Investigation of the size of the incandescent incipient soot particles in premixed sooting and nucleation flames of n-butane using LII,HIM, and 1 nm-SMPS

Laser-induced incandescence (LII) measurements were conducted to explore the ability of LII to detect small soot particles of less than 10 nm in two sooting flat premixed flames of n-butane: a so-called nucleation flame obtained at a threshold equivalence ratio Φ = 1.75, in which the incipient soot particles undergo only minor soot surface growth along the flame, and a more sooting flame at Φ = 1.95. Size measurements were obtained by modeling the time-resolved LII signals detected using 1064 nm laser excitation. Spectrally-resolved LII signals collected in the nucleation flame display a similar blackbody-like behavior as mature soot. Soot particle temperature was determined from spectrally-resolved detection. LII modeling was conducted using parameters either relevant to those of mature soot or derived from fitting the modeled results to the experimental LII data. Particle size measurements were also carried out using (1) ex situ analysis by helium-ion microscopy (HIM) of particles sampled thermophoretically and (2) online size distribution analysis of microprobe-sampled particles using a 1 nm-SMPS. The size distributions of the incipient soot particles, found in the nucleation flame and in the early soot region of the Φ = 1.95 flame, derived from time-resolved LII signals are in good agreement with HIM and 1 nm-SMPS measurements and are in the range of 2–4 nm. The thermal and optical properties of incipient soot were found to be not radically different from those of mature soot commonly used in LII modeling. This explains the ability of incipient soot particles to produce continuous thermal emissions in the visible spectrum. This study demonstrates that LII is a promising in situ optical particle sizing technique that is capable of detecting incipient soot as small as about 2.5 nm and potentially 2 nm and resolving small changes in soot sizes below 10 nm.

© 2017 American Association for Aerosol Research     

Influence of flame-generated ions on the simultaneous charging and coagulation of nanoparticles during combustion

Flames generate a large amount of chemically and thermally ionized species, which are involved in the growth dynamics of particles formed in flames. However, existing models predicting particle formation and growth do not consider particle charging, which may lead to bias in the calculated size distribution of particles. In this study, Fuchs' charging theory was coupled with a monodisperse particle growth model to study the simultaneous charging and coagulation of nanoparticles during combustion. In order to quantify the charging characteristics of nanoparticles, a high-resolution DMA was used to measure the mobilities of ions generated from a premixed flat flame operated at various conditions. The effect of temperature on ion–particle and particle–particle combination coefficients was further examined. The proposed model showed that the influence of charging on particle growth dynamics was more prominent when the ion concentration was comparable to or higher than the particle concentrations, a condition that may be encountered in flame synthesis and solid fuel-burning. Simulated results also showed that unipolar ion environments strongly suppressed the coagulation of particles. In the end, a simplified analysis of the relative importance of particle charging and coagulation was proposed by comparing the characteristic time scales of these two mechanisms.

© 2017 American Association for Aerosol Research     

多喷嘴对置式气化炉内颗粒挥发分火焰可视化研究

基于实验室规模多喷嘴对置式水煤浆气化炉及其可视化装置,研究了气化炉喷嘴平面非射流区颗粒挥发分燃烧过程。结合图像处理技术,在气化条件下对粒径小于300 μm颗粒挥发分火焰尾迹形态及变化过程进行分析。研究结果表明,颗粒挥发分火焰非典型包络型火焰,而是形成挥发分火焰尾迹。颗粒挥发分尾迹形态受颗粒脱挥发分所处阶段和颗粒相对于气流的运动状态的影响,随时间不断变化。颗粒挥发分最大火焰尺寸随颗粒粒径增加而增加。气流床气化还原性气氛条件下颗粒挥发分燃烧时间较颗粒在富氧气氛中燃烧时间显著增加。     

Evaluation of an in situ sampling probe for its accuracy in determining particle size distributions from flames

An in situ sampling probe for flames is evaluated for any biases with respect to size distributions in a given size and concentration range. Ultrafine metal particles (Pd, Pt) suspended in argon gas are added to a laminar diffusion flame, burning with argon-diluted methane. Samples of combustion aerosol are extracted from the flame via a small orifice and immediately quenched with nitrogen. The size distribution of the metal particles, recorded at various sampling positions in the flame, is found to be but little affected by the flame and the sampling procedure. The distribution of carbonaceous soot particles from an unseeded flame at a fixed sampling position is not shifted upon seeding but coexists next to the peak from the metallic flame additive. It is concluded that droplets of volatile combustion products, formed in the sampling system through homogeneous nucleation, do not significantly contribute to the soot particle signal. An investigation of the photoelectric yield of the samples implies significant changes of the surface properties of the metal particles during the combustion process. This seeding method offers a new path to study chemical processes in the flame, especially heterogeneous and catalytic reactions of flame gases with particles.     

Spatial and temporal resolution of the particle image velocimetry technique in flame speed measurements

Limitations of the spatial and temporal resolution of the particle image velocimetry (PIV) technique in velocity field measurements in a laminar flame have been investigated. The limitations are due to the need to introduce a suspension of tracer particles into the flow. For a methane-air mixture with a stoichiometry coefficient of 0.9, it is determined that at a mass fraction of TiO₂ solid particles over 0.08%, the change in the flame propagation velocity by the particles exceeds 5%. The maximum spatial resolution of PIV for which the influence of the particles is insignificant corresponds to a concentration of 0.03%; in this case, the minimum resolvable scale is limited by a value 200 times larger than the size of tracer particles. Based on analytical estimates and a comparison of measured and numerically calculated particle velocities in the flame, it is concluded that particles smaller than 2 µm adequately track the flow velocity. Under these conditions, the error of the velocity measurement is mainly determined by the limited spatial resolution of PIV. The results of the work can be used to evaluate PIV measurement errors in other experimental studies of flames.     

高温气相法可控制备纳米TiO2

通过火焰反应器结构设计，实现TiCl4高温气相氧化过程可控制备纳米TiO2粒子，新型火焰设计保证了TiCl4低温进入高温反应区，预热过程隔离保护喷嘴，避免了结疤堵塞;通过实验条件控制颗粒平均粒径和粒径分布，较低的TiCl4气相浓度、较高的载气流速有利于小粒径TiO2颗粒的生成。载气流速增加，中心TiCl4火焰形态由层流向湍流发展，焰长缩短，颗粒平均粒径减小。CH4燃气流量增加，高温反应区扩大，颗粒停留时间增加，颗粒尺寸增大。二次氧气的补充，提高了氧气与TiCl4的预混，有效地减小了产品TiO2颗粒的粒径。获得的TiO2产品平均粒径在20～80 nm之间。     

Challenges of measuring nascent soot in flames as evidenced by high-resolution differential mobility analysis

Nascent soot particles with mobility diameters ≤10 nm were measured in an ethylene/air premixed flame to shed light on the challenges and potential artifacts affecting studies on soot inception by differential mobility analysis (DMA) techniques. The size distribution functions (SDFs) of particles with charge acquired either naturally or diffusively upon ion seeding were measured at several positions in the flame using rapid-dilution probing and a high-resolution DMA for different values of the ratio of dilution ratio to residence time (DR/Δt). The SDFs are roughly bimodal with a sub-3 nm mode and a larger one that appears either downstream in the flame or for low DR/Δts. Soot nuclei smaller than 3 nm preferentially acquire positive charge, which brings into question the assumption of steady-state charging probability of flame sampled soot nuclei in the bipolar diffusion neutralizer. The approximately polarity-symmetric lognormal SDF of larger particles is attributed to nuclei coagulation. Naturally charged particles increase in number when lowering DR/Δt, suggesting either their collisional charging by flame chemi-ions or particle nucleation by condensation of neutral molecules on ions or both. The critical conditions for suppressing particle coagulation and charge redistribution in the sampling system were not achieved under most conditions, despite the fact that values of DR/Δts were more favorable to such a suppression in the present experiment as compared to other studies in the literature. As a result, the identification of this “asymptotic” regime, which is critical to determine the parent SDFs and the charge state of nascent soot in the flame, is still elusive.

© 2016 American Association for Aerosol Research     

Charge distribution uncertainty in differential mobility analysis of aerosols

The inference of particle size distributions from differential mobility analyzer (DMA) data requires knowledge of the charge distribution on the particles being measured. The charge distribution produced by a bipolar aerosol charger depends on the properties of the ions produced in the charger, and on the kinetics of charge transfer from molecular ions or ion clusters to the particles. A single parameterization of a theoretically predicted charge distribution is employed in most DMA analyses regardless of the atmospheric conditions being probed. Deviations of the actual charge distribution from that assumed in the data analysis will bias the estimated particle size distribution. We examine these potential biases by modeling measurements and data inversion using charge distributions calculated for a range of atmospheric conditions. Moreover, simulations were performed using the ion-to-particle flux coefficients predicted for a range of properties of both the particles and ions. To probe the biases over the full range of particle sizes, the measurements were simulated through an atmospheric new particle formation event. The differences between the actual charge distribution and that according to the commonly used parametrization resulted in biases as large as a factor of 5 for nucleation-mode particles, and up to 80% for larger particles. Incorrect estimates of the relative permittivity of the particles or not accounting for the temperature and pressure effects for measurements at 10 km altitude produced biases in excess of 50%; three-fold biases result from erroneous estimates of the ion mobility distribution. We further report on the effects of the relative permittivity of the ions, the relative concentrations of negative and positive ions, and truncation of the number of charge states considered in the inversion.

Copyright © 2017 American Association for Aerosol Research     

Discrete particle simulation of solids motion in a gas–solid fluidized bed

The solids motion in a gas–solid fluidized bed was investigated via discrete particle simulation. The motion of individual particles in a uniform particle system and a binary particle system was monitored by the solution of the Newton's second law of motion. The force acting on each particle consists of the contact force between particles and the force exerted by the surrounding fluid. The contact force is modeled by using the analogy of spring, dash-pot and friction slider. The flow field of gas was predicted by the Navier–Stokes equation. The solids distribution is non-uniform in the bed, which is very diluted near the center but high near the wall. It was also found that there is a single solids circulation cell in the fluidized bed with ascending at the center and descending near the wall. This finding agrees with the experimental results obtained by Moslemian. The effects of the operating conditions, such as superficial gas velocity, particle size, and column size on the solids movement, were investigated. In the fluidized bed containing uniform particles better solids mixing was found in the larger bed containing smaller size particles and operated at higher superficial gas velocity. In the system containing binary particles, it was shown that under suitable conditions the particles in a fluidized bed could be made mixable or non-mixable depending on the ratios of particle sizes and densities. Better mixing of binary particles was found in the system containing particles with less different densities and closer sizes. These results were found to follow the mixing and segregation criteria obtained experimentally by Tanaka et al.     

Characterization of a new miniCAST with diffusion flame and premixed flame options: Generation of particles with high EC content in the size range 30 nm to 200 nm

Flame-generated soot from miniCAST burners is increasingly being used in academia and industry as engine exhaust soot surrogate for atmospheric studies and instrument calibration. Previous studies have found that elemental carbon (EC) content of miniCAST soot is proportional to the mean particle size. Here, the characterization of a prototype miniCAST generator (5201 Type BC), which was designed to decouple the soot composition from the particle size and produce soot particles with high EC and BC content in a large size range, is reported. This prototype may operate either in a diffusion-flame or a partially premixed-flame mode, an option that was not available in former models. It was confirmed that soot properties, such as EC content and Ångström absorption exponent (AAE), were linked to the overall flame composition. In particular, combustion under fuel-rich conditions provided particles with size coupled to the EC fraction and AAE, i.e. smaller particles exhibited a lower EC fraction and higher AAE. In contrast, with fuel-lean diffusion flames and especially with premixed flames under near overall stoichiometric conditions small particles (down to 30?nm) with high EC/TC ratios (>60%) and low AAE (≈1.4) could be generated even without any thermal after-treatment. This new source might thus serve in the future as a useful surrogate for engine exhaust emissions and help to improve calibration procedures of common aerosol instruments.

Copyright © 2018 The Author(s). Published with license by Taylor & Francis Group, LLC     

Numerical simulation of nanoparticle synthesis in diffusion flame reactor

A computational model combining the fluid dynamics with the particle kinetics was employed to study TiO₂ nanoparticle synthesis in a diffusion flame reactor. A one-step chemical kinetics approach was used to model titanium tetraisopropoxide (TTIP) decomposition that leads to homogeneous nucleation and particle formation. The momentum, heat, and mass transfer, Brownian coagulation and diffusion, surface growth, coalescence and thermophoresis have been taken into account. Based on the particle size distributions, an efficient quadrature method of moments was allowed to approximate the general dynamics equation of particle, and the eddy dissipation concept (EDC) combustion model was used to estimate the flame temperature field. Excellent agreements between the model predictions and experimental data, with respect to the flame temperature distribution and particle kinetics, are reached. By taking the particle size and surface area as independent variables, the full distributions of volume equivalent diameters, evolution of the agglomerate number, the geometric standard deviation based on volume and agglomerate fractal nature, mean primary particle size and the number of primary particles per agglomerate are revealed. The variation of oxygen flow rate has an important influence on the temperature distribution and hence on the particle kinetics accordingly.     

Hydrocarbon Nanoparticles Formed in Flames and Diesel Engines

The formation of nanoparticles in laboratory hydrocarbon flames is reviewed in terms of particle morphology, chemical composition, and health hazards. The nascent nanoparticles in the nucleation mode have been widely reported in diverse laboratory flames, and are distinguished by their occurrence as singlet particles that form translucent images in transmission electron microscopy (TEM). Their sizes range from about 10 nm or more down to 2 to 3 nm, the limit of resolution of the TEM, and they possess a liquid-like quality. These particles are widely considered to be the precursor stage to the more readily observed carbonaceous aggregates consisting of chained primary particles that are opaque to the electron beam of the TEM. Nanoparticles sampled from the inverse diffusion flame and the particle effluent from diesel engines show a strong resemblance by GCMS analysis, and they contain many of the stabilomer PAHs and their isomers in the 200 to 302 atomic mass range. Many of these chemical species have high relative mutagenicities. Distinctive bimodal particle size distributions can be observed in both flame and engine samples. Recent TEM micrographs of diesel particulates show images of precursor-like nanoparticles, of as yet unknown chemical composition, that are formed in a diesel engine at many operating conditions.     

Atomization and combustion of canola methyl ester biofuel spray

The spray atomization and combustion characteristics of canola methyl ester (CME) biofuel are compared to those of petroleum based No. 2 diesel fuel in this paper. The spray flame was contained in an optically accessible combustor which was operated at atmospheric pressure with a co-flow of heated air. Fuel was delivered through a swirl-type air-blast atomizer with an injector orifice diameter of 300 μm. A two-component phase Doppler particle analyzer was used to measure the spray droplet size, axial velocity, and radial velocity distributions. Radial and axial distributions of NO, CO, CO₂ and O₂ concentrations were also obtained. Axial and radial distributions of flame temperature were recorded with a Pt–Pt/13%Rh (type R) thermocouple. The volumetric flow rates of fuel, atomization air and co-flow air were kept constant for both fuels. The droplet Sauter mean diameter (SMD) at the nozzle exit for CME biofuel spray was smaller than that of the No. 2 diesel fuel spray, implying faster vaporization rates for the former. The flame temperature decreased more rapidly for the CME biofuel spray flame than for the No. 2 diesel fuel spray flame in both axial and radial directions. CME biofuel spray flames produced lower in-flame NO and CO peak concentrations than No. 2 diesel fuel spray flames.     

Degree of dispersion of metal combustion products in a laminar dust flame

This paper presents the results of experimental and theoretical studies of the effect of parameters of laminar dust flames of metal particles (Al, Fe, Ti, and Zr) on the degree of dispersion of the combustion products of these metals in oxygen-containing media. Extensive experiments with Al powders showed that with variation in the mass concentrations of fuel and oxidizer, fuel particle size, type of carrier gas, and conditions of dust flame production, the most probable particle diameter varied in the range of 50–70 nm. Similar results were also obtained for other metals. The results of the experiments agree with numerical calculations. The experiments showed that the particle size of metal combustion products in laminar dust flames can be substantially increasing. The proposed method for controlling the particle size is based on the ionization of the gas phase by adding impurities to the initial fuel to affect nucleation conditions in the flame.     

Ion–Aerosol Flux Coefficients and the Steady-State Charge Distribution of Aerosols in a Bipolar Ion Environment

Fuchs’ theory, as corrected by Hoppel and Frick, is widely used to compute flux coefficients of ions to aerosol particles and the resultant charge distribution. We have identified approximations made in previous works that limit the theory's accuracy. Hoppel and Frick used two characteristic speeds or kinetic energies to calculate the flux coefficients of ions to aerosol particles in lieu of an average of the flux coefficients over the Maxwell–Boltzmann distribution of ion speeds. In the present work, we show that this approximation artificially reduces the number of multiply charged particles. Ion capture may be enhanced by three-body trapping, a process wherein an ion has a collision with a neutral gas molecule and loses sufficient kinetic energy to be captured by the particle. The gas kinetic theory approach to three-body trapping has been refined to better account for the collision between the ion and a neutral gas molecule within the potential presented by the particle. Approximations to the calculation of energy losses and the probability of ion capture have been relaxed. The possibility that an image charge may be induced on the ion as well as on the particle is allowed. While the previous work was limited to electrically conductive particles, both the ion and the particle are allowed to have any dielectric constant in the present work, and the finite size of the ions is taken into account when calculating minimum capture radii for the ion–particle interactions. The resulting ion flux coefficients differ from previous results both in the low nanometer regime and in the continuum regime. We explore the influence of key parameters on the charge distribution, including dielectric constant, temperature, and pressure, to understand how operating conditions may affect the interpretation of differential mobility analyzer measurements of particle size distributions. Finally, an empirical expression for the new charge distribution is given to facilitate rapid calculations.

© 2013 American Association for Aerosol Research     

Analysis of particle contamination in plasma reactor by 2-sized particle growth model

Rapid particle growth in the silane plasma reactor by coagulation between 2-sized particles was analyzed for various process conditions. The particle coagulation rate was calculated considering the effects of particle charge distribution based on the Gaussian distribution function. The large size particles are charged more negatively than the small size particles. Some fractions of small size particles are in neutral state or charged positively, depending on the plasma conditions. The small size particle concentration increases at first and decreases later and reaches the steady state by the balance of generation rate and coagulation rate. The large size particles grow with discharge time by coagulation with small size particles and their size reaches the steady state, while the large size particle concentration increases with discharge time by faster generation rate and reaches the steady state by the balance of generation and disappearance rates. As the diameter of small size particles decreases, the diameter of large size particles increases more quickly by the faster coagulation with small size particles of higher concentration. As the residence time increases, the concentration and size of large size particles increase more quickly and the average charges per small size and large size particle decrease.     

Examining the Relationship Between Black Carbon and Soot in Flames and Engine Exhaust

This article investigates the black carbon (BC) content of soot formed in premixed and diffusion flames and emitted by light duty gasoline and diesel vehicles. BC is measured photoacoustically and compared with particulate mass collected by filter and calculated from particle size distributions. The BC fraction of soot from rich premixed ethylene flames increases with height above the burner, but can remain well below unity in modestly sooting flames. The BC fraction produced by a propane diffusion flame soot generator (combustion aerosol standard, CAST) falls as the fuel is diluted with nitrogen, the principal means used to adjust the desired particle size. Thermally treating the soot to remove possible condensed semivolatile species does little to change these trends. Transmission electron microscopy (TEM) images show that despite low BC content, these particles display the characteristic fractal-like agglomerate morphology of soot. Particle mass spectra reveal polycyclic aromatic hydrocarbon (PAH) and fullerene fragments associated with low BC soot, which disappear as the BC fraction approaches unity. The results suggest that low BC content reflects immature solid soot that has not carbonized. Particulate matter (PM) measurements from current technology diesel and gasoline vehicles exhibit a high, >80% BC fraction. This is attributed to effective soot carbonization during the expansion and exhaust strokes of the engine, and to the substantial reductions of condensable hydrocarbons by catalytic aftertreatment. These results are discussed with respect to using light absorption-based instruments to monitor engine exhaust PM and using flame-generated soot for PM instrument calibration.     

Spatial gradients in particle reinforced polymers characterized by X-ray attenuation and laser confocal microscopy

The goal of this work is to develop techniques for measuring gradients in particle concentration within filled polymers, such as thermosetting polymer encapsulants. A high concentration of filler particles is added to such materials to tailor physical properties such as thermal expansion coefficient. Sedimentation and flow-induced migration of particles can produce concentration gradients that are most severe near boundaries. Therefore, techniques for measuring local particle concentration should be accurate near boundaries. Particle gradients in an alumina-filled epoxy resin are measured with a spatial resolution of 0.2 mm using an X-ray beam attenuation technique, but an artifact reduces accuracy near the specimen's edge. Local particle concentration near an edge can be measured more reliably using microscopy coupled with image analysis. This is illustrated by measuring concentration profiles of glass particles having 40 μm median diameter using images acquired with a confocal laser fluorescence microscope. The mean of the measured profiles of volume fraction agrees to better than 3% with the expected value and the shape of the profiles agrees qualitatively with simple theory for sedimentation of monodisperse particles. Evidence that the microscopic method can be extended to smaller particles is provided by local concentration measurements on an epoxy polymer containing particles having diameters of the order of 1 μm.     

Coagulation of Mainstream Cigarette Smoke in the Mouth During Puffing and Inhalation

The effect of Brownian coagulation on the particle size distribution of mainstream cigarette smoke subjected to conditions encountered in the mouth during human smoking has been examined experimentally and simulated with a numerical coagulation model. Smoke puffed into an artificial mouth was subjected to variable aging times and exhausted to a fast electrical mobility analyzer for particle size distribution measurement. The experimental results agreed well with the predictions of a sectional-based model of Brownian coagulation that allowed for the modeling of various continuous feed and fixed volume coagulation environments. Due to the steady input of fresh, smaller particles, particle growth during the filling of the mouth with smoke, a process intrinsic to the puffing maneuver, was significant but slower than that during fixed volume, static aging. Mouth hold times and initial smoke mass concentration were found to be strong determinants of the average particle size of smoke exiting the mouth into the respiratory tract during inhalation. The results also suggest that the smallest particles present in fresh smoke, those less than 0.1 μm diameter, are greatly reduced in number during the unavoidable mouth coagulation during puffing and virtually eliminated after 1 s of mouth hold.