Numerical investigation of soot formation mechanisms in partially-premixed ethylene–air co-flow flames

Recently, an improved chemical mechanism of PAH growth was developed and tested in soot computations for a laminar co-flow non-premixed ethylene–air diffusion flame [Dworkin et al., Combust. Flame 158(9) (2011) 1682–1695]. With the intention of testing the robustness of the solution methodology on partially-premixed systems, this work used the same algorithm as that in the study of Dworkin et al. for computations of two sets of sooting partially-premixed co-flow laminar ethylene–air flames. The results show very good qualitative and good quantitative agreement with the experimental results for soot volume fractions and soot precursors, without any changes to the parameters of the model. The soot yield was found to initially increase with decreasing primary equivalence ratios, and then to decrease for Φ < 24, reaching levels lower than the non-premixed case for Φ < 10. On the flame centerline, both PAH and acetylene-related processes were found to be important for soot growth. The initial increase in the soot yield was linked to higher inception rates. On the wings of the flame the dominant soot growth process was found to be HACA growth. The initial increase in the soot yield was mostly due to higher acetylene yield leading to faster surface growth. The primary air was also found to influence the soot oxidation process by increasing OH radicals in both the centerline and the wings region.     

Effects of water vapor addition to the air stream on soot formation and flame properties in a laminar coflow ethylene/air diffusion flame

The effects of adding water vapor to the air stream on flame properties and soot volume fraction were investigated numerically in a laminar coflow ethylene/air diffusion flame at atmospheric pressure by solving the fully elliptic conservation equations and using a detailed C₂ reaction mechanism including PAH up to pyrene and detailed thermal and transport properties. Thermal radiation was calculated using the discrete-ordinates method and a statistical narrow-band correlated-k based wide band model for the absorption coefficients of CO₂ and H₂O. Soot formation was modeled using a PAH based inception model and the HACA mechanism for surface growth and oxidation. Addition of water vapor significantly reduces radiation heat loss from the flame primarily through reduced soot loading and flame temperature. The added water vapor affects soot formation and flame properties through not only dilution and thermal effects, but also through chemical effect. The chemical effect is as significant as the dilution and thermal effects. The primary pathway for the chemical effect of water vapor is the reverse reaction of OH + H₂ ↔ H + H₂O. Our numerical results confirm that the reduced H radical concentration leads to lower PAH concentrations and consequently lower soot inception rates. In contrast, the radiation effect due to the added water vapor was found to have a minor influence on both flame structure and soot formation in the laminar diffusion flame investigated.     

Measurement of the dimensionless extinction coefficient of soot within laminar diffusion flames

The dimensionless extinction coefficient (K_e) of soot must be known to quantify laser extinction measurements of soot concentration and to predict optical attenuation through smoke clouds. Previous investigations have measured K_e for post-flame soot emitted from laminar and turbulent diffusion flames and smoking laminar premixed flames. This paper presents the first measurements of soot K_e from within laminar diffusion flames, using a small extractive probe to withdraw the soot from the flame. To measure K_e, two laser sources (635 nm and 1310 nm) were coupled to a transmission cell, followed by gravimetric sampling. Coannular diffusion flames of methane, ethylene and nitrogen-diluted kerosene burning in air were studied, together with slot flames of methane and ethylene. K_e was measured at the radial location of maximum soot volume fraction at several heights for each flame. Results for K_e at both 635 nm and 1310 nm for ethylene and kerosene coannular flames were in the range of 9–10, consistent with the results from previous studies of post-flame soot. The ethylene slot flame and the methane flames have lower K_e values, in some cases as low as 2.0. These lower values of K_e are found to result from the contributions of (a) the condensation of PAH species during the sampling of soot, (b) the wavelength-dependent absorptivity of soot precursor particles, and, in the case of methane, (c) the negligible contribution of soot scattering to the extinction coefficient. RDG calculations of soot scattering, in combination with the measured K_e values, imply that the soot refractive index is in the vicinity of 1.75–1.03i at 635 nm.     

The effect of CO2/H2O on the formation of soot particles in the homogeneous environment of a rapid compression facility

Previous investigations show that soot particle volume fraction and number density were significantly reduced by exhaust gas recirculation (EGR) diluents CO₂ and H₂O. However, these investigations were often convoluted by their experimental flame configurations and primarily focused on soot volume fraction rather than soot inception. To isolate the effects on soot inception and the corresponding chemistry, the current study measured the reactivity of CO₂ (up to 9.5% volume fraction) for both C₂H₂ (1.00% volume fraction) and CH₄ (1.85% volume fraction) fuels in homogeneous mixtures. Computed effect of H₂O on these and other fuels are also presented. Experiments were performed at high temperature (1640 K and 1770 K) and high equivalence ratios (Φ = 55 and 75) to understand the effect of CO₂ on polycyclic aromatic hydrocarbons (PAH) and formation of nascent soot particles with negligible oxygen influence. Experimental results show that CO₂ enhanced the soot inception rate when added to C₂H₂ but had an undetectable affect on CH₄. Gas chromatography confirmed that CO₂ increases CO mole fraction and reduces C₂H₂ fuel concentration. Chemical kinetic simulations showed that the C₂H₂ was being converted to soot precursors. CO₂ enhanced the soot inception rate for C₂H₂ by producing OH radicals. Images of nascent soot particles produced in the presence of CO₂ were used to determine the size of PAH molecules in the particles and particle morphology. Both attributes were similar to particles formed without CO₂. CO₂ had little impact on the long reaction pathway from CH₄ to PAH molecules because H and CH₃ radicals propagated these reactions more readily than OH radicals.     

A numerical study of high pressure,laminar, sooting,ethane–air coflow diffusion flames

A numerical study is conducted of ethane–air coflow diffusion flames at pressures from 2 to 15 atm. The model employed uses a detailed gas phase chemical kinetic mechanism that includes PAH formation and growth, and is coupled to a detailed sectional soot particle dynamics model. The model is able to accurately predict the trends observed experimentally with increasing pressure without any tuning of the model for different pressures. The model shows good agreement with the experimental data on both the flame wings and centerline regions. Peak wing and centerline soot volume fractions are found to scale with P^2.49 and P^2.02 respectively. This scaling compares well to that observed experimentally for methane–air and ethylene–air flames. As pressure is increased, the flame cross-sectional area decreases according to P^?1.0 due to a constant mass flux and a thinning of the flame, which is consistent with experimental observations. Soot formation along the wings is seen to be surface growth dominated, while PAH condensation dominates centerline soot formation. Surface growth and PAH condensation increase with increasing pressure primarily due to both of these processes being a function of surface area. This causes increases in soot volume fraction to further accelerate surface growth and PAH condensation, acting in a positive feedback manner. This positive feedback mechanism is initiated by increases in reaction rates caused by increases in gas phase density. Additionally, the significance of surface growth decreases with increasing pressure, while the role of PAH condensation increases.     

Conditional moment closure prediction of soot formation in turbulent, nonpremixed ethylene flames

Results obtained from incorporating a semiempirical soot model into a first-order conditional moment closure (CMC) approach to modeling turbulent nonpremixed flames of ethylene and air are presented. Soot formation is determined via the solution of two transport equations for soot mass fraction and particle number density, with acetylene and benzene employed as the incipient species responsible for soot nucleation, and the concentrations of these species calculated using a detailed gas-phase kinetic scheme involving 463 reactions and 70 species. The study focuses on the influence of differential diffusion of soot particles on soot volume fraction predictions. The results of calculations are compared with experimental data for three sooting ethylene flames and, in general, predictions of mixing and temperature fields within the three flames show good agreement with data. Soot volume fraction predictions are found to be in significantly better accord with data when differential diffusion is accounted for in the CMC-based soot model, supporting the importance of such effects in sooting flames, as previously noted by Kronenburg et al. in relation to methane combustion. Overall, the study demonstrates that the CMC-based soot model, when used in conjunction with a model of differential diffusion effects, is capable of accurately predicting soot formation in turbulent nonpremixed ethylene-air flames.     

LES model for sooting turbulent nonpremixed flames

In this work, an integrated Large Eddy Simulation (LES) model is developed for sooting turbulent nonpremixed flames and validated in a laboratory scale flame. The integrated approach leverages state-of-the-art developments in both soot modeling and turbulent combustion modeling and gives special consideration to the small-scale interactions between turbulence, soot, and chemistry. The oxidation of the fuel and the formation of gas-phase soot precursors is described by the Flamelet/Progress Variable model, which has been previously extended to account for radiation losses. However, previous DNS studies have shown that Polycyclic Aromatic Hydrocarbons (PAH), the immediate precursors of soot particles, exhibit significant unsteady effects due to relatively slow chemistry. To model these unsteady effects, a transport equation is solved for a lumped PAH species. In addition, due to the removal of PAH from the gas-phase, alternative definitions of the mixture fraction, progress variable, and enthalpy are proposed. The evolution of the soot population is modeled with the Hybrid Method of Moments (HMOM), an efficient statistical model requiring the solution of only a few transport equations describing statistics of the soot population. The filtered source terms in these equations that describe the various formation, growth, and destruction processes are closed with a recently developed presumed subfilter PDF approach that accounts for the high spatial intermittency of soot. The integrated LES model is validated in a piloted natural gas turbulent jet diffusion flame and is shown to predict the magnitude of the maximum soot volume fraction in the flame relatively accurately, although the maximum soot volume fraction is shown to be rather sensitive to the subfilter scalar dissipation rate model.     

Hydrogen addition to acetylene-air laminar diffusion flames: Studies on soot formation under different flow arrangements

Axisymmetric co-flowing acetylene/air laminar diffusion flames have been experimentally investigated to study the effect of hydrogen addition on soot formation and soot morphology. An acetylene-hydrogen jet burning in co-flowing air at atmospheric pressure has been studied under different flow arrangements, i.e., premixed and with separate addition of acetylene and hydrogen. Thermophoretic sampling and analysis by transmission electron microscopy are employed for soot diagnostics. Soot microstructure, primary particle size, soot volume fraction, and fractal geometry results are reported. The effect of hydrogen addition on the temperature field is moderate (maximum increase ∼100 K), the effect being greater when hydrogen is premixed with acetylene. Soot volume fraction decreases with hydrogen addition. A shift was noted in the soot volume fraction peak with change in the Reynolds and Froude numbers at the burner exit. The primary soot particle diameter is in the range of 20-35 nm. Soot particles are larger in size close to the burner for the pure acetylene flame. A reverse trend is observed with hydrogen addition. The fractal dimension of the soot aggregates is about 1.7-1.8. It is unaffected by hydrogen addition and location in the flame. Soot aggregate size tends to decrease with hydrogen addition. The results of the present study on the effect of hydrogen addition on soot volume fraction and mean primary particle size are in good correlation with the results of other investigators for ethylene-, propane-, and butane-air flames, which have been described with regard to the HACA mechanism of soot nucleation and growth and enhanced soot oxidation in fuel-rich flames by increased OH radical concentration.     

Soot formation with C1 and C2 fuels using an improved chemical mechanism for PAH growth

Recently, an improved chemical mechanism of PAH growth was developed and tested in soot computations for a laminar co-flow non-premixed ethylene–air diffusion flame. In the present work, the chemical mechanism was enhanced further to accommodate the PAH gas phase growth in methane, ethylene and ethane co-flow flames. The changes in the mechanism were tested on a methane/oxygen and two ethane/oxygen premixed flames to ensure no degradation in its application to C₂ fuels. The major soot precursors were predicted in a satisfactory matter. The robustness of the soot solution methodology was tested for different fuels by solving methane/air, ethane/air and ethylene/air co-flow laminar diffusion flames using a single solution algorithm for all three cases. The peak soot volume fractions, which varied by two orders of magnitude between fuels, were predicted within a factor of two for all flames. The computations were also able to reproduce the spatial distributions of soot and to explain the variation in soot formation pathways among the fuels. Despite a similarity in bulk properties of the flame, the soot particles in different flames exhibit significantly different growth modes. Ethylene/air flames tend to form soot earlier than methane/air flames and inception plays a bigger role in the latter.     

Towards a detailed soot model for internal combustion engines

In this work, we present a detailed model for the formation of soot in internal combustion engines describing not only bulk quantities such as soot mass, number density, volume fraction, and surface area but also the morphology and chemical composition of soot aggregates. The new model is based on the Stochastic Reactor Model (SRM) engine code, which uses detailed chemistry and takes into account convective heat transfer and turbulent mixing, and the soot formation is accounted for by SWEEP, a population balance solver based on a Monte Carlo method. In order to couple the gas-phase to the particulate phase, a detailed chemical kinetic mechanism describing the combustion of Primary Reference Fuels (PRFs) is extended to include small Polycyclic Aromatic Hydrocarbons (PAHs) such as pyrene, which function as soot precursor species for particle inception in the soot model. Apart from providing averaged quantities as functions of crank angle like soot mass, volume fraction, aggregate diameter, and the number of primary particles per aggregate for example, the integrated model also gives detailed information such as aggregate and primary particle size distribution functions. In addition, specifics about aggregate structure and composition, including C/H ratio and PAH ring count distributions, and images similar to those produced with Transmission Electron Microscopes (TEMs), can be obtained. The new model is applied to simulate an n-heptane fuelled Homogeneous Charge Compression Ignition (HCCI) engine which is operated at an equivalence ratio of 1.93. In-cylinder pressure and heat release predictions show satisfactory agreement with measurements. Furthermore, simulated aggregate size distributions as well as their time evolution are found to qualitatively agree with those obtained experimentally through snatch sampling. It is also observed both in the experiment as well as in the simulation that aggregates in the trapped residual gases play a vital role in the soot formation process.     

Chemical mechanism for high temperature combustion of engine relevant fuels with emphasis on soot precursors

This article presents a chemical mechanism for the high temperature combustion of a wide range of hydrocarbon fuels ranging from methane to iso-octane. The emphasis is placed on developing an accurate model for the formation of soot precursors for realistic fuel surrogates for premixed and diffusion flames. Species like acetylene (C₂H₂), propyne (C₃H₄), propene (C₃H₆), and butadiene (C₄H₆) play a major role in the formation of soot as their decomposition leads to the production of radicals involved in the formation of Polycyclic Aromatic Hydrocarbons (PAH) and the further growth of soot particles. A chemical kinetic mechanism is developed to represent the combustion of these molecules and is validated against a series of experimental data sets including laminar burning velocities and ignition delay times. To correctly predict the formation of soot precursors from the combustion of engine relevant fuels, additional species should be considered. One normal alkane (n-heptane), one ramified alkane (iso-octane), and two aromatics (benzene and toluene) were chosen as chemical species representative of the components typically found in these fuels. A sub-mechanism for the combustion of these four species has been added, and the full mechanism has been further validated. Finally, the mechanism is supplemented with a sub-mechanism for the formation of larger PAH molecules up to cyclo[cd]pyrene. Laminar premixed and counterflow diffusion flames are simulated to assess the ability of the mechanism to predict the formation of soot precursors in flames. The final mechanism contains 149 species and 1651 reactions (forward and backward reactions counted separately). The mechanism is available with thermodynamic and transport properties as supplemental material.     

Implementation of two-equation soot flamelet models for laminar diffusion flames

The two-equation soot model proposed by Leung et al. [K.M. Leung, R.P. Lindstedt, W.P. Jones, Combust. Flame 87 (1991) 289-305] has been derived in the mixture fraction space. The model has been implemented using both Interactive and Non-Interactive flamelet strategies. An Extended Enthalpy Defect Flamelet Model (E-EDFM) which uses a flamelet library obtained neglecting the soot formation is proposed as a Non-Interactive method. The Lagrangian Flamelet Model (LFM) is used to represent the Interactive models. This model uses direct values of soot mass fraction from flamelet calculations. An Extended version (E-LFM) of this model is also suggested in which soot mass fraction reaction rates are used from flamelet calculations. Results presented in this work show that the E-EDFM predict acceptable results. However, it overpredicts the soot volume fraction due to the inability of this model to couple the soot and gas-phase mechanisms. It has been demonstrated that the LFM is not able to predict accurately the soot volume fraction. On the other hand, the extended version proposed here has been shown to be very accurate. The different flamelet mathematical formulations have been tested and compared using well verified reference calculations obtained solving the set of the Full Transport Equations (FTE) in the physical space.     

Condensation flame of acetylene decomposition

Acetylene decomposition flame propagation was numerically analyzed and was found to be the result of the condensation reaction. Condensation processes provide reaction heat and act as a driving force for C₂H₂ flame propagation. The kinetic model reasonably predicts the level of burning velocity of the acetylene decomposition flame. The model does not demonstrate the relatively strong positive pressure dependence of burning velocity as was observed experimentally in the work of Cummings et al. [Proc. Combust. Inst. 8 (1962) 503–510]. Heat-release kinetics demonstrates a two-stage process. The first stage corresponds to heat release due to benzene formation, and the second stage of heat release corresponds to soot inception and carbonization processes. It was demonstrated that the burning velocity is sensitive to the surface growing rate constant. The use of a simplified form of presentation of the surface growing process [P.R. Lindstedt, in: Soot Formation in Combustion: Mechanisms and Models, Springer-Verlag, Berlin/New York, 1994, pp. 417–441] represents positive thermal feedback in the heat generation in a flame reaction zone.     

Nine-step phenomenological diesel soot model validated over a wide range of engine conditions

A nine-step phenomenological soot model has been implemented into the KIVA-3V code for predicting soot formation and oxidation processes in diesel engines. The model involves nine generic steps, i.e., fuel pyrolysis, precursor species (including acetylene) formation and oxidation, soot particle inception, particle coagulation, surface growth and oxidation. The fuel pyrolysis process leads to acetylene formation and it is described by a single-step reaction. The particle inception occurs via a generic gas-phase precursor species, and the precursor is the product of an irreversible reaction from acetylene. The acetylene addition reaction contributes to soot surface growth. The particle coagulation affects both particle size and number density. The oxidation of soot particles includes two mechanisms—Nagle and Strickland-Constable's O₂ oxidation mechanism and Neoh et al.'s OH oxidation mechanism. The quasi-steady state assumption is applied to an H₂–O₂–CO system for calculating OH concentration. Both acetylene and precursor species have their own consumption paths, each of which is described by a single-step oxidation reaction.Validations of the model have been conducted over a wide range of engine conditions from conventional to PCCI-like combustion. Two engine examples (a heavy-duty diesel engine and a light-duty diesel engine) are presented in this paper. The predictions are compared against measurements, and the applicability of the model to multi-dimensional diesel simulations is assessed. The model's capability of predicting the soot distribution structure in a conventional diesel flame is included in discussion as well. The work reveals that the nine-step model is not only computationally efficient but also fundamentally sound. The model can be applied to diesel engine combustion analysis and, after calibration, is suitable to be integrated with genetic algorithms for system optimization over a controllable range of operations.     

Soot formation characteristics and PAH formation process in a micro flow reactor with a controlled temperature profile

A micro flow reactor with a controlled temperature profile was examined with regard to its capabilities to investigate soot formation characteristics of rich methane/air mixtures and the formation process of polycyclic aromatic hydrocarbons (PAHs) of rich acetylene/air mixtures. In the experiment for a methane/air mixture, four kinds of flame and soot responses to equivalence ratio (1.5–4.5) and inlet mean flow velocity (5–40 cm/s) were observed: soot formation without a flame; a flame with soot formation; a flame without soot formation; and neither a flame nor soot formation. Soot formation was observed at high equivalence ratio and low flow velocity. Sooting limits depending on equivalence ratio and flow velocity (residence time) were successfully identified by the present micro flow reactor. To investigate the PAH formation process, the micro flow reactor was employed for a rich acetylene/air mixture at equivalence ratios of 4, 5 and 6 at an inlet mean flow velocity of 2 cm/s and gas sampling experiments were conducted at temperatures from 600 to 1000 K. Temperature dependence of mole fractions of benzene, styrene, naphthalene, phenanthrene, indene, acenaphthylene and biphenyl was successfully obtained and larger PAHs such as pyrene and coronene were not observed in this study. One-dimensional computation with the ABF 2.99 mechanism predicted a benzene mole fraction three times higher than the experimental result. The modification of the ABF 2.99 mechanism using recent benzene reactions greatly improved the prediction of the benzene mole fraction. The rate of production analysis was carried out and PAH formation in the micro flow reactor was investigated in detail.     

An experimental and numerical study of the effects of dimethyl ether addition to fuel on polycyclic aromatic hydrocarbon and soot formation in laminar coflow ethylene/air diffusion flames

The effects of dimethyl ether addition to fuel on the formation of polycyclic aromatic hydrocarbons and soot were investigated experimentally and numerically in a laminar coflow ethylene diffusion flame at atmospheric pressure. The relative concentrations of polycyclic aromatic hydrocarbon species and the relative soot volume fractions were measured using planar laser-induced fluorescence and two-dimensional laser-induced incandescence techniques, respectively. Experiments were conducted over the entire range of dimethyl ether addition from pure ethylene to pure dimethyl ether in the fuel stream. The total carbon mass flow rate was maintained constant when the fraction of DME in the fuel stream was varied. Numerical calculations of nine diffusion flames of different dimethyl ether fractions in the fuel stream were performed using a detailed reaction mechanism consisting of 151 species and 785 reactions and a sectional soot model including soot radiation, inception of nascent soot particle due to collision of two pyrene molecules, heterogeneous surface growth and oxidation following the hydrogen abstraction acetylene addition mechanism, soot particle coagulation, and PAH surface condensation. The addition of a relatively small amount of dimethyl ether to ethylene was found experimentally to increase the concentrations of both polycyclic aromatic hydrocarbons and soot. The synergistic effect on polycyclic aromatic hydrocarbons persists over a wider range of dimethyl ether addition. The numerical results reproduce the synergistic effects of dimethyl ether addition to ethylene on both polycyclic aromatic hydrocarbons and soot, though the magnitude of soot volume fraction overshoot and the range of dimethyl ether addition associated with the synergistic effect of soot are less than those observed in the experiment. The synergistic effects of dimethyl ether addition to ethylene on many hydrocarbon species, including polycyclic aromatic ones, and soot can be fundamentally traced to the enhanced methyl concentration with the addition of dimethyl ether to ethylene. Contrary to previous findings, the pathways responsible for the synergistic effects of benzene, polycyclic aromatic hydrocarbons, and soot in the ethylene/dimethyl ether system are found to be primarily due to the cyclization of l-C₆H₆ and n-C₆H₇ and to a much lesser degree due to the interaction between C2 and C4 species for benzene formation, rather than the propargyl self-combination reaction route, though it is indeed the most important reaction for the formation of benzene.     

Measurement and analysis of soot inception limits of oxygen-enriched coflow flames

This study explores the criteria for soot inception in oxygen-enriched laminar coflow flames. In these experiments we select an axial height in the coflow flame at which to identify the sooting limit. The sooting limit is obtained by varying the amount of inert until luminous soot first appears at this predefined height. The sooting limit flame temperature is found to increase linearly with stoichiometric mixture fraction, regardless of fuel type. To understand these results, the relationships between flame structure, temperature, and local C/O ratio is explored through the use of conserved scalar relationships. Comparison of these relationships with the experimental data indicates that the local C/O ratio is a controlling parameter for soot inception in diffusion flames (analogous to the global C/O ratio in premixed flames). Analysis of experimental results suggests that soot inception occurs when the local C/O ratio is above a critical value. The values for critical C/O ratios obtained from the analysis of experiments using several fuels are similar in magnitude to the corresponding C/O ratios for premixed flames. In addition, temperatures and PAH fluorescence were measured to identify regions in these flames most conducive to particle inception. Results indicate that the peak PAH concentration lies along a critical iso-C/O contour, which supports a theory that soot particles first appear along this critical contour, given sufficient temperature.     

Application of an enhanced PAH growth model to soot formation in a laminar coflow ethylene/air diffusion flame

A recently developed chemical kinetic scheme for C₂ fuel combustion with PAH growth has been implemented in a parallelized coflow flame solver. The reaction mechanism has been developed to include almost all reasonably well-established reaction classes for aromatic ring formation and soot particle precursor molecular weight growth. The model has recently been validated for zero- and one-dimensional premixed flame systems [N.A. Slavinskaya, P. Frank, Combust. Flame 156 (2009) 1705–1722] and has now been updated and extended to a sooting ethylene/air diffusion flame in the coflow geometry. Updates to the mechanism reflect the latest advances in the literature and address numerical stiffness that was present in diffusion flame systems. The chemical kinetic mechanism has been coupled to a sectional aerosol dynamics model for soot growth, considering PAH-based inception and surface condensation, surface chemistry (growth and oxidation), coagulation, and fragmentation. The sectional model predicts the soot aggregate number density and the number of primary particles per aggregate in each section, so as to yield information on particle size distribution and structure. Flame simulation data for the present mechanism is compared to data computed using two other reaction schemes [J. Appel, H. Bockhorn, M. Frenklach, Combust. Flame 121 (2000) 122–136; N.M. Marinov, W.J. Pitz, C.K. Westbrook, A.M. Vincitore, M.J. Castaldi, S.M. Senkan, Combust. Flame 114 (1998) 192–213]. The computed data are also compared to numerous experimental data sets. Whereas the fuel oxidation chemistry in all three mechanisms are essentially the same, the PAH growth pathways vary considerably. It is shown that soot concentrations on the wings of the flame (where soot formation is dominated by surface chemistry) can be predicted with two of the three mechanisms. However, only the present mechanism with its enhanced PAH growth routes can also predict the correct order of magnitude of soot volume fraction in the low-sooting, inception-dominated, central region of the flame. In applying this chemical mechanism, the parameter α, which describes the portion of soot surface sites that are available for chemical reaction, has been reduced to a theoretically acceptable range, thus improving the quality of the model.     

Experimental investigation and detailed modeling of soot aggregate formation and size distribution in laminar coflow diffusion flames of Jet A-1, a synthetic kerosene,and n-decane

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.