NUMBER CONCENTRATION, SIZE DISTRIBUTION AND FINE PARTICLE FRACTION OF TROPOSPHERIC AND STRATOSPHERIC AEROSOLS

1. Introduction The number concentration and size distribution of at-mospheric particles are the two major parameters for aerosol radiative forcing calculation, as the aerosol bur-dens affect the radiative transfer of solar and terrestrial emissions, while the size distribution of atmospheric aerosol is critical to all climatic influences (IPCC, 2001). However most measurements on the characteristics of the number-size distribution were carried out near the earth surface, with only a few exte…     

High-order resolution Eulerian–Lagrangian simulations of particle dispersion in the accelerated flow behind a moving shock

This paper presents a computational study of the two-dimensional particle-laden flow developments of bronze particle clouds in the accelerated flow behind a moving normal shock. Particle clouds with a particle volume concentration of 4% are arranged initially in a rectangular, triangular and circular shape. Simulations are performed with a recently developed high-order resolution Eulerian–Lagrangian method that approximates the Euler equations governing the gas dynamics with the improved high order weighted essentially non-oscillatory (WENO-Z) scheme, while individual particles are traced in the Lagrangian frame using high-order time integration schemes. Reflected shocks form ahead of all the cloud shapes. The detached shock in front of the triangular cloud is weakest. At later times, the wake behind the cloud becomes unstable, and a two-dimensional vortex-dominated wake forms. Separated shear layers at the edges of the clouds pull particles initially out of the clouds that are consequently transported along the shear layers. Since flows separated trivially at sharp corners, particles are mostly transported out of the cloud into the flow at the sharp front corner of the rectangular cloud and at the trailing corner of the triangular cloud. Particles are transported smoothly out of the circular cloud, since it lacks sharp corners. At late times, the accelerated flow behind the running shock disperses the particles in cross-stream direction the most for the circular cloud, followed by the rectangular cloud and the triangular cloud.     

Dual characters of interaction between particles and fluid

The unique characteristics of gas-solids two-phase flow and fluidization in terms of the flow structures and the apparent behavior of particles and fluid-particle interactions are closely linked to physical properties of the particles, operating conditions and bed configurations. Fluidized beds behave quite differently when solid properties, gas velocities or vessel geometries are varied. An understanding of hydrodynamic changes and how they, in turn, influence the transfer and reaction characteristics of chemical and thermal operations by variations in gas-solid contact, residence time, solid circulation and mixing and gas distribution is very important for the proper design and scale-up of fluidized bed reactors. In this paper, rather than attempting a comprehensive survey, we concentrate on examining some important positive and negative impacts of particle sizes, bubbles, clusters and column walls on the physical and chemical aspects of chemical reactor performance from the engineering application point of view with the aim of forming an adequate concept for guiding the design of multiphase fluidized bed chemical reactors.One unique phenomenon associated with particle size is that fluidized bed behavior does not always vary monotonically with changing the average particle size. Different behaviors of particles with difference sizes can be well understood by analyzing the relationship between particle size and various forces. For both fine and coarse particles, too narrow a distribution is generally not favorable for smooth fluidization. A too wide size distribution, on the other hand, may lead to particle segregation and high particle elutriation. Good fluidization performance can be established with a proper size distribution in which inter-particle cohesive forces are reduced by the lubricating effect of fine particles on coarse particles for Type A, B and D particles or by the spacing effect of coarse particles or aggregates for Type C powders.Much emphasis has been paid to the negative impacts of bubbles, such as gas bypassing through bubbles, poor bubble-to-dense phase heat & mass transfer, bubble-induced large pressure fluctuations, process instabilities, catalyst attrition and equipment erosion, and high entrainment of particles induced by erupting bubbles at the bed surface. However, it should be noted that bubble motion and gas circulation through bubbles, together with the motion of particles in bubble wakes and clouds, contribute to good gas and solids mixing. The formation of clusters can be attributed to the movement of trailing particles into the low-pressure wake region of leading particles or clusters. On one hand, the existence of down-flowing clusters induces strong solid back-mixing and non-uniform radial distributions of particle velocities and holdups, which is undesirable for chemical reactions. On the other hand, the formation of clusters creates high solids holdups in the riser by inducing internal solids circulations, which are usually beneficial for increasing concentrations of solid catalysts or solid reactants.Wall effects have widely been blamed for complicating the scale-up and design of fluidized-bed reactors. The decrease in wall friction with increasing the column diameter can significantly change the flow patterns and other important characteristics even under identical operating conditions with the same gas and particles. However, internals, which can be considered as a special wall, have been used to improve the fluidized bed reactor performance.Generally, desirable and undesirable dual characteristics of interaction between particles and fluid are one of the important natures of multiphase flow. It is shown that there exists a critical balance between those positive and negative impacts. Good fluidization quality can always be achieved with a proper choice of right combinations of particle size and size distribution, bubble size and wall design to alleviate the negative impacts.     

Experimental and numerical studies on the burning of aluminum micro and nanoparticle clouds in air

An experimental study has been conducted to determine flame propagation velocities in clouds of micro- (4.8 μm) and nano- (187 nm) aluminum particles in air at various concentrations. The experimental results show faster flame propagation in nanoparticle cloud with respect to the case of microparticles. Maximum flame temperature has been measured using a high-resolution spectrometer operating in the visible range. Analysis of combustion residual shows that nanoparticles combustion is realized via the gas-phase mechanism. A three-stage particle combustion model has been proposed based on these observations. Model parameters have been fitted to match the experimental results on the flame velocity and maximum temperature. Particle burning time is estimated from the flame simulations.     

光滑粒子模拟方法在超高速碰撞现象中的应用

简要介绍了基于黎曼解的光滑粒子法,并将改进的SPH方法应用于超高速碰撞,对二维轴对称条件下的弹丸超高速碰撞薄板问题进行了数值模拟,研究了靶板厚度、弹丸速度、弹丸形状等因素对形成碎片云的影响。通过与实验数据比较,该算法模拟的碎片云的形状及特征与实验相吻合,验证了光滑粒子法对冲击动力学问题数值模拟的有效性。     

Dispersion of a cloud of particles by a moving shock: Effects of the shape, angle of rotation, and aspect ratio

This paper discusses the particle-laden flow development from a cloud of particles in an accelerated flow behind a normal moving shock. The effects of the aspect ratio of a rectangular and ellipsoidal cloud and the cloud’s angle of attack with respect to the carrier flow are studied. Computations are performed with an in-house high-order weighted essentially non-oscillatory (WENO-Z) finite-difference scheme-based Eulerian-Lagrangian solver that solves the conservation equations in the Eulerian frame, while particles are traced in the Lagrangian frame. Streamlined elliptically shaped clouds exhibit a lower dispersion than blunt rectangular clouds. The averaged and root-mean-square locations of the particle coordinates in the cloud show that the cloud’s streamwise convection velocity increases with decreasing aspect ratio. With increasing rotation angle, the cross-stream dispersion increases if the aspect ratio is larger than unity. The particle-laden flow development of an initially moderately rotated rectangle is qualitatively and quantitatively comparable to the dispersion of an initially triangular cloud.     

The discrete heat source approach to dust cloud combustion

In the present paper, combustion of dust clouds from the discrete point heat source method has been addressed. Time-place temperature profile generated by single particle burning has been obtained to study the dust combustion. The summation of the temperature profiles of burned and burning particles predict the temperature in the preheating zone so that the ignition time of layer in flame front can be determined. Consequently the flame propagating speed was obtained based on the dust concentration corresponding to particles spacing and particle diameter. This method has been validated with aluminum dust cloud combustion. Decrease in the dust concentration leads to the lean limit of dust combustion. Increase in particles diameter or reduction in the dust concentration causes higher lean limit and also reduction in the flame propagating speed. Adding the ignition energy as igniter to this system, provides the path to study the effects of ignition energy in the dust combustion.     

不同粒径PMMA粉尘云火焰温度特性研究

为揭示粒径分布对聚甲基丙烯酸甲酯（polymethyl methacrylate,PMMA）粉尘云火焰温度的影响,本文分别采用热电偶和高速比色测温法测量了开敞空间不同粒径PMMA粉尘云的火焰温度特性。结果表明:相比30 μm粉尘粒子,100 nm粉尘粒子热解/挥发速率较快,燃烧更加充分,粉尘云火焰的最高温度可达1 551℃,而30 μm粉尘云火焰最高温度仅为1 108℃;在微米尺度,随着PMMA粉尘粒径的增大,火焰最高温度和高温火焰区面积先增大后减小;20 μm粉尘粒子由于其分散性较好,裂解气化特征时间尺度与燃烧反应特征时间尺度较接近,燃烧反应充分,火焰最高温度和高温火焰区面积均最大。     

天然橡胶磨粒侵蚀过程中的表面化学效应

研究了天然橡胶分别在含石英砂的清水、聚丙烯酰胺溶液和氢氧化钠溶液冲击作用下磨粒侵蚀过程中所发生的表面化学效应，并且利用扫描电子显微镜、Ｘ射线光电子能谱仪和傅立叶表面红外仪，对天然橡胶在磨损前后的表面形貌、元素的化学状态和官能团进行了观察与分析。     

固定床化学链燃烧氧化反应的数值模拟

基于Chimera网格采用有限体积法模拟了450个颗粒随机填充固定床中的化学链燃烧的氧化反应过程,并采用三维瞬态N-S方程,结合压力Poisson方程方法,详细分析了床层入口Re=5时的颗粒内部和外部的传质传热过程．模拟结果揭示了在大颗粒的固定床中,颗粒内部有效扩散系数对颗粒内部的传质起着决定性作用,而且颗粒表面的浓度梯度决定了总反应速率;另外,有惰性芯的结构化颗粒能有效地改善颗粒内部总的反应速率,颗粒的转化速率,并且能使床层很快地达到热平衡．模拟结果能更好地帮助我们认识固定床化学链反应器中的反应和组分传递机理．     

清水介质条件下天然橡胶磨损45#钢的机理研究

为了提高流体机械和石油机械中橡胶－金属摩擦副的使用寿命，针对目前人们大都只着眼于改善橡胶耐磨性的现状，对清水介质条件下天然橡胶磨损４５＃钢的机理进行了研究．用扫描电子显微镜和Ｘ射线光电子能谱仪，对天然橡胶和４５＃钢表面磨损前后的微观形貌、元素的组成和结合能进行了分析，用傅立叶表面红外分析仪分析了天然橡胶磨损前后表面官能团的变化．结果表明：在给定的试验条件下，天然橡胶磨损４５＃钢的物理过程主要是滞留在摩擦表面的铁屑和添加剂颗粒对钢的微切削作用；摩擦界面发生的力化学反应主要有橡胶表面的分子链力裂解和氧化降解，橡胶大分子自由基与Ｆｅ反应生成Ｆｅ－高分子化合物，橡胶大分子链中的羧基与Ｆｅ的反应，以及Ｆｅ的氧化反应．橡胶磨损４５＃钢的过程是Ｆｅ与橡胶及介质之间发生力化学反应在４５＃钢表面形成化学反应膜，反应膜在微切削作用下脱落造成钢的磨损．这种磨损机理主要属于摩擦界面材料的化学－力学自催化破坏机理     

Dispersion modeling by kinematic simulation: Cloud dispersion model

A new technique has been developed to compute mean and fluctuating concentrations in complex turbulent flows (tidal current near a coast and deep ocean). An initial distribution of material is discretized into any small clouds which are advected by a combination of the mean flow and large scale turbulence. The turbulence can be simulated either by kinematic simulation (KS) or direct numerical simulation. The clouds also diffuse relative to their centroids; the statistics for this are obtained from a separate calculation of the growth of individual clouds in small scale turbulence, generated by KS. The ensemble of discrete clouds is periodically re-discretized, to limit the size of the small clouds and prevent overlapping. The model is illustrated with simulations of dispersion in uniform flow, and the results are compared with analytic, steady state solutions. The aim of this study is to understand how pollutants disperses in a turbulent flow through a numerical simulation of fluid particle motion in a random flow field generated by Fourier modes. Although this homogeneous turbulent is rather a “simple” flow, it represents a building block toward understanding pollutant dispersion in more complex flow. The results presented here are preliminary in nature, but we expect that similar qualitative results should be observed in a genuine turbulent flow.     

Large-eddy Simulation of Pilot-assisted Pulverized-coal Combustion in a Weakly Turbulent Jet

Large-eddy simulation has been performed to investigate pilot-assisted pulverized-coal combustion in a weakly turbulent air jet. An advanced pyrolysis model, the chemical percolation devolatilization (CPD) model, has been incorporated into the LES framework to predict the local, instantaneous pyrolysis kinetics of coal particles during the simulation. Prediction on volatile species generation is thus improved, which provides an important initial condition for gas-phase volatile and solid-phase char combustion. For gas-phase combustion, the partially stirred reactor (PaSR) model is employed to model the combustion of volatile species, taking into account subgrid turbulence-chemistry interactions. For heterogeneous solid-phase char combustion, both the intrinsic chemical reaction on the internal surface of a char particle and the diffusion of gaseous oxidant through the film layer around the particle have been incorporated by using a kinetic/diffusion surface reaction model. The LES results show overall good agreements with experimental data. Sensitivity analysis has been performed to better understand the impact of parameter uncertainties on the LES results.     

Surrogate atmospheric dust particles generated from dune soils in laboratory: Comparison with field measurement

Desert dust strongly influences the climate and the environment by altering the radiation budget, participating in atmospheric chemical reactions, and engaging the biogeochemical cycle. Studying its impacts requires knowledge of the physical and chemical properties of the original particles from the source origin. Unfortunately, the field collection of atmospheric samples in the desert is impractical. Fine surface soil is thus considered an alternative material from which to obtain the information applicable to dust particles from the desert soil. In this study, a laboratory system was set up to generate dust particles with simulated natural wind erosion processes from surface soils or directly from desert surfaces. Surface soils and field dust were simultaneously collected during a local blowing dust event. The comparison between the laboratory-generated dust and the field dust showed a consistent trend in particle size distribution and chemical composition. The chemical compositions were found to have explainable differences with the Certified Reference Materials for Asian dust (CJ-2). The laboratory-generated dust particles with the system could be applied as surrogates for soil-emitted dust in desert areas.     

Cloud dispersion models

A new technique has been developed to compute mean and fluctuating concentrations in complex turbulent flows. An initial distribution of material is discretised into any small clouds which are advected by a combination of the mean flow and large scale turbulence. The turbulence can be simulated either by Kinematic Simulation or by a stochastic model for the motion of each cloud centroid. The clouds also diffuse relative to their centroids; the statistics for this are obtained from a separate calculation of the growth of individual clouds in small scale turbulence, generated by Kinematic Simulation. The ensemble of discrete clouds is periodically rediscretised, to limit the size of the small clouds and prevent overlapping. The model is illustrated with simulations of dispersion in uniform flow and in a coastal flow, and the results are compared with analytic, steady state solutions where available.     

Cloud dispersion models

A new technique has been developed to compute mean and fluctuating concentrations in complex turbulent flows. An initial distribution of material is discretised into any small clouds which are advected by a combination of the mean flow and large scale turbulence. The turbulence can be simulated either by Kinematic Simulation or by a stochastic model for the motion of each cloud centroid. The clouds also diffuse relative to their centroids; the statistics for this are obtained from a separate calculation of the growth of individual clouds in small scale turbulence, generated by Kinematic Simulation. The ensemble of discrete clouds is periodically rediscretised, to limit the size of the small clouds and prevent overlapping. The model is illustrated with simulations of dispersion in uniform flow and in a coastal flow, and the results are compared with analytic, steady state solutions where available.     

Positron emission particle tracking-Application and labelling techniques

The positron emission particle tracking (PEPT) technique has been widely used in science and engineering to obtain detailed information on the motion and flow fields of fluids or granular materials in multiphase systems, for example, fluids in rock cracks, chemical reactors and food processors; dynamic behaviour of granular materials in chemical reactors, granulators, mixers, dryers, rotating kilns and ball mills. The information obtained by the PEPT technique can be used to optimise the design, operational conditions for a wide range of industrial process systems, and to evaluate modelling work. The technique is based on tracking radioactively labelled particles (up to three particles) by detecting the pairs of back-to-back 511 ke V -γ-rays arising from annihilation of emitted positrons. It therefore involves a positron camera, location algorithms for calculating the tracer location and speed, and tracer labelling techniques. This paper will review the particle tracking technique from tracking algorithm, tracer labelling to their application.     

Numerical investigation of the expansion of a cloud of dispersed particles or drops under the influence of an explosion

The results are given of a numerical investigation into the expansion of clouds of fine disperse particles or drops suspended in air under the influence of a blast wave. Two possible situations are simulated: an explosion in front of a cloud and an explosion in the region of a cloud (explosion with simultaneous formation of a drop region behind the shock front). The influence that the determining parameters have on the range and extension of the expansion zone is analyzed, and the results of the numerical experiments are generalized by means of empirical dependences.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 1, pp. 82–90, January–February, 1982.We thank K. I. Kozorezov for drawing our attention to the subject of the investigation and A. L. Gonor for a helpful discussion.     

Role of surface roughness in the magnesite flotation and its mechanism

Surface roughness has a significant influence on mineral flotation. The assisting effect of surface roughness on minerals flotation is extensively investigated from its physical properties (e.g., the existing form of asperity and its size), however, the associated effect on mineral flotation based on the differences in surface chemical property caused by surface roughness has been rarely touched. With such a question in mind, in this study, we investigated the flotation recoveries of two batches of magnesite particles with varying degree of surface roughness produced by two different mills, and associated the flotation performances to their surface chemical properties (amount of adsorption sites for the collector) via a series of detections, including Scanning Electron Microscope-Energy Dispersive Spectrometry (SEM-EDS) observations, X-ray photoelectron spectroscopy (XPS) analysis, adsorption capacity tests, and contact angle measurements. Finally, we concluded that rougher magnesite particles could provide more active sites (Mg²⁺) for a larger capacity of sodium oleate (NaOL), thereby improving the hydrophobicity and floatability.     

Positron emission particle tracking—Application and labelling techniques

The positron emission particle tracking (PEPT) technique has been widely used in science and engineering to obtain detailed information on the motion and flow fields of fluids or granular materials in multiphase systems, for example, fluids in rock cracks, chemical reactors and food processors; dynamic behaviour of granular materials in chemical reactors, granulators, mixers, dryers, rotating kilns and ball mills. The information obtained by the PEPT technique can be used to optimise the design, operational conditions for a wide range of industrial process systems, and to evaluate modelling work. The technique is based on tracking radioactively labelled particles (up to three particles) by detecting the pairs of back-to-back 511 keV γ-rays arising from annihilation of emitted positrons. It therefore involves a positron camera, location algorithms for calculating the tracer location and speed, and tracer labelling techniques. This paper will review the particle tracking technique from tracking algorithm, tracer labelling to their application.