Design and Characterization of a Fluidized Bed Aerosol Generator: A Source for Dry,Submicrometer Aerosol

A fluidized bed aerosol generator has been designed and built for the purpose of generating a constant output of dry, submicrometer particles with a large number density. The output of the fluidized bed for generating aerosol particles from dry soot powder has been characterized using a differential mobility analyzer and a condensation particle counter. The particle size distribution is bimodal, with a mode in the submicrometer diameter size range and a mode in the supermicrometer diameter size range. The larger diameter mode is fully separated from the smaller mode and can thus be easily removed from the aerosol flow using impaction techniques. The distribution in the submicrometer size range is nearly log-normal, with a count median diameter falling between 0.1 and 0.3 micrometers. A number density of greater than 10⁵ particles cm^-3 of soot particles in the submicrometer range can be produced, constant to within 25% (1 standard deviation) over a 4 h time period. The number density of particles produced in the submicrometer range was found to vary with the ratio of soot to bronze beads in the bed mixture, whether or not a feed system was used, and nitrogen flow rate through the fluidized bed and feed system.     

Analysis of Fluidized Beds for the Simultaneous Aerosol Separation and Heat Recovery

Abstract

A mathematical model is developed to describe the performance of fluidized beds for the simultaneous heat recovery and aerosol separation. This new concept is analyzed in light of the various transport processes taking place within the bed. A two-phase model is developed for the system in which heat and aerosol particles are transferred from the bubble phase to the emulsion phase. In addition to aerosol separation via diffusion, interception, impaction and electrostatic precipitation, thermophoretic collection is also analyzed. The results indicate that high thermal and separation efficiencies can be obtained.     

Deposition of Charged Aerosol Particles on a Substrate by Collimating Through an Electric Field Assisted Coaxial Flow Nozzle

Electroaerodynamic (EAD) jet printing, where aerodynamic force is coupled with electrostatic force in order to obtain a wide range of controlled pattern sizes, is introduced. Charged and sheathed aerosol particles yield a high deposition rate even at low velocity owing to the force of their electrostatic attraction to the substrate. In this study, two coaxial nozzles (inner diameters of 6 mm and 100 μm) were designed and tested theoretically and experimentally in order to observe the effects of electrostatic force, particle size, and air flow rate on particle trajectory and dot pattern size. A higher sheath air flow rate (higher Stokes number) caused the aerosol jet stream to be focused. For Stokes numbers higher than 1, the effect of applied voltage on pattern size was less than that of the sheath air flow rate. However, for Stokes number lower than 1, the pattern size was affected by both the applied voltage and the sheath air flow rate. After incorporating all data, the diameter of the particle deposition area (W_p) was expressed as a function of nozzle diameter (W), sheath air flow rate (Q_sheath), aerosol flow rate (Q_aerosol), Stokes number (Stk), and Electrostatic number (Es). Three different equations were obtained for Stk < 1, for 1 ≤ Stk < 5, and for Stk ≥ 5, respectively. These equations would be used to predict pattern width for given conditions of aerosol and sheath flow rates, particle size, electric field, and nozzle size.

Copyright 2013 American Association for Aerosol Research     

A Particle-into-Liquid Collector for Rapid Measurement of Aerosol Bulk Chemical Composition

We report on a new instrument developed for rapid automated on-line and continuous measurement of ambient aerosol bulk com- position.The general approach is based on earlier devices (Khlystov et al. 1995; Simon and Dasgupta 1995) in which ambient particles are mixed with saturated water vapor to produce droplets easily collected by inertial techniques. The resulting liquid stream is analyzed with an ion chromatograph to quantitatively measure the bulk aerosol ionic components. In this instrument, a modified ver- sion of a particle size magnifier (Okuyama et al. 1984) is employed to activate and grow particles comprising the fine aerosol mass. A single jet inertial impactor is used to collect the droplets onto a vertical glass plate that is continually washed with a constant water diluent flow of nominally 0.10 ml min^-1     

Effect of Scattering by Fluidization of Electrically Conductive Beads on Electrical Field Intensity Profile in Microwave Dryers

Abstract:

A technology to apply a fluidized bed of electrically conductive beads is proposed to improve uniformity of the electric field intensity in microwave dryers, which are required for uniform heating of wet media. The principle of this effectiveness lies in a dynamic random scattering of microwave due to motion of the conductive beads in the bed. The electrically conductive beads were prepared by wrapping aluminum foil around styrene foam balls with cellophane tape. The diameter and density were 13 mm and 123 kg/m³, respectively. The minimum fluidizing velocity of these beads agreed with the one predicted by the Wen–Yu equation when the distributor was a porous plate. However, the fluidization of the beads took place at a lower gas rate than the minimum fluidizing velocity due to a jet flow from the small pores for the distributor of a perforate plate. The intensity in the applicator of a commercial microwave oven became the most uniform when the beads were fluidized with a uniform holdup profile along the height of the bed placed in front of the applicator walls. The dynamic effect making the intensity uniform by the fluidization was advanced by increasing the area of apparent reflection by the fluidized bed and the holdup of beads and was superior to a conventional stirring blade.     

Novel Aerosol/Gas Inlet for Aircraft-Based Measurements

A novel inlet has been designed for selective sampling of gas and aerosol phases of volatile species from high-speed aircraft. A multistage flow system brings the flow nearly isokinetically towards the sampling port. Two small airfoil-shaped "blades" are placed close to the sample port to provide the flow conditions required for aerosol and gas sampling. Aerosols are sampled when these blades are positioned to operate the inlet as a counterflow virtual impactor (CVI). The design enables sampling of particles as small as 0.1 w m from a high-speed aircraft under stratospheric conditions, a substantial improvement over that possible with previous CVI designs. For gas sampling, one of the blades is moved by a stepper motor to occlude the inlet opening and gas is sampled perpendicular to the bulk flow. Boundary layer suction is used to prevent the sampled gas from coming in contact with the impactor walls. This is one of the first designs of an inlet that enables gas sampling free of wall contact. The inlet was flown on the NASA ER-2 aircraft during the SOLVE 2000 campaign to study aerosol/gas partitioning of nitric acid in the lower stratosphere. Data from the flight tests show that the inlet flow characteristics are broadly in agreement with computational fluid dynamics (CFD) simulations.     

Low Pressure Aerosol Flow Reactor

In this work we report the development of a novel low pressure aerosol flow reactor for the determination of the kinetic parameters of fast heterogeneous processes. The experimental apparatus consists of a spray atomizer to introduce aerosols into a low pressure zone; a fast flow reactor for kinetic measurements and an IR spectrometer and mass spectrometer for concentration measurements. The surface area distribution and number density of the aerosol particles are determined from their infrared spectra and the decay kinetics are determined by monitoring the disappearance rates of the gas phase species (with a mass spectrometer) as a function of the aerosol properties. We report the application of this apparatus to the investigation of the uptake of acetone by liquid water aerosols (0.1–20 μ m diameter) at room temperature and a pressure of 35 Torr. These measurements yielded a value of the mass accommodation coefficient, α, of 3.6 _{? 2} ^{+ 3.1} × 10 ^{? 3} .     

Regimes of Unstable Expansion and Diffusion Combustion of a Hydrocarbon Fuel Jet

Results of an experimental study of hydrodynamics and diffusion combustion of hydrocarbon jets are presented. Various regimes of instability development both in the jet flame proper and inside the source of the fuel jet are considered. The experiments are performed for the case of subsonic gas jet expansion into the air from a long tube 3.2 mm in diameter in the range of Reynolds numbers from 200 to 13 500. The fuel is the propane–butane mixture in experiments with a cold jet (without combustion) and pure propane or propane mixed with an inert dilutant (CO₂ or He) for the jet flame. The mean velocity and velocity fluctuations in the near field of the jet without combustion are measured. Among four possible regimes of cold jet expansion (dissipative, laminar, transitional, and turbulent), three last regimes are investigated. The Hilbert visualization of the reacting flow is performed. The temperature profiles in the near field of the jet are measured by a Pt/Pt–Rh thermocouple. An attached laminar flame is observed in the transitional regime of propane jet expansion from the tube. In the case of combustion of C₃H₈ mixtures with CO₂ or with He in the range of Reynolds numbers from 1900 to 3500, the transitional regime is detected in the lifted flame. Turbulent spots formed in the tube in the transitional regime exert a significant effect on the flame front position: they can either initiate a transition to a turbulent flame or lead to its laminarization.     

Experimental investigation of electrostatic effect on particle motions in gas‐solid fluidized beds

The excess accumulation of charges in the fluidized bed has a severe impact on hydrodynamics. Due to lack of effective experimental methods, electrostatic effects on hydrodynamics have mostly been studied using numerical simulation. By injecting a trace of liquid antistatic agents into a fluidized bed, charges were controlled and electrostatic influences on particle motions were investigated. The average particle–wall impact angles are acquired by developing multiscale wavelet decomposition of acoustic emission signals. The impact angles are significantly influenced by both charge levels and gas velocities. If the electric force is reduced and/or fluid drag is increased, friction dominates the particle–wall interactions. Under a larger gas velocity where fluid drag dominates, charges elimination causes no significant variation in particle impact angles, but particle velocities increase as well as at lower gas velocities. In addition, existence of electrostatic charges influences the ranges of bubble growing zone and jet impacting zone. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3628–3638, 2015     

外加电场下气固流化床的数值模拟

工业流化床中通常存在显著的静电效应,会影响流化床的流体力学特性,并常常带来一定的安全隐患,而引入外加电场可以达到"化害为利"的作用。为了获知外加电场下流化床内的流体力学特性,本文采用CFD耦合静电模型及解耦求解的方式,研究了外加电场下气固流化床内的轴向电势分布和颗粒的运动规律。研究结果表明,传统流化床的轴向电势分布呈Z形,与实验结果一致,引入直流电场后,颗粒自身产生的电场正电势区域明显增大,而负电势区域也有小幅增大。直流电场可以使得壁面处小颗粒的聚集减少,但在床中心出现明显的颗粒聚集。在交流电场下,小颗粒在壁面处的分布比正常流化床时的稀疏。通过对颗粒自身产生电场的分析,发现近分布板区和料位区的电场方向不同,近分布板区的电场指向床中心,而料位区的电场指向壁面。     

Experimental Study of Electrostatic Aerosol Filtration at Moderate Filter Face Velocity

Aerosol collection efficiency was studied for electrostatically charged fibrous filters (3M Filtrete?, BMF-20F). In this study, collection efficiencies at moderate filter face velocities (0.5–2.5 m/s) representative of some high volume sampling applications was characterized. Experimental data and analytical theories of filter performance are less common in this flow regime since the viscous flow field assumption may not be representative of actual flow through the filter mat. Additionally, electrostatic fiber charge density is difficult to quantify, and measurements of aerosol collection efficiency are often used to calculate this fundamental parameter. The purpose of this study was to assess the relative influence of diffusion, inertial impaction, interception, and electrostatic filtration on overall filter performance. The effects of fiber charge density were quantified by comparing efficiency data for charged and uncharged filter media, where an isopropanol bath was used to eliminate electrostatic charge. The effects of particle charge were also quantified by test aerosols brought into the equilibrium Boltzmann charge distribution, and then using an electrostatic precipitator to separate out only those test particles with a charge of zero. Electrostatically charged filter media had collection efficiencies as high as 70–85% at 30 nm. Filter performance was reduced significantly (40–50% collection efficiency) when the electrostatic filtration component was eliminated. Experiments performed with zero charged NaCl particles showed that a significant increase in filter performance is attributable to an induction effect, where electrostatic fiber charge polarizes aerosol particles without charge. As filter face velocity increased the electrostatic filtration efficiency decreased since aerosol particles had less time to drift toward electrostatically charged fibers. Finally, experimental data at 0.5 m/s were compared to theoretical predictions and good agreement was found for both electrostatic and nonelectrostatic effects.

© 2013 American Association for Aerosol Research     

Jet Assisted Aerosol Chemical Vapor Deposition for Optical Fiber Synthesis

Various kinds of high quality optical fibers are routinely fabricated by the modified chemical vapor deposition (MCVD), in which fine particles are generated through the oxidation of chemical precursor and deposited in a silica tube reactor. Efficiency, rate, and uniformity of particle deposition determine the quality and cost of optical fibers; therefore efforts to enhance aerosol deposition performance should be important for further improving both quality and cost. Here we propose a jet assisted aerosol chemical vapor deposition method utilizing gas jets in the conventional MCVD silica tube reactor for the purpose of enhancing the efficiency, rate, and uniformity of particle deposition. High temperature helium gas is injected radially through an electrically heated thin tube inserted inside the silica tube. High temperature gas jets push particles generated in a tube toward the tube wall and therefore shorten the axial length of particle trajectories before deposition and cause particles to experience higher thermophoretic force. As a result, deposition efficiency (and rate) was found to considerably increase compared to the conventional method, and the uniformity was also significantly improved.     

Ammonium Chloride Aerosol Nucleation and Growth in a Cross-Flow Impinging Jet Reactor

The nucleation and growth of an ammonium chloride aerosol starting from gaseous ammonia and hydrogen chloride was investigated experimentally and with the use of a mathematical model. The reactor was composed of 2 opposed jets perpendicular to a main stream and was operated under laminar/transition flow conditions. The reactants were segregated when they entered the reactor. The parameter observed was the particle size distribution of the aerosol described by its moments. Considerable scatter in the experimental results complicated their analysis, but some important trends could be identified. The model results were strongly affected by the fluid mechanics model, which influenced the predicted mixing inside the reactor. This work shows the importance of fluid mixing in controlling the aerosol size distribution.     

Synthesis of Titanium Dioxide Aerosol Gels in a Buoyancy-Opposed Flame Reactor

Aerosol gels are a novel class of materials with potential to serve in various energy and environmental applications. In this work, we demonstrate the synthesis of titanium dioxide (TiO₂) aerosol gels using a methane-oxygen coflow diffusion flame reactor operated in down-fired configuration (fuel flow in the direction opposite to buoyancy forces). Titanium tetraisopropoxide was fed as a precursor to the flame under different operating conditions. Control of the monomer size and crystalline phase of TiO₂ gel particles was achieved by adjusting the flame operating conditions, specifically the flame temperature, which was shown to significantly influence the phase transformation and rate of particle growth and sintering. The resulting materials were characterized for their physical and optical properties. Results showed that the TiO₂ aerosol gels had effective densities in the range 0.021–0.025 g/cm³, which is 2 orders of magnitude less than the theoretical mass density of TiO₂. The monomer size distribution, crystalline phase, and UV-Vis absorbance spectra of the gels showed distinct characteristics as a function of flame temperature.

Copyright © 2015 American Association for Aerosol Research     

Numerical simulation of horizontal jet penetration in a three-dimensional fluidized bed

The introduction of reactant gas as a jet into a fluidized bed chemical reactor is often encountered in various industrial applications. Understanding the hydrodynamics of the gas and solid flow resulting from the gas jet can have considerable significance in improving the reactor design and process optimization. In this work, a three-dimensional numerical simulation of a single horizontal gas jet into a cylindrical gas-solid fluidized bed of laboratory scale is conducted. A scaled drag model is proposed and implemented into the simulation of a fluidized bed of FCC particles. The gas and particles flow in the fluidized bed is investigated by analyzing the transient simulation results. The jet penetration lengths of different jet velocities have been obtained and compared with published experimental data as well as with predictions of empirical correlations. The predictions by several empirical correlations are discussed. A good agreement between the numerical simulation and experimental results has been achieved.     

电晕放电在气固流化床内静电控制中的应用

通过冷模实验,对比研究了施加电晕放电前后气固流化床内场强、颗粒荷质比和静电势的变化。实验发现,施加负电晕放电前,场强、颗粒荷质比和静电势均为正值,且随着气速的增大而增大;施加负电晕放电后,场强、颗粒荷质比和静电势均出现不同程度的下降,且下降幅度随着气速的增大而增大,达到平衡静电势所用的时间也随气速的增大而增大。实验结果表明,电晕放电可以用于气固流化床内的静电控制。     

EXPERIMENTAL AND NUMERICAL STUDY ON GAS FLOW IN THE GRID ZONE OF JETTING-FLUIDIZED BEDS

A grid model describing the gas flow and interchange in the grid zone of jetting fluidized beds is proposed. Based on this model, longitudinal gas concentration profiles in the jet and annulus are calculated. The longitudinal gas concentration distribution is also experimentally investigated in a jetting fluidized bed with an inside diameter of 50 mm at the ambient temperature, and a jetting fluidized bed with an inside diameter of 80 mm at high temperatures. Comparison between the calculated and experimental results has shown that the experimental profiles can be qualitatively predicted by the grid model. The results indicated that the concentration in the grid zone depends on the gas exchange between the jet and the annulus, and the net gas flow from the jet to the annulus. The gas exchange rate is mainly affected by the inlet gas velocity from the nozzle. The present study is thought to be helpful to understand the grid gas behavior in the jetting fluidized bed coal gasifier.     

大型射流流化床混沌特性的研究

1 INTRODUCTIONJetting fluidized beds have been widely applied in suchprocesses as catalytic and flame reactions,combustionand gasification of coal,treatment of waste,cleaningof dusty gases,coating and granulation.The flowcharacteristics of jetting fluidized beds are relevant tothe stable gas jet and the high rates of heat transfer     

A New Aerosol Flow System for Photochemical and Thermal Studies of Tropospheric Aerosols

For studying the formation and photochemical/thermal reactions of aerosols relevant to the troposphere, a unique, high-volume, slow-flow, stainless steel aerosol flow system equipped with UV lamps has been constructed and characterized experimentally. The total flow system length is 8.5 m and includes a 1.2 m section used for mixing, a 6.1 m reaction section and a 1.2 m transition cone at the end. The 45.7 cm diameter results in a smaller surface to volume ratio than is found in many other flow systems and thus reduces the potential contribution from wall reactions. The latter are also reduced by frequent cleaning of the flow tube walls which is made feasible by the ease of disassembly. The flow tube is equipped with ultraviolet lamps for photolysis. This flow system allows continuous sampling under stable conditions, thus increasing the amount of sample available for analysis and permitting a wide variety of analytical techniques to be applied simultaneously. The residence time is of the order of an hour, and sampling ports located along the length of the flow tube allow for time-resolved measurements of aerosol and gas-phase products. The system was characterized using both an “inert” gas (CO ₂ ) and particles (atomized NaNO ₃ ). Instruments interfaced directly to this flow system include a NO _x analyzer, an ozone analyzer, relative humidity and temperature probes, a scanning mobility particle sizer spectrometer, an aerodynamic particle sizer spectrometer, a gas chromatograph-mass spectrometer, an integrating nephelometer, and a Fourier transform infrared spectrophotometer equipped with a long path (64 m) cell. Particles collected with impactors and filters at the various sampling ports can be analyzed subsequently by a variety of techniques. Formation of secondary organic aerosol from α-pinene reactions (NO _x photooxidation and ozonolysis) are used to demonstrate the capabilities of this new system.     

Evaporative liquid jets in gas–liquid–solid flow system

Some aspects of the fundamental characteristics of evaporative liquid jets in gas–liquid–solid flows are studied and some pertinent literature is reviewed. Specifically, two conditions for the solids concentration in the flow are considered, including the dilute phase condition as in pneumatic convey and the dense phase condition as in bubbling or turbulent fluidized beds. Comparisons of the fundamental behavior are made of the gas–solid flow with dispersed non-evaporative as well as with evaporative liquids.For dilute phase conditions, experiments and analyses are conducted to examine the individual phase motion and boundaries of the evaporative region and the jet. Effects of the solids loading and heat capacity, system temperature, gas flow velocity and liquid injection angle on the jet behavior in gas and gas–solid flows are discussed. For dense phase conditions, experiments are conducted to examine the minimum fluidization velocity and solids distribution across the bed under various gases and liquid flow velocities. The electric capacitance tomography is developed for the first time for three-phase real time imaging of the dense gas–solid flow with evaporative liquid jets. The images reflect significantly varied bubbling phenomenon compared to those in gas–solid fluidized beds without evaporative liquid jets.