The Evolution of Soot Morphology in an Atmospheric Laminar Coflow Diffusion Flame of a Surrogate for Jet A-1 (original) (raw)
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The evolution of soot morphology in a laminar coflow diffusion flame of a surrogate for Jet A-1
An experimental study is performed to investigate the evolution of soot morphology in an atmospheric pressure laminar coflow diffusion flame of a three-component surrogate for Jet A-1. The laser extinction measurement method and the rapid thermocouple insertion technique are used to obtain soot volume fraction profiles and temperature profiles, respectively. Thermophoretic sampling followed by transmission electron microscopy and atomic force microscopy is used to study the morphology of soot particles at different locations inside the flame. Soot formation on the centerline appears to be different from conventional models. Liquid-like particles, which are transparent at the wavelength of 623 nm, are formed and grow up to a volume equivalent diameter of d p = 60 nm at temperatures below T = 1500 K. When the temperature exceeds 1500 K, transition of the transparent particles to the mature agglomerated particles happens immediately, i.e. in less than 12 ms. The volume of the liquid-like particles just before the start of their transformation to solid is about five times larger than the volume of mature primary particles. This significant size difference suggests that a large liquid-like particle does not transform into a single primary particle. In addition, multiple dark nuclei are observed in the liquid-like particles prior to carbonization. The significant size discrepancy and the presence of multiple dark nuclei may indicate that primary particle formation and agglomeration on the centerline happen inside the liquid-like particles. In contrast to the centerline, on another streamline with a significantly different temperature history, soot particles form from relatively small liquid-like particles. These particles have the same size as mature primary particles. Carbonization happens early on the streamline. A single dark nucleus grows inside each liquid-like particle and primary particles agglomerate after carbonization is completed. Most of the currently used computational soot models consider a single evolution process for all of the streamlines inside the flame which may not be an accurate assumption. This study shows that soot evolution processes may be different across the flame and are a function of temperature and the concentration of specific species inside the flame.
Fuel, 2019
While the majority of studies explore soot formation in relatively simple, one-dimensional flames, most realworld flames consist of complex flows defined by large-scale turbulent eddies, recirculating flow patterns, and buoyancy effects. The effects of complex flow on soot physicochemical properties are poorly understood. This work employs an inverted gravity flame reactor (IGFR) to compare differences in soot growth between a onedimensional laminar diffusion flame and a recirculating flame. Computational fluid dynamics (CFD) and experimental observations show particle oscillations between (i) a rich region with a high concentration of surface growth species, and (ii) a high-temperature oxidation region. Transmission electron microscopy (TEM) shows a significant difference in final primary particle diameter, where the one-dimensional flame produces primary particles 10-25 nm in diameter and the recirculating flame produces primary particles 25-75 nm in diameter. Additionally, larger primary particles from the recirculating flame contain both single and multiple cores. We propose that due to the spheroidal shape of the large primary particles, the secondary surface growth is primarily a result of polyaromatic hydrocarbon (PAH) condensation during re-entrainment of mature soot into the fuelrich region followed by subsequent liquid layer carbonization in the high-temperature environment of the flame front. The recirculating flow patterns in the IGFR and repeated particle growth/oxidation cycle can serve as a model for soot formation in the large-scale flames with complex flow patterns, such as forest fires, coal fire plants, and other sources.
International Journal of Heat and Mass Transfer, 2006
Temperature histories of nanosecond pulsed laser heated soot particles of different primary particle size distributions were calculated using a single primary particle based heat and mass transfer model under conditions of a typical atmospheric laminar diffusion flame. The critical peak soot particle temperatures beyond which soot particle sublimation cannot be neglected were identified to be about 3300-3400 K. Knowledge of this critical soot particle temperature is required to conduct low-fluence laser-induced incandescence experiments in which soot sublimation is avoided. After the laser pulse, the temperature of smaller primary soot particles decreases faster than that of larger ones as a result of larger surface area-to-volume ratio. Unlike the common belief that the peak soot particle temperature is independent of the primary particle diameter, the numerical results indicate that this assumption is valid only when soot sublimation is negligible and for primary soot particle diameters greater than about 20 nm. The effective temperature of a soot particle ensemble having different primary particle diameters in the laser probe volume was calculated based on the ratio of the total thermal radiation intensities of soot particles at 400 and 780 nm to simulate the experimentally measured soot particle temperature using two-color optical pyrometry. In the non-sublimation regime, the initial effective temperature decay rate after the peak soot temperature is related to the Sauter mean diameter of the primary soot particle diameter distribution. At longer times, the effective temperatures of soot particle ensembles start to display different decay rates for different soot primary particle diameter distributions. A simple approach was proposed in this study to infer the two parameters of lognormal distributed primary soot particle diameter. Application of this approach was demonstrated in an atmospheric laminar ethylene diffusion flame with the inferred primary soot particle diameter distribution compared with independent ex situ measurement.
Soot particle formation in laminar diffusion flames
Langmuir, 1987
Soot formation processes in laminar diffusion flames are described. Experimental results obtained at the National Bureau of Standards are reviewed and show that fuel molecules are rapidly converted to acetylene and other key precursors (such as the vinyl radical) in flame positions adjacent to and on the fuel-rich side of the high-temperature reaction zone. These molecules react rapidly to form small aromatic species, such as benzene, and eventually small soot particles. This soot particle inception process occurs in a time period on the order of about 1 ms. Once formed, particles may grow rapidly by surface chemistry if they travel through flame regions with high concentrations of growth species such as acetylene. A systematic variation of the fuel flow rate shows that the final particle size, as well as the volume fraction occupied by soot particles, is determined by the particle residence time in this growth region. During the time that surface growth is occurring, particle agglomeration leads to a decrease in total soot particle number concentration. As particles continue along streamlines, they eventually cross the high-temperature reaction zone where they may be oxidized. The oxidative process is also affected by fuel flow rate in the sense that the larger amount of soot formed in flames with relatively high fuel flow rates results in an increase in the radiative transfer from the flame. This reduces the flame temperature and, hence, the rate of oxidation.
A fully-coupled soot formation model is developed to predict the concentration, size, and aggregate structure of soot particles in the atmospheric pressure laminar coflow diffusion flames of a three-component surrogate for Jet A-1, a three-component surrogate for a Fischer-Tropsch Synthetic Paraffinic Kerosene (SPK), and n-decane. To model the chemical structure of the flames and soot precursor formation, a detailed chemical kinetic mechanism for fuel oxidation, with 2185 species and 8217 reactions, is reduced and combined with a Polycyclic Aromatic Hydrocarbon (PAH) formation and growth scheme. The mechanism is coupled to a highly detailed sectional particle dynamics model that predicts the volume fraction, structure, and size of soot particles by considering PAH-based nucleation, surface growth, PAH surface condensation, aggregation, surface oxidation, fragmentation, thermophoresis, and radiation. The simulation results are validated by comparing against experimental data measured for the flames of pre-vaporized fuels. The objectives of the present effort are to more accurately simulate the physical soot formation processes and to improve the predictions of our previously published jet fuel soot formation models, particularly for the size and aggregate structure of soot particles. To this end, the following improvements are considered: (1) addition of particle coalescence submodels to account for the loss of surface area, reduction of the number of primary particles, and increase of primary particle diameters upon collision, (2) consideration of a larger PAH molecule (benzopyrene instead of pyrene) for nucleation and surface growth to enhance the agreement between the soot model and the measured chemical composition of soot particles, and (3) implementation of a dimerization efficiency in the soot inception submodel to account for the collisions between PAH molecules that do not lead to dimerization. The results of two different particle coalescence submodels show that this process is too slow to account for the growth of primary particles, mainly because of the limited rate of particle collisions. Soot volume fraction predictions on the wings and at lower flame heights are considerably improved by using benzopyrene, due to the different distribution of the soot forming PAH molecule in the flame. The computed number of primary particles per aggregate and the diameters of primary particles agree very well with the experimentally measured values after implementing the dimerization efficiency for PAH collisions, because of the reduced rate of soot inception compared to growth by PAH condensation. Concentrations of major gaseous species and flame temperatures are also well predicted by the model. The underprediction of soot concentration on the flame centerline, observed in previous studies, still exists despite minor improvements.
The present numerical and experimental study aims at enhancing the quantitative accuracy of soot formation predictions in atmospheric pressure, laminar, sooting, coflow diffusion flames of jet fuel, improving upon a previously published study that used a polycyclic aromatic hydrocarbon (PAH) nucleation based soot model. That model used a conventional acetylene-based PAH growth reaction scheme to calculate PAH concentrations in a Jet A-1 surrogate flame. Its results were compared to the experimental data for a real Jet A-1 flame. In the central region of the flame, that model, similar to many soot models in the literature, underpredicted soot concentration by more than one order of magnitude. The following improvements are made in the present work: (1) flame temperature and soot volume fraction profiles are experimentally measured in a flame using surrogate Jet A-1 rather than real Jet A-1, so that more direct comparisons can be made, (2) a novel reaction scheme for PAH growth (mechanism 2 in the present work), with a more comprehensive set of pathways for aromatic ring formation and growth, is used to model soot formation, and (3) the empirical soot surface growth parameter, a, is updated. The simulation data, using mechanism 2, are compared to the measurement results and another set of computational data, using the less detailed acetylene-based mechanism (mechanism 1). It is shown that only mechanism 2 can predict the correct order of magnitude of the centerline soot concentrations, reproducing them within a factor of one to five. Soot particles are shown to be exposed to similar temperatures and acetylene concentrations with both mechanisms. Hence, this improvement on the centerline is because of the enhanced PAH growth model, which produces higher levels of pyrene and increases the soot particle nucleation rate.
Combustion and …, 2008
The evolution of the soot particle size distribution function (PSDF) and particle morphology are studied for premixed ethylene-oxygen-argon flat flames at equivalence ratio φ = 2.07 over the maximum flame temperature range of 1600-1900 K. Experiments were carried out using an in-situ probe sampling method in tandem with a scanning mobility particle sizer (SMPS), yielding the PSDF for various distances from the burner surface. The morphology of the particles was examined by transmission electron microscopy (TEM) and atomic force microscopy (AFM). Within the particle size range that can be detected, the PSDF transitions from an apparent unimodal PSDF for high temperature flames (T f > ∼1800 K) to a bimodal PSDF at lower temperatures (T f < ∼1800 K). The bimodal PSDF has a noticeable trough that separates the nucleation tail and log-normal mode. This mode-transition trough had been previously thought to occur at a fixed particle size, but these results show a continuous shift of the trough location towards smaller sizes with increasing flame temperature. TEM images show that the particles are spherical, even when the PSDF is bimodal, suggesting that the bimodality occurs as the primary particles undergoes mass and size growth, and is not a result of particle aggregation. Atomic force microscopy of substrate deposited particles shows that particles spread and form hill like structures upon impact with the substrate surface, indicating that the particles are liquid-like at the time of impact.
Different non-intrusive optical and intrusive non optical diagnostic methods are used to measure flame and soot properties in a laminar coflow diffusion flame in order to compare and analyze the sensitivity of each technique to soot particles with different age and morphology. Flame temperature is measured using rapid thermocouple insertion (RTI) method and also by measuring soot spectral emissions (SSE). Soot volume fraction (f v) is measured quantitatively with laser extinction (LE), time resolved laser induced incandescent (TiRe-LII) and SSE methods and qualitatively from the transmission electron microscope (TEM) images of the thermophoretically sampled soot particles. Particle internal/aggregate nanostructure, and primary particle diameter are also analyzed based on TEM images from the sampled particles and TiRe-LII. It is shown that the optical methods are only sensitive to mature soot particles with solid appearance and cannot detect either temperature or f v in regions where liquid like nascent soot particles are dominant. f v measured by LE and TiRe-LII agree well while the values measured by SSE are lower. This discrepancy is attributed to the high sensitivity of f v measured by SSE to the measured temperature values. Temperature profiles measured by SSE are considerably higher than the values measured by RTI. It is shown that not considering the change of the surface emissivity of the thermocouple junction due to particle deposition for estimating radiation loss in regions where nascent or mature soot particles are dominant contributes to this discrepancy. Primary particle sizes measured based on TEM images and TiRe-LII agree reasonably well. Soot aggregate fractal dimension is shown to decrease as the soot particles age and become more mature.