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.     

Application of a modified Eulerian model to study the particle deposition on a vertical surface in turbulent flow

In this study the v2-f model was used with the two-phase Eulerian approach to predict the particle deposition rate on a vertical surface in a turbulent flow. The standard Eulerian particle model was adopted from the literature and modified, considering the majority of particle transport mechanisms in the particle deposition rate. The performance of the modified model was examined by comparing the rate of particle deposition on a vertical surface with the experimental and numerical data in a turbulent channel flow available in the literature. The model took into account the effects of drag force, lift force, turbophoretic force, electrostatic force, inertia force and Brownian/turbulent diffusion on the particle deposition rate. Electrostatic forces due to mirror charging and charged particles under the influence of an electric field were considered. The predictions of the modified particle model were in good agreement with the experimental data. It was observed that when both electrostatic forces are present they are the dominant factor in the deposition rate in a wider range of particle sizes.     

Highly swirling transient flows in spray dryers and consequent effect on modeling of particle deposition

Cotton turf and hotwire measurements were used to experimentally assess the highly swirling flow characteristics in a spray dryer fitted with a rotary disc atomizer. The numerical simulation captured key features of the flow field. Analysis revealed that rapid rotation of an atomizing disc tends to centrifugally split the central inlet jet. The flow field exhibited significant long time-scale transient behaviour. However, this centrifugally split jet resulted in a balanced upward recirculation region in the chamber as expected by the jet-feedback mechanism. Detailed analysis using a ‘transient air-steady particle’ approach revealed that this approach is applicable for such highly swirling transient flows as well. However, caution should be exercised when interpreting the deposition results, particularly in regions of low particle velocities. This is anticipated be an important consideration in future attempts to simulate the predominantly transient flows in spray dryers.     

环吹风温湿度的控制

介绍了环吹风与纺丝的关系。环吹风温湿度对车间前纺丝束冷却、降温起着重要作用。从温湿度,装置环吹风的控制原理等方面分析了影响环吹送风温湿度的因素。指出了分汽包蒸气压力的正常调整和异常处理方法。对环吹风温湿度的稳定起到良好效果。     

分离技术在室内空气污染物净化过程中的应用

论述了室内空气污染物的主要来源、种类、危害、净化分离的方法以及常见分离净化设备,并对几种常见的分离净化方法特性,适用场合进行了分析比较。     

Surface roughness and its influence on particle adhesion using atomic force techniques

The surface force interactions between individual 8 μm diameter spheres and atomically flat substrates have been investigated using atomic force techniques. The lift-off force of glass, polystyrene, and tin particles from atomically smooth mica and highly oriented pyrolitic graphite substrates was determined as a function of the applied loading force in an inert nitrogen environment. While the relative magnitudes of the measured lift-off force were found to scale as expected between the various systems studied, the absolute values were a factor of ～50 smaller than expected from the Johnson, Kendall, and Roberts theory. The surface topography of representative spheres was characterized with atomic force microscopy, allowing a quantitative assessment of the role that surface roughness plays in the adhesion of micrometer-size particles to substrates. Taking into account the radius of curvature of the asperities measured from the atomic force scans, agreement between the measured and theoretical estimates for the lift-off forces was improved, with the corrected experimental forces about a factor of 3 smaller than theoretical expectations.     

Shape of a particle curtain falling in stagnant air

The dynamics of a free falling particle curtain were analysed and studied in the laboratory. In the experiments a steady, uniform, high-voidage stream of particles was fed through a rectangular slit to form a curtain across the entire width of a horizontal duct of cross-section of 0.15 × 0.60 m. The thickness of the particle stream at the inlet was varied over the range 2-10 cm by varying the width of the rectangular slit. The particle flow rates were maintained constant at approximately 0.040 kg/s. The shape of the particle curtain was recorded using a high-speed video camera. Our results indicated a low voidage curtain led to diverging curtain; increasing the initial voidage of the particle stream caused the curtain to become less divergent; further increase the voidage of the particle stream caused the curtain to become increasingly convergent. The shape of particle curtain was analysed using an Eulerian-Eulerian computational fluid dynamics (CFD) model. Reasonable agreement was found between the experiments and the CFD model predictions.     

Cold weather installation of wood flooring by adhesive bonding and its impact on indoor air quality

This study discusses the effect of installation of wood flooring by adhesive bonding under cold weather conditions on indoor air quality (IAQ) and analyzes an onsite strategy that can ensure the adhesion strength of the wood flooring and improve IAQ. To examine the temperature effects of adhesive bonding on volatile organic compound (VOC) emissions from wood flooring, the emission rates from flooring composites were compared with those from adhesive and plywood flooring as individual materials under temperature variations in a small-scale chamber. In addition, the effect of strategies, including room heating to enhance adhesion performance, and the ventilation on material emissions and indoor concentrations, were examined in onsite apartment units equipped with a radiant floor heating system. The results of the small-scale chamber test showed that VOC emissions increased notably when the temperature was elevated, particularly in the adhesive-bonded plywood flooring compared to plywood flooring alone. The results of the onsite field test showed that the flooring bonded onto the radiant floor heating system, which supplied direct heat to the flooring, caused emission of a large amount of VOCs due to the combined effects of the bonding installation and temperature-dependent characteristics. Overall, the results indicate that heating the room in conjunction with ventilation is an effective strategy to ensure adhesion strength and to improve IAQ under cold weather adhesive bonding conditions when the material temperatures can be properly maintained.     

Effects of drying temperature and relative humidity on spaghetti characteristics

This study aimed at investigating the effect of drying conditions on spaghetti properties, i.e., its color, surface structure, rupture strength, rehydration characteristics, texture, and sauce retention capacity. The effects of temperature and humidity were independently examined under constant drying conditions, which were compared to those applied industrially, where the temperature and relative humidity are changed stepwise with time. The knowledge obtained in this study is considered useful for reasonably determining the drying conditions for producing spaghetti with desired properties.     

3-D Monte Carlo simulation of particle deposition on a permeable surface

Monte Carlo simulations are performed to investigate deposition phenomena of sticky particles that undergo diffusive, convective, and gravitational influences on a highly permeable surface. Deposited structures are studied in 3-D space using a Pelcet number as the key parameter. Fractal dimensions of the particle trajectory and deposit structure indicate that a transition from diffusion-limited to convection (and gravitation)-limited transport occurs at the Peclet number of Pe = 10^− 0.84 = 0.146. Analysis of the lateral density shows three distinct regions of deposit structures (defined in this study): founding, grown, and progressing regions. A penetration depth is defined to explain the relationship between particle transport and the evolution of deposit structures.     

Fibrous particle deposition in human nasal passage: The influence of particle length, flow rate, and geometry of nasal airway

Man-made vitreous fibers (MMVFs) have been used as a substitute for asbestos in industrial and residential applications. This shift has raised the concerns of the potential hazards associated with inhalation of these fibers. The human nose is an important protective organ that captures harmful particles and then clears them from human respiratory tract. However, studies have shown that some or even most of the inhalable fibrous particles can penetrate human nose and deposit into the deep lung. The understanding of fibrous particle deposition in the human nasal passage has important occupational health and possible drug delivery applications. To study the deposition pattern and influential factors, three realistic human nasal models were used and a dielectrophoretic classifier was applied to generate test aerosol of glass fibers with a narrow length distribution. These models were made by using stereolithography based on MRI data from two human subjects. Regional and total deposition efficiencies were measured for five different flow rates: 4, 8, 12, 15, and 18 Lpm and four different fiber length ranges: 10–19, 20–29, 30–39, and 40–. This study found that deposition of glass fibers (with about diameter) in human nasal passage is mainly due to inertial impaction and these fibers orientated themselves normal to the flow direction before deposition occurs. An effective aerodynamic diameter is defined such that the deposition efficiencies of glass fibers are comparable with those of spherical particles. Non-dimensional parameters were defined and an empirical model based on the experimental results is proposed to calculate fibrous particle deposition efficiency in human nose. Empirical expressions were also developed to estimate the pressure drop across the nasal model. Thus, empirical equations are now available for the prediction of total deposition in the human nasal tract for the fibrous particles under constant inspiring flow rates. In addition, this study suggested that these equations can also be used to predict the deposition of spherical particles.     

Characterization of nickel manganite NTC thermistor films prepared by aerosol deposition at room temperature

NiMn₂O_4+δ thermistor thick films have been successfully deposited by the so-called Aerosol Deposition Method (ADM) at room temperature on alumina substrates, Si-wafers, as well as on special planar four-wire interdigital electrode structures for high-precision electrical characterization. The NTCR films are homogeneous, completely dense and scratch resistant. Both as-deposited and tempered, the NTCR films exhibit a cubic spinel structure. Between 25 °C and 90 °C, the NTCR film resistance behaves as it is typical for variable range hopping (VRH) with parabolic density of states. As a result of moderate film tempering, the thermistor constant B and the specific resistance at 25 °C (ρ₂₅) decrease from 4250 K to 4020 K and 65 Ω·m to 40 Ω·m respectively, and are close to bulk values. In combination with the excellent reproducibility of the ρ₂₅ and B values, AD processing of films appears to be a promising alternative for classical ceramic bulk processes.     

Advantageous and detrimental effects of air sparging in membrane filtration: Bubble movement, exerted shear and particle classification

In various membrane applications air scour is applied to minimise fouling and to remove cake layers. Optimisation of module design and operating conditions (e.g., geometry and aeration intensity) requires knowledge of the most suited hydrodynamic conditions for the filtration task. However, many fundamentals of this multiphase flow in membrane modules are still unknown and difficult to access experimentally. Using experimental and numerical investigations it was shown that air sparging can have advantageous but also detrimental effects: depending on membrane plate spacing, wall shear can decrease with bubble size. Additionally, particle classification or segregation which increases the cake’s hydraulic resistance must be taken into account. Based on such findings, it will be possible to derive optimum bubble sizes, membrane spacing, aeration intensities and start-up strategies.     

Convective heat and mass transfer coefficient measurements and correlations for particle beds using temperature and humidity step changes with air flow through a test bed

In this paper, transient responses of a porous urea particle bed subject to a step change in the inlet temperature or humidity for a forced convective air flow through the particle bed are investigated to determine the convective heat and mass transfer coefficients inversely by comparing the measured time constant with the predicted characteristic time constant, which is a function of the convection coefficients and Reynolds number. The experimental results show, that although both the time constants for temperature and humidity step changes are dependent on Reynolds number, the temperature response time constant (35-1300 s) is much larger than the humidity response time constant (4-25 s) for the Reynolds number range of 300-5. The surface adsorption of water vapor is very rapid but the absorption inside the porous urea particle is slowed by a very low internal effective diffusion coefficient within the particles whereas the very low Biot number for heat transfer in the particles implies a complete thermal interaction with the air flow throughout each particle and a much larger time constant. Empirical correlations of the Chilton-Colburn j-factor and Nusselt number versus Reynolds number are compared with the correlations of other researchers. These new correlations, which include an uncertainty analysis, imply much lower convective coefficients than those reported previously in the literature.     

考虑地面积尘时置换通风的数值模拟

引言 置换通风系统始于北欧,较多应用于工业通风.近几年来,由于其合理的温度分布、较高的通风效率和明显的节能特性,置换通风系统已经越来越多地应用到民用建筑中.在北欧新建的办公建筑中,约有50%～70%的部分采用了置换通风系统[1].     

Deposition of micron-sized particles on flat surfaces: effects of hydrodynamic and physicochemical conditions on particle attachment efficiency

An experimental study of micron-sized particle deposition on flat surfaces is presented, aimed at delineating the effects of hydrodynamic and physicochemical interactions on particle transport and attachment efficiency, and at obtaining a better understanding of the particle sticking probability, a concept employed in modelling particulate fouling of industrial heat exchangers. Dilute particle suspensions are employed in a parallel-plate-laminar-flow channel, and hydrodynamic and physicochemical conditions are systematically varied. Deposition rates are determined by optical microscopy and image analysis techniques. It is observed that if gravity forces are present (in a horizontal channel) they control deposition at low wall shear stresses. As the hydrodynamic wall shear stress increases particle deposition rates are significantly reduced due to the effect of hydrodynamic lift or drag forces inhibiting transport or attachment. In general, for hydrodynamic conditions similar to those encountered in industrial heat exchangers, it appears that the particle sticking probability is significantly lower than unity.     

Electrochemical deposition and characterization of NiFe coatings as electrocatalytic materials for alkaline water electrolysis

In this study, nickel (Cu/Ni), iron (Cu/Fe) and nickel-iron (Cu/NiFe) composite coatings with various chemical compositions were electrochemically deposited on a copper electrode and characterized using cyclic voltammetry (CV), atomic absorption spectroscopy (AAS), scanning electron microscopy (SEM) and atomic force microscopy (AFM) techniques in view of their possible applications as electrocatalytic materials for the hydrogen evolution reaction (HER) in an alkaline medium. The electrocatalytic activity of the coatings for the HER was studied in 1 M KOH solution using cathodic current-potential curves and electrochemical impedance spectroscopy (EIS) techniques. The presence of nickel along with iron increases the electrocatalytic activity of the electrode for the HER when compared to nickel and iron coatings individually. The HER activity of the composite coatings depends on the chemical composition of the alloys. The Cu/NiFe-3 electrode (with a molar concentration ratio of Ni²⁺:Fe²⁺ of 4:6 in the plating bath) was found to be the best suitable cathode material for the HER in an alkaline medium under the experimental conditions studied. Furthermore, the electrocatalytic activity of the Cu/NiFe-3 electrode for the HER was tested for extended periods of time in order to evaluate the change in the electrocatalytic activity of the electrode with operation time. The HER was activation controlled and has not been changed after electrolysis. A constant current density of 100 mA cm⁻² was applied to the electrolysis system, and the corrosion behavior of the Cu/NiFe-3 electrode was investigated after different operation times using EIS and linear polarization resistance (LPR) techniques. For comparison, the corrosion behavior of a Cu/NiFe-3 electrode to which current was not applied was also investigated. The corrosion tests showed that the corrosion resistance of the Cu/NiFe-3 cathode changed when a cathodic current was applied to the electrolysis system.     

温湿度变化对固体火箭发动机粘接界面的影响

固体火箭发动机的粘接界面通常在贮存和使用过程中容易受到多种环境因素的影响。采用加速老化的方法研究钢壳体发动机粘接界面在湿热老化条件下的性能变化。双因素显著性分析可对界面扯离力的测试影晌进行量化评估。结果表明湿热环境下界面力学性能下降明显,湿度变化为主要原因。     

Effects of relative humidity and particle type on the performance and service life of automobile cabin air filters

Cabin air filters are the main barrier for protecting automobile passengers from on road particulate matter. There are many studies about the evaluation of their performance in terms of filtration efficiency. However, the knowledge about the loading capacity of them is still lacking. Meanwhile, there has been no quantitative method to estimate the proper filter service life time. This study focuses on testing the loading capacity of different types of cabin air filters under the conditions of different relative humidity values and particle types. The results indicate that when the relative humidity increases, the activated carbon coated filters can adsorb significant amounts of water with no significant increase of the pressure drop. The normal fibrous filters show in contrast negligible water adsorbance. Compared with the filters loaded by Arizona road dust only, loading the filters by Arizona road dust and soot particles simultaneously will result in the steeper loading curves as well as the shift of most penetrating particle size to the smaller diameter. Finally, a new method to estimate the proper service life time of the cabin air filters is suggested based on the loading curves.

© 2016 American Association for Aerosol Research