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Fractal and projected structure properties of soot aggregates

The structure of soot aggregates was investigated, emphasizing the fractal properties as well as the relationships between the properties of actual and projected soot images. This information was developed by considering numerically simulated soot aggregates based on cluster-cluster aggregation as well as measured soot aggregates based on thermophoretic sampling and analysis by transmission electron microscopy (TEM) of soot for a variety of fuels (acetylene, propylene, ethylene, and propane) and both laminar and turbulent diffusion flame conditions. It was found that soot aggregate fractal properties are relatively independent of fuel type and flame condition, yielding a fractal dimension of 1.82 and a fractal prefactor of 8.5, with experimental uncertainties (95% confidence) of 0.08 and 0.5, respectively. Relationships between the actual and projected structure properties of soot, e.g., between the number of primary particles and the projected area and between the radius of gyration of an aggregate and its projected image, also are relatively independent of fuel type and flame condition.

Structure of Overfire Soot in Buoyant Turbulent Diffusion Flames at Long Residence Times

Abstract The structure of soot was investigated within the fuel-lean (overfire) region of overventilated buoyant turbulent diffusion flames burning in still air. The study was limited to the long residence time regime where characteristic flame residence times are roughly more than an order of magnitude longer than the laminar smoke point residence time and soot generation factors (the mass of soot emitted per unit mass of fuel carbon burned) are relatively independent of flame residence times. Both gaseous and liquid fuels were used to provide a range of H/C ratios (1–2.7) and fuel types (alkynes, alkenes, alkanes, aromatics, and alcohols) as follows: toluene, acetylene, benzene, propylene, ethylene, n -heptane, propane, and isopropanol. Measurements included transmission electron microscopy to find primary particle diameters, the number of primary particles per aggregate and aggregate geometrical and fractal dimensions. The results show that the structure of soot varies with fuel type but is relatively independent of both position in the overfire region and flame residence time for the long residence time regime. Mean primary particle diameters were 30–51 nm and the mean number of primary particles per aggregate were 255–552, with the larger values associated with the more heavily sooting fuels. Aggregate fractal dimensions, however, were less dependent on fuel type, only varying in the range 1.70–1.79. The structure measurements are used to estimate the optical properties of overfire soot, based on a recent approximate theory for polydisperse aggregates, finding significant differences between aggregate and Rayleigh scattering properties in the visible and near-infrared portions of the spectrum, even though the primary particles are well within the Rayleigh scattering regime.

Soot volume fraction and temperature measurements in laminar nonpremixed flames using thermocouples

Abstract Thermocouple particle densitometry (TPD), a new method for measuring absolute soot volume fraction in flames which was suggested by Eisner and Rosner, has been successfully implemented in several laminar nonpremixed flames. This diagnostic relies on measuring the junction temperature history of a thermocouple rapidly inserted into a soot-containing flame region, then optimizing the fit between this history and one calculated from the principles of thermophoretic mass transfer. The TPD method is very simple to implement experimentally, yields spatially resolved volume fractions directly, can easily measure small volume fractions, and does not depend on the prevailing soot particle size, morphology, or optical characteristics. p]In a series of methane and ethylene counterflow flames whose soot volume fractions varied by more than an order of magnitude, the TPD results agreed to within experimental error with our own laser extinction measurements. In axisymmetric methane and ethylene co-flowing flames, the shape of TPD profiles agreed well with published laser extinction measurements, but the TPD concentrations were significantly larger in the early regions of the ethylene flame and throughout the methane flame; these discrepancies are probably attributable to visible light-transparent particles that are detectable with TPD but not with laser extinction. The TPD method is not applicable to the upper regions of these co-flowing flames since OH concentrations there suffice to rapidly oxidize any soot particles that deposit. Gas temperatures were obtained simultaneously with volume fraction by averaging the junction temperature history shortly after insertion. The error in these temperatures due to soot deposition-imposed changes in the junction diameter and emissivity were assessed and found to be moderate, e.g., less than 60 K near the centerline of the ethylene coflowing flame where the volume fraction was 6 ppm and the gas temperature was 1550 K.

Simultaneous measurements of soot volume fraction and particle size/microstructure in flames using a thermophoretic sampling technique

A new particle volume fraction measurement technique was developed using electron microscope analysis of thermophoretically sampled particles/aggregates based on a theoretical treatment of particle deposition to a cold surface immersed in a flame. This experimental method, referred to as the thermophoretic sampling particle diagnostic (TSPD), can yield all particle parameters of principal interest (particle volume fraction, particle and aggregate sizes, and fractal properties) without requiring knowledge of particle bulk density and refractive index. To assess its reliability, the TSPD technique was implemented at various heights on the centerline of a soot-containing coflowing ethylene/air nonpremixed laminar flame. Inferred soot volume fractions agreed with previous laser extinction and thermocouple particle densitometry measurements within experimental uncertainties at sampling positions where only aggregates of mature particles were present. However, TSPD-soot volume fractions were about a factor of 3 higher than light extinction results in the lower part of the flame. This significant difference was evidently a result of the presence of translucent precursor soot particles, which do not absorb as much visible light as mature particles, but can be quantified with the electron microscope. Clearly, this ability of TSPD to separately measure the concentration and morphology of each type of soot is a significant advantage over other available diagnostics, making it extremely valuable for studying particle formation in flames.

Range of validity of the Rayleigh-Debye-Gans theory for optics of fractal aggregates.

The range of validity of the Rayleigh-Debye-Gans approximation for the optical cross sections of fractal aggregates (RDG-FA) that are formed by uniform small particles was evaluated in comparison with the integral equation formulation for scattering (IEFS), which accounts for the effects of multiple scattering and self-interaction. Numerical simulations were performed to create aggregates that exhibit mass fractallike characteristics with a wide range of particle and aggregate sizes and morphologies, including x(p) = 0.01-1.0, ‖m - 1‖ = 0.1-2.0, N = 16-256, and D(f) = 1.0-3.0. The percent differences between both scattering theories were presented as error contour charts in the ‖m - 1‖x(p) domains for various size aggregates, emphasizing fractal properties representative of diffusion-limited cluster-cluster aggregation. These charts conveniently identified the regions in which the differences were less than 10%, between 10% and 30%, and more than 30% for easy to use general guidelines for suitability of the RDG-FA theory in any scattering applications of interest, such as laser-based particulate diagnostics. Various types of aggregate geometry ranging from straight chains (D(f) ≈ 1.0) to compact clusters (D(f) ≈ 3.0) were also considered for generalization of the findings. For the present computational conditions, the RDG-FA theory yielded accurate predictions to within 10% for ‖m - 1‖ to approximately 1 or more as long as the primary particles in aggregates were within the Rayleigh scattering limit (x(p) ≤ 0.3). Additionally, the effect of fractal dimension on the performance of the RDG-FA was generally found to be insignificant. The results suggested that the RDG-FA theory is a reasonable approximation for optics of a wide range of fractal aggregates, considerably extending its domain of applicability.

Soot Morphology and Optical Properties in Nonpremixed Turbulent Flame Environments

Abstract Motivated by the importance of soot to the emission of particulates and other pollutants from combustion processes, current understanding of soot morphology and optical properties is reviewed, emphasizing nonpremixed flame environments. The understanding of soot morphology in flames has grown rapidly in recent years due to the development of methods of thermophoretic sampling and analysis by transmission electron microscopy (TEM). The results show that soot consists of nearly spherical primary particles, having diameters generally less than 60 nm, which collect into open structured aggregates that are mass fractal objects. Aggregates grow by cluster/cluster aggregation to yield broad aggregate size distributions with the largest aggregates containing thousands of primary particles and reaching dimensions of several urn. The optical properties of soot aggregates generally are not suited for the Rayleigh and Mie scattering approximations which has led to the development of approximate Rayleigh-Deby...

Quantitative analysis of in situ optical diagnostics for inferring particle/aggregate parameters in flames: Implications for soot surface growth and total emissivity

Abstract An in situ particulate diagnostic/analysis technique is outlined based on the Rayleigh-Debye-Gans polydisperse fractal aggregate (RDG/PFA) scattering interpretation of absolute angular light scattering and extinction measurements. Using proper particle refractive index, the proposed data analysis method can quantitatively yield all aggregate parameters (particle volume fraction, f v , fractal dimension, D f , primary particle diameter, d p , particle number density, n p , and aggregate size distribution, pdf( N )) without any prior knowledge about the particle-laden environment. The present optical diagnostic/interpretation technique was applied to two different soot-containing laminar and turbulent ethylene/air nonpremixed flames in order to assess its reliability. The aggregate interpretation of optical measurements yielded D f , d p , and pdf( N ) that are in excellent agreement with ex situ thermophoretic sampling/transmission electron microscope (TS/TEM) observations within experimental uncertainties. However, volume-equivalent single particle models (Rayleigh/Mie) overestimated d p by about a factor of 3, causing an order of magnitude underestimation in n p . Consequently, soot surface areas and growth rates were in error by a factor of 3, emphasizing that aggregation effects need to be taken into account when using optical diagnostics for a reliable understanding of soot formation/evolution mechanism in flames. The results also indicated that total soot emissivities were generally underestimated using Rayleigh analysis (up to 50%), mainly due to the uncertainties in soot refractive indices at infrared wavelengths. This suggests that aggregate considerations may not be essential for reasonable radiation heat transfer predictions from luminous flames because of fortuitous error cancellation, resulting in typically a 10 to 30% net effect.

Carbon Monoxide and Soot Emissions from Liquid-Fueled Buoyant Turbulent Diffusion Flames

Carbon monoxide concentrations, soot concentrations, and mixture fractions were measured in the fuel-lean (overfire) region of liquid-fueled buoyant turbulent diffusion flames burning in still air. Pool-fire configurations were studied with the liquids burning from horizontal round wicks, considering both sooting (toluene, benzene, n-heptane, and isopropanol) and nonsooting (methanol and ethanol) fuels. Flame heights and characteristic residence times also were measured, both for the turbulent flames and at the normal smoke point (for the sooting fuels). Carbon monoxide and soot generation factors (mass of CO or soot emitted per unit mass of fuel carbon burned) were uniform throughout the overfire region and were relatively independent of flame residence times (which were generally an order of magnitude longer than the normal smoke point residence times of the sooting fuels). Processes of carbon monoxide and soot emission for the nonalcobols are closely related, based on the good correlation between their emission factors: 0.37 kg CO/per kg soot with a standard deviation of 0.09. However, nonsooting methanol and ethanol/air flames still emitted low levels of CO so that there is a component of CO emissions that is not associated with soot.

Soot formation in weakly buoyant acetylene-fueled laminar jet diffusion flames burning in air☆☆☆

Abstract The structure and soot properties of weakly buoyant, acetylene-fueled, laminar jet diffusion flames were studied experimentally for combustion in air at pressures of 0.125–0.250 atm. Properties along the axis, where soot processes are similar to behavior within nonbuoyant diffusion flames, were emphasized. The following measurements were made: soot volume fractions using laser extinction, temperatures using both thermocouples and multiline emission, soot structure using thermophoretic sampling and analysis by transmission electron microscopy, concentrations of major gas species using sampling and analysis by gas chromatography, and velocities using laser velocimetry. As distance increased along the axis of the present acetylene-fueled flames, significant soot formation began when temperatures exceeded roughly 1250 K, and ended when fuel-equivalence ratios decreased to roughly 1.7, where the concentration of acetylene became small. This behavior allowed observations of soot growth and nucleation for acetylene concentrations of 6 × 10−6-1 × 10−3 kg-mol/m3 and temperatures of 1000–2100 K. Over this range of conditions, soot growth rates were comparable to past observations of new soot in premixed flames, and after correction for effects of soot oxidation yielded essentially first-order growth with respect to acetylene concentrations with a negligible activation energy, and an acetylene/soot collision efficiency of 0.53%. Present measurements of soot nucleation rates also suggested first-order behavior with respect to acetylene concentrations but with an activation energy of 32 kcal/gmol and with rates that were significantly lower than earlier estimates in the literature. Nevertheless, uncertainties about effects of soot oxidation and age on soot growth, and about effects of surface area estimates and translucent objects on soot nucleation, must be resolved in order to adequately define soot formation processes in diffusion flames.

Numerical characterization of the morphology of aggregated particles

Abstract The structures of both cluster–cluster and particle–cluster fractal-like aggregates were investigated in the present study. Statistically significant populations of numerically simulated aggregates having appropriate fractal properties and prescribed number of primary particles per aggregate were generated in order to characterize three-dimensional morphological properties of aggregates, such as fractal dimension, fractal pre-factor, coordination number distribution function, and distribution of angles between triplets. Effects of aggregation mechanisms (i.e., cluster–cluster or particle–cluster) and aggregate size were taken into consideration. In addition, the morphological properties of aggregates undergoing partial sintering and restructuring were also investigated. To fulfill these objectives, aggregates were initially built without considering sintering or restructuring effects. Partial sintering of primary particles was then considered by introducing a penetration coefficient that allows touching particles to approach each other. Restructuring of aggregates was modeled during the process of building the cluster–cluster aggregates. For each pair of clusters that were attached together due to the normal aggregation procedure, a further mechanism was included that allowed the cluster to collapse until a more compact and stable position was achieved. The population studied was composed of ca. 450 simulated aggregates having a number of primary particles per aggregate between 8 and 1024. Calculations were performed for aggregates having a penetration coefficient in the range of 0–0.25 with and without restructuring. The following properties were investigated: fractal dimension, fractal pre-factor, coordination number distribution function, angle between triplets, and aggregate radius of gyration.