Soot Number Density Research Articles

Soot Production and Radiative Heat Transfer in Opposed Flame Spread over a Polyethylene Insulated Wire in Microgravity

Flame spread over an insulated electrical wire is identified as a fire scenario in space vehicles. In such microgravity configurations, the contribution of thermal radiation from gaseous participating species and soot to the wire burning rate and flame spread is not fully understood and the present paper addresses this question both experimentally and numerically. A non-buoyant opposed-flow flame spread configuration over a nickel–chrome wire coated by Low Density PolyEthylene (LDPE) is considered with an O2/N2 oxidizer composed of 19% of oxygen in volume and a flow velocity of 200 mm/s. Flame spread rate, pyrolysis rate, stand-off distance, soot volume fraction, and soot temperature are experimentally determined based on optical diagnostics that capture the flame spread in parabolic flights. The numerical model uses the measured spread and pyrolysis rates as input data and solves transport equations for mass, momentum, species, energy, and soot number density and mass fraction in an axisymmetric flame-fixed coordinate system in conjunction with a simple degradation model for the LDPE and a state-of-the-art radiation model. The model considers two assumptions. First, pure ethylene results from the decomposition of LDPE and, second, an acetylene/benzene based-soot model, initially validated for C1–C3 hydrocarbons, can be extended with minor modifications to model soot production of LDPE. Comparisons between model predictions and experimental data in terms of flame structure and soot volume fraction support these assumptions. The major finding of this study is that radiation contributes negatively to the surface heat balance along the LDPE molten surface and the coating ahead of the molten front. This shows that the convective heat transfer from the flame is the main contribution to sustain the pyrolysis process and the flame spread is mainly ensured owing to the combined contribution of convection from flame and conduction inside the condensed phase. The maximum incident radiative flux along the molten ball is 17.5 kW/m2 and is reached at the molten ball trailing edge whereas the radiant fraction is about 0.25. Neglecting flame self-absorption affects these values by less than 5%, showing that the optically-thin approximation is valid for this flame. In addition, soot radiation dominates the radiative heat transfer in this flame, contributing for about two-third of the total radiation. Finally, model results show that the usually-used thermally-thin assumption throughout the LDPE coating is not strictly valid.

Experimental Investigation of Soot Oxidation under Well-Controlled Conditions in a High-Temperature Flow Reactor

ABSTRACTOxidation by molecular oxygen of freshly produced soot from a flat ethylene-air-flame is investigated under well-controlled conditions in a flow reactor at 773–1273 K and atmospheric pressure. Soot particles are characterized before and after oxidation by electrical mobility technique. In addition to the size-classified soot number density, the molecular gas-phase species are obtained simultaneously by molecular-beam mass spectrometry. Oxidation behavior of soot particles as well as some hydrocarbon species sampled from the soot source was determined quantitatively. Oxygenated species are formed as molecular intermediates during the oxidation process inside the reactor. Different particle size distributions have been investigated by varying the sampling position and equivalence ratio of the flame. Oxidation of soot starts at different temperatures, but is clearly separated from oxidation of flame species starting earlier at lower temperatures. Soot oxidation rates were calculated and comparison with the Nagle-Strickland-Constable model indicates that the investigated flame-sampled soot is more reactive than graphite under the investigated conditions. The presented dataset may help to validate existing soot models.

Experimental determination of distributions of soot particle diameter and number density by emission and scattering techniques**Project supported by the National Key Research and Development Program of China (Grant No. 2017YFB0601900), the National Natural Science Foundation of China (Grant Nos. 51827808, 51821004, and 51406095), the Fundamental Research Funds for the Central Universities, China (Grant Nos. 2018ZD03 and 2017ZZD005), and Science and

A diagnostics method was presented that uses emission and scattering techniques to simultaneously determine the distributions of soot particle diameter and number density in hydrocarbon flames. Two manta G-504C cameras were utilized for the scattering measurement, with consideration of the attenuation effect in the flames according to corresponding absorption coefficients. Distributions of soot particle diameter and number density were simultaneously determined using the measured scattering coefficients and absorption coefficients under multiple wavelengths already measured with a SOC701V hyper-spectral imaging device, according to the Mie scattering theory. A flame was produced using an axisymmetric laminar diffusion flame burner with 194 mL/min ethylene and 284 L/min air, and distributions of particle diameter and number density for the flame were presented. Consequently, the distributions of soot volume fraction were calculated using these two parameters as well, which were in good agreement with the results calculated according to the Rayleigh approximation, demonstrating that the proposed diagnostic method is capable of simultaneous determination of the distributions of soot particle diameter and number density.

A Transient Study on the Development of Temperature Field and Soot under Reduced Gravity in a Methane Air Diffusion Flame

The transient behavior of flame in terms of temperature development and soot formation has been investigated numerically with an in-house developed code at normal gravity, reduced gravity of 0.5 G, and 0.0001 G (microgravity) in a methane air co flow laminar diffusion flame. The intermediate data of the field variables have been recorded at suitable intervals of time from ignition to convergence. Enhanced soot volume due to more residence time at lower gravity increases the radiative heat loss causing shrinkage of high temperature borne zone. Both soot volume and soot number register a build up and then a decay till steady state. The corresponding periods increase with the reduction of gravity. At microgravity, both the soot volume and soot particle number reach the peak at the same time and also almost stabilize concomitantly, unlike in other two gravity levels. The peak value of soot volume fraction, under 0.5 G and microgravity at steady state, become as high as 3.5 and 7 times respectively of the values at normal gravity due to surface growth. The soot number density, however, increases only 1.2 and 1.35 times respectively. The relatively lower factor of increase of soot number density is due to coagulation.

Kinetic modeling of soot formation in premixed burner-stabilized stagnation ethylene flames at heavily sooting condition

A detailed kinetic mechanism of soot formation and oxidation is revised and extended to include temperature-dependent collision efficiencies. The collision efficiency for various particle size is studied and compared with experimental data and molecular dynamics simulations for the PAH dimerization where the experimental data are not available. This revised kinetic model is validated in comparison with the premixed burner-stabilized stagnation ethylene flames at heavily sooting conditions. The results showed that quasi-one-dimensional numerical simulations can capture the flame structure and predict soot formation quite satisfactorily. The predicted particle size distribution function (PSDF) is in reasonable agreement with experimental results, but the model only partially reproduces the distinct separation between nucleation and coagulation modes observed experimentally. This leads to some discrepancies in the prediction of soot number density, while the predicted soot volume fraction, which is dominated by the large particles of the PSDF, is in generally good agreement with the experimental data. There is an overestimation of the initial soot volume fraction in the flame region close to the burner, which is a consequence of the over-prediction of the amount of young particles. Therefore, the prediction of PAHs formation and their condensation on soot, which controls the nucleation rate, will require further attention. The comparison between the temperature-dependent model and the model neglecting the temperature dependency showed that the temperature-dependent model could improve the prediction of soot number density, which is controlled by small particles.

Effect of soot model, moment method, and chemical kinetics on soot formation in a model aircraft combustor

The simulation of turbulent sooting flames requires a host of models, of which the two critical components are the chemical kinetics that describe soot precursor evolution and the description of the soot population. The purpose of this study is to understand the sensitivity of soot predictions in a realistic aircraft combustor to model choices for these components. Two different chemistry mechanisms, three different statistical approaches, and two different soot inception models are considered. The simulations show that acetylene-based soot inception produces very high soot volume fraction, with the soot particles present predominantly in the inner recirculation zone of the swirl-stabilized combustor. The PAH-based nucleation models lead to soot generation in the shear layers emanating from fuel injection. The two advanced statistical approaches (Hybrid and Conditional Quadrature Method of Moments) also show significant differences. While the Hybrid method produces lower soot number density, it also generates larger soot particles due to a faster predicted rate of coagulation. The Conditional Quadrature approach produces much higher soot number density, but its particle sizes are smaller compared to the Hybrid method for all kinetic mechanisms considered. This experimental combustor is strongly dominated by surface growth based soot mass addition. As a result, even if nucleation/condensation rates are different, the final soot mass yield is comparable for PAH-based soot models. These results indicate the importance of not only the chemical mechanism, which may be less important in this surface growth dominated combustor, but also the soot statistical model, to which coagulation and the soot surface area are relatively sensitive.

Detailed investigation of soot formation from jet fuel in a diffusion flame with comprehensive and hybrid chemical mechanisms

A numerical and experimental study is performed to investigate soot formation from jet fuel in a laminar coflow diffusion flame. The combustion chemistry of the fuel is simulated using (1) the MURI jet fuel surrogate (Dooley et al. 2012) with a modestly reduced Ranzi mechanism (Ranzi et al. 2012), and (2) the recently proposed HyChem model (Xu et al. 2018) combined with the KAUST PAH mechanism 2 (Wang et al. 2013). The two reaction mechanisms are coupled with a sectional soot model to simulate a coflow diffusion flame of methane doped with the MURI jet fuel surrogate. The combined laser extinction and two-angle elastic light scattering method is used to perform non-intrusive in situ measurements of soot volume fraction, primary particle diameter and number density. The good agreement including soot particle size and number density between the experimental data and the simulation results computed with the reduced Ranzi mechanism demonstrate the robustness of the soot model to changes in fuel composition, as the model parameters are unchanged with a previous numerical study of soot formation of n-propylbenzene/n-dodecane mixtures (Zhang and Thomson, 2018). The computation with the combined HyChem/KAUST mechanism predicts similar results as the computation with the detailed chemistry of the reduced Ranzi mechanism for fuel breakdown, thus the basic premise of the HyChem model that the fuel decomposition process can be greatly simplified with the lumped reaction steps is supported. The results also show that by adding a PAH growth scheme to the HyChem model, the approach can be used to predict soot formation from jet fuel combustion in a laminar coflow diffusion flame. Finally, the dependency of the soot prediction on PAH chemistry is discussed and it is suggested that more experimental data is needed to validate the PAH mechanism and improve the predictive accuracy of the model.

A Multi-Moment Sectional Method (MMSM) for tracking the soot Number Density Function

A new Multi-Moment Sectional Method (MMSM) is proposed for tracking the evolution of the soot Number Density Function (NDF). Unlike conventional sectional methods, in MMSM, multiple statistical moments are solved within each section, and the size distribution within each section is reconstructed from a polynomial profile or, for the last section, an exponential profile. MMSM can be reduced to a method of moments with a single section to minimize the computational cost and reduced to a sectional method when the number of moments per section is one. The MMSM approach is combined with a detailed univariate soot model and applied to the computation of the soot NDF in an ethylene laminar premixed flame. Compared to a pure sectional method, MMSM is shown to achieve a higher rate of convergence for the predicted total number density and particle size distribution.

Toward the accurate prediction of soot in engine applications

Soot emissions from internal combustion engines represent a major challenge to engine manufactures with ever most stringent emission regulations, not only in soot mass yielded but also in soot particle number. For example, a particulate number standard has been introduced in 2011 with Euro 5b for diesel engines and in 2014 with Euro 6 for petrol engines (a limit of 6 × 1011/km). Soot models provide a detailed insight into soot evolution processes and are thus an essential tool in today’s advanced engine designs. Therefore, continuous efforts are made to develop more physically based engine soot models and improve the prediction accuracy. The primary objective of this work is to identify and demonstrate the critical parameters for accurate soot predictions in internal combustion engine applications using the high-fidelity detailed soot model from an engineering point of view. A detailed soot model based on sectional method was used to solve the soot process in diesel and spark ignition direct injection gasoline engines. A series of sensitivity analyses were carried out to evaluate the importance and significance of wall boundary conditions, wall film formation and vaporization, multi-component fuel surrogate, and soot transport process in engine exhaust on soot predictions. The predicted results were compared in details to engine-out measurements in terms of soot mass, number density, and size distributions under various operating conditions. The model results demonstrate that the correct description of the spray–wall interaction and wall film vaporization, as well as the soot transport processes in full engine cycle, is critical for achieving reliable predictive capabilities in engine simulations, especially for spark ignition direct injection gasoline engines. The findings should help engineers in this field for more accurate soot predictions in engine simulations.

Characteristics of evolution of in-cylinder soot particle size and number density in a diesel engine

Particulate matter (PM) emission from diesel engines has been a critical issue due to its environmental impact. The soot size and number density inside the cylinder can severely affect the size and mass of PM emission emitted from the diesel exhaust pipe. In the present study, the temporal and spatial variations of the mean and instantaneous in-cylinder soot size and number density in a single cylinder direct injection diesel engine have been simulated numerically at different operating conditions, injection system parameters, fuel types and piston bowl geometry using AVL-FIRE code. The results show that the engine load has great effects on the mean soot particle size and number density with crank angle while the engine speed and the fuel injection system parameters have no obvious effects on them. It is observed that there is an obvious shift to larger particles at higher load. The mean soot number densities firstly increase with the increase of loads, but decrease at later crank angle. The in-cylinder soot size distribution shows a unimodal shape at different crank angle under all operating conditions. The soot particle numbers are concentrated from the start of combustion to 40 °CA after top dead center (ATDC). The spatial soot particle size distribution in the combustion chamber is closely related to engine load, impinging angle and airflow motion. The piston bowl diameter has large effect on the mean soot size and number density during the latter phase of the expansion stroke. The results also indicate that both in-cylinder mean soot size and number from biodiesel-diesel blends are affected not only by the oxygen content, but also engine operating conditions.

Soot primary particle size dependence on combustion pressure in laminar ethylene diffusion flames

A multi-probe thermophoretic soot sampling system, installed inside a high pressure combustion chamber, was used to collect soot samples at elevated pressures from ethylene–air laminar diffusion flames. Ethylene was diluted with nitrogen at a ratio of 1/3 by mass, and a constant mass flow rate of ethylene-nitrogen mixture was maintained at pressures of 5, 10, 15, and 20 bar. Selected mass flow rate of ethylene, 0.72 mg/s, provided stable and non-smoking laminar flames with a height of about 16 mm at all pressures considered. Soot samples were collected on transmission electron microscope grids at three heights above the burner rim, 3, 8, and 12 mm. The images of the soot particles were captured by transmission electron microscope and the primary soot particle diameters were determined using an automated edge detection method. The mean primary soot particle diameter increased from the sampling height of 3 mm to 8 mm, which corresponds to the mid-height of the flames where the peak soot volume fractions are observed. The soot diameters decreased from the mid-height of the flame to the sampling location of 12 mm, near the tip of the flame. The mean diameter of the primary soot particles increased with increasing pressure up to 15 bar; at 20 bar, the mean soot diameter seemed to reach a plateau or start decreasing. Measured soot volume fractions at sampling locations of 3 and 8 mm above the burner rim indicate that, in view of the measured mean soot diameters at the same locations, soot number density should be increasing intensely with pressure.

Experimental and numerical investigation on soot volume fractions and number densities in non-smoking laminar n-heptane/n-butanol coflow flames

In this work, the soot volume fractions (SVF) and number densities in laminar coflow flames of n-heptane/n-butanol blends were experimentally and numerically investigated. Five blends, B00, B25, B50, B75, and B100, based on the volume fraction of n-butanol in the liquid fuel, were employed to explore the soot behavior when n-butanol is added to the fuel stream. Mass flow rates of the blends were adjusted to keep the mole flow of carbon constant. The fuel stream was heavily diluted with N2 to carry the fuel vapor. Measured visible heights are 5.7 cm, 5.6 cm, 5.3 cm, 5.1 cm and 4.8 cm for the five target flames, respectively. A thermophoretic probe was used to sample particles at different height of the flames. SVF, soot number density, average primary diameter, and number of primary particles per aggregate were measured and calculated through transmission electron microscopy (TEM) images. CoFlame code was used to calculate the soot formation in the coflow flames with a newly developed n-heptane/n-butanol/polycyclic aromatic hydrocarbon (PAH) skeletal model and a fixed sectional soot method. The qualitative characteristics of SVF and number densities obtained from TEM images were well captured by the calculation, although the visible heights were underpredicted. Calculated peaks of soot mass fraction (SMF) and number densities show a decrease as more n-butanol is added, and all the peaks are reached around the height of 3.2 cm. Inception by the dimerization of pyrene (A4) plays the most important role in the evolution of soot number densities, while hydrogen-abstraction-carbon-addition (HACA) process dominates the soot mass addition. Lower mole fractions of A4 and C2H2 are responsible for the suppression of SMF and number densities with the addition of n-butanol. Condensation of A4 and fragmentation play small roles in the increase of SMF and number densities, respectively. Finally, all the particles are completely oxidized, first mainly by OH, and then by O2.

Stochastic Simulation of Soot Formation Evolution in Counterflow Diffusion Flames

Soot generally refers to carbonaceous particles formed during incomplete combustion of hydrocarbon fuels. A typical simulation of soot formation and evolution contains two parts: gas chemical kinetics, which models the chemical reaction from hydrocarbon fuels to soot precursors, that is, polycyclic aromatic hydrocarbons or PAHs, and soot dynamics, which models the soot formation from PAHs and evolution due to gas-soot and soot-soot interactions. In this study, two detailed gas kinetic mechanisms (ABF and KM2) have been compared during the simulation (using the solver Chemkin II) of ethylene combustion in counterflow diffusion flames. Subsequently, the operator splitting Monte Carlo method is used to simulate the soot dynamics. Both the simulated data from the two mechanisms for gas and soot particles are compared with experimental data available in the literature. It is found that both mechanisms predict similar profiles for the gas temperature and velocity, agreeing well with measurements. However, KM2 mechanism provides much closer prediction compared to measurements for soot gas precursors. Furthermore, KM2 also shows much better predictions for soot number density and volume fraction than ABF. The effect of nozzle exit velocity on soot dynamics has also been investigated. Higher nozzle exit velocity renders shorter residence time for soot particles, which reduces the soot number density and volume fraction accordingly.

An LES-PBE-PDF approach for predicting the soot particle size distribution in turbulent flames

In this article, we combine the large eddy simulation (LES) concept with the population balance equation (PBE) for predicting, in a Eulerian fashion, the evolution of the soot particle size distribution in a turbulent non-premixed hydrocarbon flame. In order to resolve the interaction between turbulence and chemical reactions/soot formation, the transport equations for the gas phase scalars and the PBE are combined into a joint evolution equation for the filtered pdf associated with a single realization of the gas phase composition and the soot number density distribution. With view towards an efficient numerical solution procedure, we formulate Eulerian stochastic field equations that are statistically equivalent to the joint scalar-number density pdf. By discretizing the stochastic field equation for the particle number density using an explicit adaptive grid technique, we are able to accurately resolve sharp features of evolving particle size distributions, while keeping the number of grid points in particle size space small. Compared to existing models, the main advantage of our approach is that the LES-filtered particle size distribution is predicted at each location in the flow domain and every instant in time and that arbitrary chemical reaction mechanisms and soot formation kinetics can be accommodated without approximation. The combined LES-PBE-PDF model is applied to investigate soot formation in the turbulent non-premixed Delft III flame. Here, the soot kinetics encompass acetylene-based rate expressions for nucleation and growth that were previously employed in the context of laminar diffusion flames. In addition, both species consumption by soot formation and radiation based on the assumption of optical thinness are accounted for. While the agreement of our model predictions with experimental measurements is not perfect, we indicate the benefits of the LES-PBE-PDF model and demonstrate its computational viability.

Numerical Study of Inlet Turbulators Effect on the Thermal Characteristics of a Jet Propulsion-Fueled Combustor and Its Hazardous Pollutants Emission

This work numerically studies the effects of inlet air and fuel turbulators on the thermal behavior of a combustor burning the jet propulsion (JP) (kerosene-surrogate) fuel and its resulting pollutants emission including the nanoparticulate soot aerosols and aromatic compounds. To model the soot formation, the method employs a semi-empirical two-equation model, in which the transport equations for soot mass fraction and soot number density are solved considering soot nanoparticles evolutionary process. The soot nucleation is described using the phenyl route in which the soot is formed from the polycyclic aromatic hydrocarbons. Incorporating a detailed chemical mechanism described by 200 species and 6907 elementary reactions, the flamelets and their lookup table library are precomputed and used in the context of steady laminar flamelet model (SLFM). Thus, the current finite-volume method solves the transport equations for the mean mixture fraction and its variance and considers the chemistry–turbulence interaction using the presumed-shape probability density functions (PDFs). To validate the utilized models, a benchmark combustor is first simulated, and the results are compared with the measurements. Second, the numerical method is used to investigate the effects of embedding different inflow turbulators on the resulting flame structure and the combustor pollutants emission. The chosen turbulators produce mild to severe turbulence intensity (TI) effects at the air and fuel inlets. Generally, the results of current study indicate that the use of suitable turbulators can considerably affect the thermal behavior of a JP-fueled combustor. Additionally, it also reduces the combustor polycyclic aromatic hydrocarbon (PAH) pollutants emission.

Predictions of soot formation and its effect on the flame temperature of a pulverized coal-air turbulent jet

The distributions of soot volume fraction, soot number density, and soot particle size in a coal-air turbulent jet flame was numerically investigated using a soot formation model developed in our previous study. In addition, the effect of radiation from different radiative media on the flame temperature was assessed and discussed. Validated by the reported experimental measurements, the distribution of soot volume fraction in the turbulent air-coal jet flame was well predicted. It was also found that soot particles were formed and accumulated in the high temperature zone with remarkable oxygen deficient. In the same time, the temperature of the coal-air jet flame was reduced remarkably by soot radiation but barely by soot formation. For the simulated flame, at a downstream location of 200mm from the jet exit, the heat loss caused by the thermal radiation of coal particles, soot particles and gas species resulted in a temperature decrease of 262K, 238K and 102K respectively. Different radiative media induced remarkably different distributions of absorption coefficient, and the emitting heat loss from the media was determined by the local absorption coefficient and temperature.

Numerical investigation on soot particles emission in compression ignition diesel engine by using particulate mimic soot model

Research via computational method, specifically by detailed-kinetic soot model offers much more advantages than the simple model as more detailed formation/oxidation process is taken into consideration, thus providing better soot mass concentration, soot size, soot number density as well as information regarding other related species. In the present computational study, investigation of in-cylinder soot concentration as well as other emissions in a single cylinder diesel engine has been conducted, using a commercial multidimensional CFD software, CONVERGE CFD. The simulation was carried out for a close-cycle combustion environment from inlet valve closing (IVC) to exhaust valve opening (EVO). In this case, detailed-kinetic Particulate Mimic (PM) soot model was implemented as to take benefit of the method of moment, instead of commonly implemented simple soot model. Analyses of the results are successfully plotted to demonstrate that the soot size and soot mass concentration are strongly dependent on the detailed soot formation and oxidation process rates. The calculated of soot mass concentration and average soot size at EVO provide the end value of 29.2 mg/m 3 and 2.04 × 10 −8 m, respectively. Besides, post-processing using EnSight shows the qualitative results of soot concentration along simulation period in the combustion chamber.

Investigation of mass and energy coupling between soot particles and gas species in modelling ethylene counterflow diffusion flames

A numerical model is developed aiming at investigating soot formation in ethylene counterflow diffusion flames. The mass and energy coupling between soot solid particles and gas-phase species is investigated in detail. A semi-empirical two-equation model is chosen for predicting soot mass fraction and number density. The model describes particle nucleation, surface growth, and oxidation. A detailed kinetic mechanism is considered for the gas phase and the effect of considering radiation heat losses is also evaluated. Simulations were done for a range of conditions that produce low-to-significant amounts of soot using three strategies: first by changing the strain rate imposed on the flow field, second, by changing the oxygen content in the oxidant stream, and third, by changing the pressure. Additionally, the effect of the transport model chosen was analysed. The results showed that, for the flames studied and within the limits of the present work, the soot and gas radiation terms are of primary importance for numerical simulations. Additionally, it was shown that the soot mass and thermodynamic properties coupling terms are, in general, a second-order effect, with an importance that increases as soot amount increases. As a general recommendation, the radiation terms have always to be considered, whereas full coupling has to be employed only when the soot mass fraction, YS, is equal to or larger than 0.008. If a higher precision is required, with errors less than 1%, full coupling should be taken into account for YS ≥ 0.002. For lower soot amounts, the coupling through soot mass and thermodynamic properties may be neglected as a first approximation, but an error on the total mass conservation will be present. Additionally, discrepancies from considering different transport models (detailed or simplified) are larger than those found from not fully coupling the phases.

Simultaneous measurements of acetylene and soot during the pyrolysis of ethylene and benzene in a shock tube

Acetylene is one of the most important precursors of soot and contributes to soot growth by the hydrogen-abstraction acetylene-addition (HACA) mechanism. In this work, we undertake time-resolved simultaneous measurements of acetylene and soot behind reflected shock waves at temperatures of 1600–2200K and pressures of 3–5bar. Acetylene mole fraction time-histories are measured from the absorption of a quantum-cascade laser operating around 13.6µm. The soot volume fraction, particle size and number densities are calculated from the extinction and scattering of a cw Nd:Yag laser at 532nm. Acetylene and soot are generated from the pyrolysis of 1% benzene in argon, 2.35% ethylene in argon, and binary mixtures of ethylene with propane/methane in argon. We note that acetylene time-histories exhibit a two-stage growth during the pyrolysis of benzene, which can be correlated to the initial rapid increase of soot volume fraction and a later plateauing. In comparison to ethylene pyrolysis, the pyrolysis of benzene results in larger values of the soot volume fraction, particle diameter and number density. We compare the measured data against the values simulated using the method-of-moments routine in Chemkin-Pro and a detailed PAH mechanism based on KM2 [1] and AramcoMech 1.3 [2]. Large discrepancies are observed between the measured and predicted values of the soot parameters. The data obtained from our experiments may assist future validation and development of soot mechanisms.

Modeling soot oxidation with the Extended Quadrature Method of Moments

Modeling the oxidation of soot particles in flames is a challenging topic both from a chemical point of view and regarding the statistical treatment of the evolution of the soot number density function (NDF). The method of moments is widely-used for the statistical modeling of aerosol dynamics in various applications, and a number of different moment methods have been established and successfully applied to the modeling of soot formation and growth. However, a shortcoming of existing moment methods is the lack of an accurate, numerically robust, and computationally efficient way to treat soot oxidation, especially regarding the prediction of the particle number density. In this work, the recently developed Extended Quadrature Method of Moments (EQMOM) is integrated with a physico-chemical soot model and combined with a treatment for particle removal by oxidation. This leads to a modeling framework for the simulation of coupled inception, growth, coagulation, and oxidation of soot in flames. In EQMOM, the moment equations are closed by reconstructing the soot NDF with a superposition of continuous kernel functions. Various standard distribution functions can be used as kernel functions, and the algorithm has been implemented here using gamma and lognormal distributions. It is shown that and discussed why gamma distributions are more suitable as kernel functions than lognormal distributions in order to accurately predict soot oxidation. The integrated model is validated by comparisons with analytical solutions for the NDF, results from Monte Carlo simulations of soot formation and oxidation in flames, and experimental data.