Penetration of freeway ultrafine particles into indoor environments

High concentrations of ultrafine particles have been reported to exist near major freeways. Many urban residences are located in close proximity to high-density roadways. Consequently, indoor environments near freeways may experience significant concentrations of outdoor ultrafine particles. Given that people spend over 80% of their time indoors, understanding transport of ultrafine particles from outdoor to indoor environments is important for assessing the impact of exposure to outdoor particulate matter on human health. Four two-bedroom apartments within 60 m from the center of the 405 Freeway in Los Angeles, CA were used for this study. Indoor and outdoor ultrafine particle size distributions in the size range of 6–220 nm were measured concurrently under different ventilation conditions without indoor aerosol generation sources. The size distributions of indoor aerosols showed less variability than the adjacent outdoor aerosols. Indoor to outdoor ratios for ultrafine particle number concentrations depended strongly on particle size. Indoor/outdoor (I/O) ratios also showed dependence on the nature of indoor ventilation mechanisms. Under infiltration conditions with air exchange rates ranging from 0.31 to 1.11  ${\mathrm{h}}^{-1}$, the highest I/O ratios (0.6–0.9) were usually found for larger ultrafine particles (70–100 nm), while the lowest I/O ratios (0.1–0.4) were observed for particulate matter of 10–20 nm. Data collected under infiltration conditions were fitted into a dynamic mass balance model. Size-specific penetration factors and deposition rates were determined for all studied residences. Results from this research have implications concerning personal exposure to freeway-related ultrafine particles and possible associated health consequences.     

微孔分散法制备超细CaCO3

引言 超细CaCO3的粒径范围通常为0.02～0.1μm[1],具有纳米尺度效应和特殊的性能,作为一种优质填料和白色颜料,广泛应用在橡胶、塑料、造纸、涂料、油墨、医药等众多行业.超细Ca-CO3的商业产品主要由湿化学沉淀法合成的,即用CO2气体和Ca(OH)2/H2O体系进行化学反应来制备.     

Preparation of ultrafine calcium carbonate particles with micropore dispersion method

Ultrafine particles of CaCO₃ were synthesized by dispersing the mixture of CO₂ and N₂ into the Ca(OH)₂/H₂O slurry with a micropore-plate. Because the micropore is micrometers scale, process of momentum transfer, mass transfer and reaction was significantly enhanced. The carbonation process of Ca(OH)₂/H₂O system was monitored with pH and conductivity. Operation conditions were investigated on the specific surface area of particles, such as initial slurry concentration and volume, gas flowrate and concentration, and temperature. The crystal structure of particles was characterized with BET, IR, TEM, SEM, etc. Results showed ultrafine particles were calcite with general shape of cube, whose size was about 40 nm and specific surface area was more than 25 m²/g. This preparation method is easy to operate.     

An improved model for particle deposition in porous foams

Porous foam provides an inexpensive, light-weight and effective medium to capture physiologically-relevant aerosol fractions. It can be manufactured to have a wide range of properties relevant to aerosol deposition. A series of laboratory experiments were conducted to measure particle penetration though porous foam media of varying pore size and foam length. Both solid and liquid aerosols (0.01–10 μm diameter) were tested using a Sequenzial Mobility Particle Sizer or Aerodynamic Particle Sizer to count and size particles penetrating the foam. With this data, an existing semi-empirical model was improved upon to predict particle penetration through a foam of a given fiber diameter, and thickness. The model is based on three dimensionless parameters (St, Ng, Pe) that account for inertial, gravitational, and diffusive modes of deposition, respectively.     

超声波在超细粉体分散中的应用

超细粉体具有较大的比表面积及表面活性 ,易于吸附而发生凝聚 ,影响了它在改性聚酯中的应用。研究了通过超声波处理超细粉体 ,打开凝聚体 ,使它均匀地分散在分散剂中 ,从而生产出可纺性好的改性聚酯。     

Thermophoretic deposition of ultrafine particles in a turbulent pipe flow: Simulation of ultrafine particle behaviour in an automobile exhaust pipe

Thermophoresis of ultrafine particles in a turbulent pipe flow was studied using high-temperature and high-concentration polydisperse ultrafine particles. Experimental conditions were designed to simulate particle behaviour in an automobile exhaust pipe, with a particular focus on establishing similar particle residence time. From the experimental results, thermophoresis was found to be a dominant mechanism for ultrafine particle deposition in the turbulent pipe flow. A previous thermophoretic deposition model was found to be inadequate with respect to estimating the results of the experimental conditions. In this study, the experimental data and the computational analysis results reflect the necessity of establishing a new formula for thermophoretic deposition for high-concentration polydisperse ultrafine particles in a pipe flow.     

Direct numerical simulation of turbulent particle dispersion in an unbaffled stirred-tank reactor

Turbulent dispersion of inertial particles in a flat-bottom stirred-tank reactor equipped with an eight-blade Rushton impeller is investigated using accurate numerical techniques (Verzicco et al., 2004, Flow in an impeller-stirred tank using an immersed-boundary method. A.I.Ch.E. Journal, 50(6), 1109-1118.). Direct Numerical Simulation of the turbulent flow field in the vessel is obtained using a second-order finite-difference scheme coded in a cylindrical reference frame, and an immersed-boundary approach is used to simulate the motion of the impeller. The flow scales are resolved explicitly down to the Kolmogorov scale. To give a comprehensive picture of the turbulence structure in the vessel, angle-resolved averages of turbulent kinetic energy, turbulent energy dissipation rate and Kolmogorov time-scales are evaluated in vertical planes aligned with the blade and mid-way between two blades. The dispersion of heavy particles of different diameter is then investigated by Lagrangian tracking. The particle-to-fluid mass loading ratio is low enough to assume one-way coupling momentum transfer between continuous and dispersed phase. Three sets of particles, characterized by different response time, are investigated and, for each set, two equal, randomly distributed swarms are initially released above and below the impeller, which is placed mid-way between top and bottom of the tank. Statistics calculated after 3 impeller revolutions are used to evaluate the evolution of particle dispersion in the flow and to quantify their preferential accumulation into specific regions of the tank.     

Refinement and testing of the drift-flux model for indoor aerosol dispersion and deposition modelling

The drift-flux approach for predicting aerosol deposition within enclosed spaces has been refined and implemented within a commercial computational fluid dynamics (CFD) solver. The drift-flux model is tested against a previously reported study of aerosol deposition within a ventilated chamber to illustrate the performance of the approach for unsteady particle concentrations. The model has been made more general by accounting for deposition to surfaces at any angle to the gravitational vector. The algorithms for the calculation of the deposition velocity have been revised to offer improved precision. The model shows good performance when compared with the measured particle concentration decay rates. Comparison with well-mixed models shows obvious differences from the measured and drift-flux CFD approach. The dependence of point concentration measurements of decay rates on sampling time is explored and the limitations for estimating steady-state deposition behaviour are highlighted. An important aspect of aerosol ventilation of incompletely mixed enclosed spaces is illustrated. The drift-flux approach is shown to perform very well at reproducing the unsteady particle concentration decay observed.     

Modeling of submicron particle filtration in an electret monolith filter with rectangular cross-section microchannels

Electret monolith filters have the advantage of low pressure drop and high filtration efficiency. In such filters, the filtration of submicron aerosol particles occurs as air passes through millions of microchannels. This article investigates the flow and filtration mechanisms in a representative rectangular microchannel of an electret monolith filter. An improved incompressible lattice Boltzmann method with Bhatnagar–Gross–Krook (traditionally shortened as LBGK) and lattice velocity D3Q15 model is employed to simulate no-slip and slip flows in the rectangular microchannels of a monolith filter. We considered mono-disperse submicron particles and one-way coupling (particle motion was affected by the flow, but the presence of particles did not affect the flow). Based on flow computations, the effects of key dimensionless parameters (Reynolds number, Knudsen number, Stokes number and the dimensionless length of the channel) on the total capture efficiency of mono-disperse submicron particles were investigated. Our results indicate that the optimal monolith filter should be characterized by a Knudsen number between 0.022 and 0.044, and that the dimensionless length of the channel should be between 4 and 8.

Copyright © 2016 American Association for Aerosol Research     

Analysis of particle dispersion in a turbulent flow considering particle rotation

Non-spherical particles exist widely in natural and industrial fluid systems and the motions of nonspherical particles are significantly different from that of spherical particles. In this paper, a simplified model of non-spherical particles considering particle drag correction, lift, and rotation was established.Based on the Eulerian–Lagrangian simulation, the dispersion characteristics of spherical and nonspherical particles with different Stokes numbers in a high-speed turbulent jet were anal...     

Data-driven prediction model of indoor air quality in an underground space

Several data-driven prediction methods based on multiple linear regression (MLR), neural network (NN), and recurrent neural network (RNN) for the indoor air quality in a subway station are developed and compared. The RNN model can predict the air pollutant concentrations at a platform of a subway station by adding the previous temporal information of the pollutants on yesterday to the model. To optimize the prediction model, the variable importance in the projection (VIP) of the partial least squares (PLS) is used to select key input variables as a preprocessing step. The prediction models are applied to a real indoor air quality dataset from telemonitoring systems data (TMS), which exhibits some nonlinear dynamic behaviors show that the selected key variables have strong influence on the prediction performances of the models. It demonstrates that the RNN model has the ability to model the nonlinear and dynamic system, and the predicted result of the RNN model gives better modeling performance and higher interpretability than other data-driven prediction models.     

Validation of the Langevin particle dispersion model against experiments on turbulent mixing in a T-junction

The fluctuating fluid velocities seen by particles entrained in a turbulent fluid have recently been modeled using a stochastic model based normalized Langevin Continuous Random Walk (CRW). This model has been quite successful in predicting particle dispersion in mildly complex flows. In the present study, we aim at validating the CRW model further against data collected in a challenging 3D geometry. We consider turbulent fluid mixing downstream of a T-junction using a hybrid Euler-Lagrange approach whereby tracer particle trajectories are computed and mixing of the streams deduced from the relative concentration of particles originating from the two inlet branches of the Tee. In a first simulation, RANS Reynolds Stress Model (RSM) is used to obtain the mean flow field, whereas the fluid fluctuations are specified from a CRW. Simulation results are compared to experimental data on mixing of two isothermal streams consisting of tap and de-ionized water, respectively. It is found that RSM-CRW yields strong under-prediction of the mixing. Closer look at the results shows that the Reynolds stresses, which are required inputs to the CRW, are poorly predicted with RSM. Detached Eddy Simulations (DES) are subsequently performed to provide the mean flow field, and the DES-CRW model predictions are found to compare quite well with the experimental data.     

Monte Carlo N-particle tracking of ultrafine particle flow in bent microtubes

The problem of large pressure-differential-driven laminar convective–diffusive ultrafine aerosol flow through bent microtubes is of interest in several contemporary research areas including; release of contents from pressurized containment vessels, aerosol sampling equipment, advanced scientific instruments, gas-phase microheat exchangers, and microfluidic devices. In each of these areas, the predominant problem is the determination of the fraction of particles entering the microtube that is deposited within the tube and the fraction that is transmitted through. Due to the extensive parameter restrictions of this class of problems, a Lagrangian particle tracking method making use of the coupling of the analytical stream line solutions of Dean for convective motion and a sampling of a Gaussian distribution for diffusive motion is a useful tool in problem characterization. This method is a direct analog to the Monte Carlo N-Particle method of particle transport extensively used in nuclear physics and engineering. In this work, 10-nm-diameter particles with a density of 1 g/cm³ are tracked within microtubes with toroidal bends with pressure differentials ranging between 0.2175 and 0.87 atmospheres. The tubes have radii of 25 microns or 50 microns and the radius of curvature is either 1 m or 0.3183 cm. The carrier gas is helium, and temperatures of 298 K and 558 K are considered. Numerical convergence is considered as a function of time step size and of the number of particles per simulation. Particle transmission rates and deposition patterns within the bent microtubes are calculated.

Copyright © 2016 American Association for Aerosol Research     

An improved CFD model of gas flow and particle interception in a fiber material

An improved CFD model of gas flow and particle interception in a fiber material which fiber size is Y-shape was developed in this work.The porous medium model was used to build the model of the whole size of fiber filter medium.Mixture model was adopted.The algorithm of particle interception in the whole size of fiber filter medium was derived and UDF (User Defined Function) that described kinds of particle filtering mechanisms in filter fibrous media was added to the Fluent default conservation equation as source term for simulation.The inertial resistance of the filter was taken into consideration,which provided a more precise measurement of the smoke flow and the particle interception in the filter under higher smoke speed conditions.The commercial software,Fluent 6.3,was used to simulate the smoke flow and particle interception in the filter in a single suction.The velocity and pressure profiles of smoke or nicotine particle in the filter,as well as nicotine particle volume fraction profile were well simulated.Finally.the comparisons of nicotine particle filtration efficiency between Fluent simulation results in this work and experimental results,as well as the model prediction in the literature were made to validate the simulation model.The comparisons showed that the particle entrapment model from simulation results was in good agreement with that from the experimental results.In addition,the Fluent simulation results are closer to reality both at the beginning and the end of the smoke process compating with the model predicted results in the literature.     

Modeling particle deposition and distribution in a chamber with a two-equation Reynolds-averaged Navier–Stokes model

A Lagrangian simulation for aerosol particle transport and deposition in a chamber is developed. The eddy interaction model (EIM) is adopted to generate the instantaneous turbulent fluctuating velocity field. It is found that a satisfactory result can be obtained only when the near-wall grid is sufficiently fine and the near-wall turbulent kinetic energy is damped effectively according to its component normal to the wall. Seven particle size groups ranging from 0.01 to are studied. A comparison between the current numerical model and a semi-empirical expression indicates that improved deposition fraction results were obtained. The particle deposition and particle fate in the chamber are also presented.     

基于改进遗传算法的PEMFC电堆温度模型建模

针对质子交换膜燃料电池（PEMFC）系统过于复杂、难以建模,而已建立的数学模型由于电堆使用的材料、结构不同,模型参数相差甚远,无法统一给出一个具有代表性的高精度的PEMFC电堆模型,难以满足PEMFC控制系统设计和应用的要求.本文从实际应用角度出发,利用传热学、流体动力学知识建立了一个千瓦级的PEMFC电堆温度模型.由于模型中一些关键参数无法由实验精确测出,但它们对模型精度的影响极大,所以需要找出一个能够高精度辨识模型参数的方法来解决这个问题.文章中提出的改进遗传算法具有良好的全局和局部的搜索、优化能力,能够高精度地辨识这些参数.仿真和实验结果验证了模型的实用性和可靠性.     

Modification of polypropylene fibres with cationic polypropylene dispersion for improved dyeability

Polypropylene (PP) fibres are important hydrophobic fibres which are used in the production of functional textiles such as sports textiles. The absence of functional groups and low polarity make PP fibres difficult to dye, thus mass coloration during fibre extrusion is the major technique applied today. However, the disadvantage of mass coloration is the low flexibility and the demand to produce high volumes. A new method to modify the surface of PP fibres utilises the deposition and thermal fixation of cationic PP dispersion. Through padding and thermal fixation of a cationic PP dispersion, dyeable 100% PP fibres can be obtained. The effects of fixation temperature, and of the amount of dispersion used on the modified fibres were studied using Fourier Transform‐infrared spectroscopy, laser scanning microscopy, dyeing experiments with CI Acid Red 151, and by determining selected fastness properties. The results indicate the potential of this new method to produce surface‐modified 100% PP fibres, which can be dyed in conventional acid‐dyeing processes and therefore used in fibre blends, for example in combination with wool.