oil system, should be expected or not. We show that SM explains well the few experimental data available in the literature and identifies specific behaviors associated with limiting SM values being either very large or close to 1.
查看更多>>摘要：? 2022 Elsevier LtdParticle dispersion and deposition in turbulent pipe flows were studied numerically. The mean flow solution was calculated via a Reynolds Stress Model. Three different stochastic models were used to evaluate the effect of instantaneous fluctuations on entrained particles including the standard Discrete Random Walk (DRW) model available in Ansys-Fluent and two Continuous Random Walk (CRW) schemes that solve a normalized Langevin equation to better account for the effects of inhomogeneous turbulence on particle entrainment. Special attention is paid to the inconsistent directional vector resulting from solving the Langevin equation in non-rectangular geometries in location-specific coordinate systems. Concentration, axial distributions of deposition velocity, and deposition velocity versus particle relaxation time were compared with existing experiments and direct numerical simulation (DNS) using two different geometries. The validity of DNS data for near wall corrections were discussed. A third geometry representative of the hot section of a gas turbine was employed to study the capability of DRW and CRW on modeling spatially-developing flows. Comparisons were drawn between different stochastic models with an all-stick condition and experimental results. It was shown that the classical DRW model tends to overpredict the effect of entrance and does not yield good agreement in the deposition statistics. The CRW model shows a promising capability of modeling particle dispersion and deposition. However, the accuracy of CRW relies heavily upon the fidelity of the underlying mean flow solution and root-mean-square turbulence statistics.
查看更多>>摘要：? 2022 Elsevier LtdThe Hygroscopic Tandem Differential Mobility Analyzer (H-TDMA) measures the hygroscopicity of atmospheric particles, and many atmospheric processes that change this hygroscopicity also change the atmospheric size distribution. Two assumptions made during H-TDMA inversion create spurious hygroscopic trends as a function of the changing inlet size distribution. These two assumptions—that the particles exiting the first Differential Mobility Analyzer (DMA1) are singly charged and that the inlet size distribution has a slope of zero (flat)— generate Multi-Charge Dispersion (MCD) bias and Slope bias, respectively. First, we use a model, named TAO, to show that the inlet size distribution could theoretically change the measured ammonium sulfate hygroscopicity by 10%–20% as a function of diameter or experimental time with no change in relative humidity. Secondly, we show experimentally that aerosol emitted from the flaming combustion of grass creates MCD bias. In this experiment, we measure the CPC response of the first three charges and invert these responses using a new routine named Junior. Junior's inversion of each charge shows that one growth factor distribution describes all measured diameters (no growth dependence on diameter). As in the modeling study above, previous publications of this aerosol system, using traditional inversion assumptions, report a decrease in hygroscopicity as DMA1 diameter increases. Unlike traditional inversions, Junior's inversion does not assume the particles are singly charged nor does it make the flat inlet size distribution assumption. Instead, both the inlet size distribution and each charge's CPC response are measured quantities. Thus, the discrepancy between our inversion results and previous publications is likely due to the traditional inversion routine assumptions. This underscores the importance for accounting for Slope and MCD bias during inversions. Experimental results should be carefully analyzed when reporting hygroscopic trends with respect to diameter or experimental time when using the traditional inversion assumptions.
查看更多>>摘要：? 2022 The Author(s)Experiments on sulphuric acid nucleation in low oxygen atmospheres were done in order to investigate the role of nucleation in the Archean atmosphere. Nucleation initiated by photolysis of SO2 and subsequent reaction between atomic O and SO2 was measured with a PSM and a separate CPC. The parameters were <10 ppm O2 with varying levels of SO2 (4 levels from 40 to 105 ppb), RH (3 levels from 0 to 51%), UV light (254 nm, 4 levels from 55 to 100% power), and ionization (2 levels: Background (～3 cm?3 s?1) and increased w. gamma sources (～42 cm?3 s?1)). We find that nucleation is possible under these conditions and that the measured formation rates correlate positively with all varied parameters. This suggests that the sulphuric acid nucleation system could have played a role in the Archean atmosphere.
查看更多>>摘要：? 2022Concerns regarding noxious emissions from internal combustion engines have increased over the years. There is a strong need to understand the nature of sub-23 nm particles and to develop measurement techniques to evaluate the feasibility of new regulations for particle number emissions in the sub-23 nm region (down to at least 10 nm). This paper presents the results of three EU-funded projects (DownToTen, PEMs4Nano and SUREAL-23) which supported the understanding, measurement and regulation of particle emissions below 23 nm and have successfully developed sub-23 nm particle measurement devices, specifically laboratory systems and mobile devices for RDE tests. The new technology was validated in chassis dyno tests and on the real road. The results show that sub-23 nm particles are mainly generated at the engine start and during acceleration phases. The innovations show that the technology is mature and robust enough to serve as a basis for regulating sub-23 nm particles.
查看更多>>摘要：? 2022 The AuthorPenetration of 3–10 nm particles through a metal grid has been measured using different experimental methodologies. Two basic schemes have been compared: (i) the upstream-downstream (UD), where particle penetration is determined from comparison between the aerosol number concentrations measured at two sampling points, one placed upstream, the other downstream, of the filter; and (ii) the filter-dummy (FD), in which penetration is determined from comparison of number concentrations measured at the outlet of two geometrically identical chambers, one empty and the other containing the filter. The test aerosol facing the grid was either monodisperse (MONO) or polydisperse (POLY). The UD method requires the additional measurement of particle penetration (without filter) through the flow path between the two sampling points and it is thus a more time consuming strategy, but it has the advantage that fabrication of an additional dummy holder is not needed. UD + MONO and FD + MONO gave results in good agreement with the well-established fan filter model theory of Cheng and Yeh. The results obtained with the FD + POLY method, the fastest one, were less precise and accurate. As far as diffusional capture of particles is concerned, the MONO method (using either the UD or FD basic schemes) should be preferred for accurate measurement of filter efficiency; the POLY method should perhaps be more suitable for a quick evaluation of the overall performance of a given filtration system.
查看更多>>摘要：? 2022 Elsevier LtdFundamental observations of particle size distributions are needed to develop models that predict the fate and transport of radioactive materials in the atmosphere following a nuclear incident. The extent of material transport is influenced by the time scales of particle formation processes (e.g., condensation, coagulation). In this study, we investigated the influence of cooling time scales on size distributions of uranium, aluminum, and iron oxide particles that are synthesized separately under identical run conditions inside the controlled environment of an argon plasma flow reactor. Two distinct temperature distributions are imposed along the flow reactor by varying the argon flow rate downstream of the plasma torch. The vaporized reactants of uranium, aluminum, and iron are cooled from about 5000 K to 1000 K before they are collected on silicon wafers for ex situ scanning electron microscope analysis. The microscope images show that the sizes of the largest aluminum and iron oxide particles heavily depend on the cooling time scales, whereas significant size variation with cooling rate is not observed for uranium oxide particles. In addition, the size distribution of aluminum oxide particles exhibits the broadest range among all three metal oxides studied. We performed simulations of particle size distributions using a kinetic model that couples gas phase oxidation chemistry with particle formation processes, including nucleation, condensation, and coagulation. The model results demonstrate the strong sensitivity of particle size distribution to different cooling histories (i.e., temperature vs residence time) along the flow reactor. The kinetic model also helps identify directions for future research to improve the predictions.
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