A Method To Employ the Aerodynamic Particle Sizer Factory Calibration Under Different Operating Conditions

The dimensionless aerodynamic particle sizer (APS) response function (normalized particle velocity against particle Stokes number) first reported by Chen et al. (1985) is explored for much larger solid particles (diameters to 35 μm) over a similar range of instrument pressures (624–l740 mm Hg) and flow rates (4.2–6.0 L/min). An essentially unique response function is found for low and intermediate Stokes numbers under a variety of operating conditions, including the use of argon as the carrier gas. For large particles, however, non-Stokesian drag effects introduce systematic differences among calibration sets so that a unique response function no longer applies. The largest differences are observed between calibrations performed in air and argon, although even in this case the sizing error amounts to < 12% for a 20-μm polystyrene latex sphere. For intermediate Stokes numbers, a direct consequence of this work is that a reference calibration (channel number against Stokes number) can be used under different ambient conditions by setting the APS to operate at the same nozzle velocity as used in the reference calibration. With the single-velocity method, the factory-supplied calibration relating channel number to aerodynamic diameter can be used for air over a reasonable range of ambient temperatures and pressures. The same calibration can be used with an argon carrier gas provided that the aerodynamic diameters reported by the APS software are adjusted by the square root of the gas viscosity ratio. For the single-velocity mode of operation, a generalization of a correction proposed by Wang and John (1987, 1989) can be made and is shown to reduce by one half the sizing error introduced by non-Stokesian drag.     

A Study of Density Effect and Droplet Deformation in the TSI Aerodynamic Particle Sizer

Effects of particle density and droplet deformation on the performance of a TSI aerodynamic particle sizer (APS) were studied using polystyrene latex (PSL), dioctyl phthalate (DOP), ammonium fluorescein (AF), fused aluminosilicate (FAP), and fused cerium oxide (FCO) monodisperse aerosols. Results indicated that, because of the sensitivity of the instrument, periodic cleaning of the APS inner nozzle is needed to maintain the consistency of its calibration curve. Density effects were experimentally confirmed with PSL, AF, FAP, and FCO aerosols of particle densities ranging from 1.05 to 4.33 g/cm³. Results, however, showed that this effect can only be experimentally detected for particles of density greater than 2 g/cm³ and aerodynamic diameter greater than 5 (μm. Effects of droplet deformation were studied with DOP.     

Effects of Gas Density and Viscosity on Response of Aerodynamic Particle Sizer

The performance of the aerodynamic particle sizer (APS) in an atmosphere that differs from standard calibration conditions was studied. The response of the APS for monodisperse polystyrene latex and dioctylphthalate particles ranging from 2.4 to 15.1 μm in diameter was measured in air, argon, and nitrous oxide atmospheres. The measurements indicated that particles in argon and in nitrous oxide accelerate faster than those in air. In order to postulate the mechanisms and interpret the observed results, the effects of both gas viscosity and gas density on instrument performance were considered. The particle density correction factor previously given by Wang and John (1987) (Aerosol Science and Technology. 6:191–198) was extended to include gas property effects. Good agreement was obtained between the new factor and the experimental data. Existing data obtained at a low pressure in air were also compared successfully with the developed model. Expected applications of the work are the use of the APS in an arbitrary gas or at an arbitrary temperature and pressure without a new calibration.     

Particle attrition mechanism with a sonic gas jet injected into a fluidized bed

High velocity gas jets in fluidized beds provide substantial particle attrition: they are used industrially to control the particle size in fluid bed cokers and to grind products such as toner, pharmaceutical or pigment powders. One method to control the size of the particles in the bed is to use an attrition nozzle, which injects high velocity gas and grinds the particles together. An important aspect of particle attrition is the understanding and modeling of the particle breakage mechanisms. The objective of this study is to develop a model to describe particle attrition when a sonic velocity gas jet is injected into a fluidized bed, and to verify the results using experimental data. The model predicts the particle size distribution of ground particles, the particle breakage frequency, and the proportion of original particles in the bed which were not ground. It was found that the particle breakage frequency can be used to predict the attrition results in different bed sizes. A correlation was also developed, which uses the attrition nozzle operating conditions such as gas density and equivalent speed of sound to predict the mass of particles broken per unit time.     

喷管型气体分布器和颗粒物性对提升管内气固两相流动行为的联合影响

在两套均采用喷管型气体分布器的循环流化床实验装置上分别采用河沙和FCC颗粒系统测试了提升管内的轴向压力梯度分布和局部颗粒浓度,研究了气体分布器结构和颗粒直径及密度对上行气固两相流动行为的共同影响.结果表明,当表观气速小于8.0 m·s-1时,粗重的河沙颗粒在喷管型气体分布器效应逐渐消失的过程中会出现不同程度的减速,而细轻的FCC颗粒在本实验的测试范围内则不存在上述现象.当采用喷管型气体分布器时,粗重的河沙颗粒在加速过程中,不仅其颗粒浓度显著高于FCC,而且其沿径向分布的不均匀程度也明显大于FCC;但在充分发展段,河沙的颗粒浓度不仅比FCC低,而且在径向的分布也更为均匀.     

喷动床中气体停留时间分布的研究

In a spouted bed of 80mm in ID and 1700mm in height, the gas residence time distributions at different radial positions in both spout and annular area were measured with five different kinds of particles as spouting material, air as spouting gas, and hydrogen as tracer. The effects of superficial gas velocity, operating pressure, particle size and its category on gas residence time distribution were discussed. It was found that the gas velocity profile in spout was more uniform than that in annulus. It could be concluded that the gas flow in the spout could be treated as a plug-flow, while that in the annulus inhibited a strong non-ideal flow behavior. Increasing the superficial gas velocity and decreasing the operating pressure, the particle density and its size gave rise to spouting disturbance, thus the measured tracer concentrations vs. time curves fluctuated. The variances of residence time distribution curves could be taken as a measure of the gas fluctuation degree.     

Some aspects on gas-solid flow in a U-bend: Numerical investigation

Pneumatic drying is widely used in many engineering applications. It has been shown in earlier research [Fyhr C. and Rasmuson A., Mathematical model of a pneumatic conveying dryer, AIChE Journal, Vol. 43, pp 2889-2902, 1997.] that the U-bends in the pneumatic conveying dryer system significantly influence drying behavior since they create enhanced slip velocities between suspended material and the drying medium. On one hand, this slip will increase external heat and mass transfer rates, thereby enhancing drying conditions. On the other hand, increasing the number of bends will cause an increase in pressure drop. Use of the suitable mean gas velocity and the suitable bend radius ratio will result in a better design and improved operating conditions.Two-phase CFD calculations, using a Eulerian-Eulerian model and commercial program Fluent 6.0, were employed to calculate the gas and particle flows in a U-bend. Variables studied include: particle diameter, particle density, particle volume fraction, gas velocity and bend radius ratio. The numerical calculations were validated against experimental data from the literature. The density and diameter of particle vary from 600 up to 1000 kg/m³ and from 0.00025 up to 0.001 m, respectively. The gas velocity and particle volume fraction vary from 10 up to 25 m/s and from 0.001 up to 0.01 m³/m³, respectively. The bend radius ratio varies from 3 up to 8 m/m. The slip velocity is affected by all the studied parameters, in particular, particle diameter, gas velocity and bend radius ratio; whereas the total pressure drop is strongly affected by gas velocity and bend radius ratio. A low mean gas velocity will give a lower total pressure drop and longer particle residence time. A small bend radius ratio will produce a faster dispersion of particles, which benefits drying, but on the other hand, will increase the total pressure drop. Thus, optimizing gas velocity and bend radius ratio is important in reducing energy consumption.     

三维上流式反应器床层流动和返混特性

采用内径为280 mm的上流式反应器,以空气模拟气相、甘油和水混合溶液模拟渣油。用3种不同粒径的氧化铝球形工业催化剂颗粒为填充颗粒,考察了不同模拟物系的颗粒粒径、颗粒密度、液相黏度、不同床层的高径比和不同操作条件对上流式反应器内床层压降及其波动、床层轴向返混的影响规律。得到模拟工业运行物系和操作条件的上流式反应器床层总压降关联式,相对误差在12%以内。床层总压降均随床层高径比、颗粒密度和液相黏度增加而增大,但随颗粒粒径的增大而减小,床层压降波动随表观气速增加而增大。填充颗粒粒径越小、颗粒密度越小、高径比越大,床层内轴向返混越严重;床层内压降和轴向返混均随表观气速的增加而增大。     

RESS法制备微细颗粒的喷嘴内流动特性的数值模拟

为了对RESS法制备微细颗粒过程中喷嘴内流体规律进行研究,通过对超临界流体快速膨胀法（rapid expansion of supercritical solution,RESS）流动过程的研究与分析,建立了喷嘴内超临界流体流动数学模型。对喷嘴内流场和温度场进行研究,考察了预膨胀压力、预膨胀温度、长径比等操作参数对RESS过程的影响,模拟结果表明,喷嘴内部的密度曲线在喷嘴入口段,几乎没有发生变化,而在直管段和出口膨胀段超临界流体密度发生急剧下降;随着长径比的增大,喷嘴内密度曲线变陡;随着长径比的增大,喷嘴出口处流体的温度都变小,过饱和度变大,结晶颗粒使得更为细小。该模型和模拟过程能够为实现制备均一微细颗粒的实际操作条件和优化过程参数奠定基础。     

Powder processing in a plasma jet: A proposed model

A theoretical study is reported on the use of d.c. and r.f. plasma jets as chemical reactors for the processing of minerals in the form of a fine powder. The temperature and flow fields of the jet are calculated by solving the integral boundary layer equations. Single particles trajectories are obtained by solving the Basset-Odar equations. A multi-particle model is then developed for a feed of known particle size and injection velocity distributions under low loading conditions. Calculators are made on the thermal decomposition of molybdenum disulphide (5 to 30-micron equivalent diameter). The parameters investigated are the free jet velocity, the mean injection velocity, and the reactor ambient conditions. The results are presented as the probability density distributions of the gas loading, particle temperature, and conversion at different levels downstream of the nozzle.     

Spray cone angle and its correlation in a high pressure fuel spray

An experimental study was conducted to determine the effects of nozzle orifice size and operating parameters on the cone angle of a liquid fuel spray injected into a gaseous environment. High fuel pressures in the range 55 to 130 MPa and background gas density ranging from atmospheric to 40 kg/m³ were used in the study. The results show the spray cone angle to depend on the orifice dimensions as well as on the operating conditions such as the injection velocity and the background gas density. A correlation is derived to predict spray cone angles in terms of Reynolds and Weber numbers. The predicted results are shown to agree well with the experimental findings of other investigators.     

HIGH-SPEED PARTICLE BEAM GENERATION: SIMPLE FOCUSING MECHANISMS

Modern chemical characterization instruments employ an aerosol inlet that transmits atmospheric aerosols to the low pressure source region of a time-of-flight mass spectrometer, where particles are ablated and ionized using high energy irradiation. The ions when analyzed in the mass spectrometer yield information about the elemental composition of airborne aerosols. Often, the rate at which particles are analyzed is limited by the transmission rate of the inlet used. Depending on their size, particles are lost during sampling usually due to inertial effects or diffusion. Often simple capillaries and conical nozzles are used as primary focusing elements in the formation of high-speed particle beams. Due to the basic nature of the focusing mechanism, such elements transmit particles efficiently over a narrow size range. This size range strongly depends on the nozzle geometry and operating conditions. In this work, numerical techniques are used to (a) simulate fluid and particle transport in axi-symmetric nozzles, (b) help understand and identify the mechanisms by which particle beams are formed in capillaries and conical nozzles, and (c) illustrate the contrasting nature of the beams thus formed. Particle focusing is also simulated in some typical inlets to validate the predictions and illustrate the merits and drawbacks of each design.     

Jet penetration depth in a two-dimensional spout-fluid bed

The jet penetration depth was proposed to be an important parameter to describe the jet action during the chemical process of spout-fluid bed coal gasification. A two-dimensional cold model of a spout-fluid bed coal gasifier with its cross section of and height of 2000 mm was established to investigate the jet penetration depth. Four types of Geldart group D particles were used as bed materials. A multi-channel pressure sampling system and a high-resolution digital CCD camera were employed for experimental investigations. The effects of spouting gas velocity, spout nozzle diameter, static bed height, particle property and fluidizing gas flow rate on the jet penetration depth have been systematically studied by pressure signal analysis and image processing. Experimental results indicate that the jet penetration depth increases with increasing spouting gas velocity and spout nozzle diameter, while it decreases with increasing particle density, particle diameter, static bed height and fluidizing gas flow rate. Additional, a new correlation considered all of the above effects especially static bed height and fluidizing gas flow rate, was developed for predicting the jet penetration depth in spout-fluid beds. The correlation was compared with published experimental data or correlations, which was in well agreement with the present experimental results and some other references.     

Water electrolysis and pressure drop behaviour in a three-dimensional electrode

Water electrolysis from acidified solution was used as a model system to investigate the net contribution of hydrogen bubbles to the pressure drop increase in a three-dimensional electrode. A bed of silvered glass beads in both fixed and fluidized state was used, assuming an unchanging particle surface during the experiments. Pressure drop behaviour with time was measured for different experimental conditions and presented relative to the pressure drop determined for a bubble free bed. Parameters, such as current density, electrolyte velocity and particle size, greatly influence the relative pressure drop behaviour in the three-dimensional electrode. A sudden increase in the pressure drop occurs with the appearance of a gas phase in the bed, reaching a constant value (plateau) after a certain time; this plateau corresponds to steady state conditions. The pressure drop increases with increasing current density. This increase is in the range 40-150% relative to the bubble free electrolyte flow through the bed. Electrolyte flow-rate also strongly influences the pressure drop in the hydrogen evolving fixed bed electrode. It was observed that the relative pressure drop decreases with increasing electrolyte velocity. At higher flow rates, peaks occur on the pressure drop-time curves, indicating the existence of channeling inside the bed in which spouting occurs. The time to reach the pressure drop plateau decreases with increasing electrolyte velocity as do the time intervals corresponding to maximum pressure drop values. At the minimum fluidization velocity the peaks disappear and the relative pressure drop decreases with time, tending to approach a constant value. For hydrogen evolution in the fluidized bed, the pressure drop is lower than that measured in the absence of gas, and reason for this decrease being the gas hold-up in the bed.     

Deposition Uniformity and Particle Size Distribution of Ambient Aerosol Collected with a Rotating Drum Impactor

Rotating drum impactors (RDI) are cascade type impactors used for size and time resolved aerosol sampling, mostly followed by spectrometric analysis of the deposited material. They are characterized by one rectangular nozzle per stage and are equipped with an automated stepping mechanism for the impaction wheels. An existing three-stage rotating drum impactor was modified, to obtain new midpoint cutoff diameters at 2.5 μm, 1 μm, and 0.1 μm, respectively. For RDI samples collected under ambient air conditions, information on the size-segregation and the spatial uniformity of the deposited particles are key factors for a reliable spectrometric analysis of the RDI deposits. Two aerodynamic particle sizers (APS) were used for the determination of the RDI size fractionation characteristics, using polydisperse laboratory room air as quasi-stable proxy for urban ambient air. This experimental approach was suitable for the scope of this study, but was subject to numerous boundary conditions that limit a general use. Aerodynamic stage penetration midpoint diameters were estimated to be 2.4 and 1.0 μm for the first two RDI stages. Additionally, the spatial uniformity and geometrical size distribution of the deposited aerosol were investigated using micro-focus synchrotron radiation X-ray fluorescence spectrometry (micro-SR-XRF) and transmission electron microscopy (TEM), respectively. The size distribution of the particles found on the TEM samples agreed well with the results from the APS experiments. The RDI deposits showed sufficient uniformity for subsequent spectrometric analysis, but in the 2.5–10 μm size range the particle area density was very low. All of the applied methods confirmed the theoretical cutoff values of the modified RDI and showed that compared to other cascade impactors, the determined stage penetration sharpness was rather broad for the individual impactor stages.     

THE EFFECT OF TEMPERATURE ON FLUIDIZED BED BEHAVIOUR

Most published correlations for the minimum fluidizing gas velocity have been derived from tests under ambient conditions and increasing discrepancy is found in their application over wider ranges of operating conditions. Up to 1000°C the Ergun equation is reliable but it requires a knowledge of the particle shape factor and bed voidage for its application. Bed voidage is found to vary with temperature for laminar gas flow conditions.

Paralleling changes in gas flow conditions with operating temperature are changes in bed-to-surface heat transfer coefficients. There is a distinct transition from the interphase gas convective to the particle convective component of heat transfer being the dominant mechanism as the operating temperature increases and Re_mf reduces through 12,5 at Ar ～ 26000. This is thought to be a consequence of change in bed bubbling behavior.     

Mass Concentration of Diesel Particle Emissions from Photoacoustic and Opacity Measurements

In this report we compare the capabilities of two optical devices—a conventional opacity meter and a recently developed photoacoustic instrument—for measurement of diesel particulate emissions. Emission measurements were performed using two vehicles (built in 1978 and 1980 and equipped with 5.7 liter diesel engines) operated at a variety of steady-state conditions and also over the federal test procedure driving cycle. The light absorbed by diesel particles heats them thereby increasing the ambient gas pressure. The change in pressure, which can be related to the particle mass concentration, is measured in the photoacoustic instrument. This is about 100 times more sensitive than the opacity meter so that it can be used to measure particulate emissions even in diluted exhaust. Further, its operation in the infrared region ensures that particle size variations do not affect its calibration against mass concentration. However, the observed optical data, both in the visible and in the infrared region, are dependent on engine operating conditions, which indicates that other particle characteristics, such as shape, are significant. Consequently, the measurement of absolute mass concentrations with the photoacoustic instrument has some of the uncertainties experienced with opacity meters.     

原子吸收分光光度法测定水泥中高含量钙时的几种分析计算方法

研究并比较了用原子吸收分光光度法测试水泥中高含量组份钙时的几种分析计算方法、即工作曲线法、工作曲线弯曲部分直接作图法、工作曲线弯曲部分校直汁算法、工作曲线线性拟合法。在硼酸-氟硼酸密封高压分解试样的溶样条件下,对几种分析计算方法的精度及回收率等进行了实验,并测试了一些水泥试样。本文所讨沦的分析计算方法是对原子吸收分析法用于高含量组份分析时的一般性讨论,具有较广泛的适用性。     

DROPLET DISTORTION IN ACCELERATING FLOW

Several commercial instruments size particles based on their acceleration in a high-velocity flow field. Previous work suggested that droplet distortion in these instruments resulted in inaccurate sizing. Liquid aerosol droplet shape distortion produced in an accelerating flow field was therefore computed through analytical solution of the Navier–Stokes equation for comparison to experiment. A high-Reynolds-number empirical approximation to the pressure external to the droplet was used in these calculations. Within the droplet, the longest-lived excitations correspond to a quadrupolar distortion of shape. Droplet excitations were obtained in terms of aerosol diameter, viscosity, surface tension and density. At the largest viscosities considered (as in many oils), only a damped relaxation was found, whereas at lower viscosities and high surface tension (as in water) damped capillary oscillations were predicted as possible, given rapid shifts in the surrounding air flow. In order to compute the effect of airflow varying in time, an approximate Green's function was used. The Green's function in the frequency domain was approximated using only a pair of poles, thereby accounting for only the longest-lived excitations. In application of the theory to compute aerosol distortion on passage through an aerodynamic particle sizer (APS) acceleration nozzle, the change in air velocity was found to be so gradual that no oscillations were induced for droplets as small as 20-μm diameter. Measurements of droplet undersizing in the APS compared favorably with the theoretical predictions. The theoretical results were also consistent with photographs of distorted oleic acid and only slightly distorted water droplets emerging from a nozzle.     

THE EFFECT OF TEMPERATURE ON FLUIDIZED BED BEHAVIOUR

Most published correlations for the minimum fluidizing gas velocity have been derived from tests under ambient conditions and increasing discrepancy is found in their application over wider ranges of operating conditions. Up to 1000°C the Ergun equation is reliable but it requires a knowledge of the particle shape factor and bed voidage for its application. Bed voidage is found to vary with temperature for laminar gas flow conditions.

Paralleling changes in gas flow conditions with operating temperature are changes in bed-to-surface heat transfer coefficients. There is a distinct transition from the interphase gas convective to the particle convective component of heat transfer being the dominant mechanism as the operating temperature increases and Re_mf reduces through 12,5 at Ar ∼ 26000. This is thought to be a consequence of change in bed bubbling behavior.