Modeling and measurements of size distributions in premixed ethylene and benzene flames

This study demonstrates the major differences in the evolution of the particle size distributions (PSDs), both measured and modeled, of soot in premixed benzene and ethylene flat flames. In the experiments, soot concentration and PSDs were measured by using a scanning mobility particle sizer (SMPS, over the size range of 3-80 nm). The model employed calculations of gas phase species coupled with a discrete sectional approach for the gas-to-particle conversion. The model includes reaction pathways leading to the formation of nano-sized particles and their coagulation to larger soot particles. The particle size distribution, both experimental and modeled, evolved from a single particle mode (the nucleation mode) to a bimodal size distribution. An important distinction between the results for the ethylene and benzene flames is the behavior of the nucleation mode which persists at all heights above the burner (HAB) for ethylene whereas it was greatly suppressed at greater HAB for the benzene flames. The explanation for the decreased nucleation mode at higher elevations in the benzene flame is that the aromatics are consumed in the oxidation zone of the flame. Fair predictions of particle-phase concentrations and particle sizes in the two flames were obtained with no adjustments to the kinetic scheme. In agreement with experimental data, the model predicts a higher formation of particulate in the benzene flame as compared with the ethylene flame.     

Implementation of an advanced fixed sectional aerosol dynamics model with soot aggregate formation in a laminar methane/air coflow diffusion flame

An advanced fixed sectional aerosol dynamics model describing the evolution of soot particles under simultaneous nucleation, coagulation, surface growth and oxidation processes is successfully implemented to model soot formation in a two-dimensional laminar axisymmetric coflow methane/air diffusion flame. This fixed sectional model takes into account soot aggregate formation and is able to provide soot aggregate and primary particle size distributions. Soot nucleation, surface growth and oxidation steps are based on the model of Fairweather et al. Soot equations are solved simultaneously to ensure convergence. The numerically calculated flame temperature, species concentrations and soot volume fraction are in good agreement with the experimental data in the literature. The structures of soot aggregates are determined by the nucleation, coagulation, surface growth and oxidation processes. The result of the soot aggregate size distribution function shows that the aggregate number density is dominated by small aggregates while the aggregate mass density is generally dominated by aggregates of intermediate size. Parallel computation with the domain decomposition method is employed to speed up the calculation. Three different domain decomposition schemes are discussed and compared. Using 12 processors, a speed-up of almost 10 is achieved which makes it feasible to model soot formation in laminar coflow diffusion flames with detailed chemistry and detailed aerosol dynamics.     

Influence of soot particle aggregation on time-resolved laser-induced incandescence signals

Laser-induced incandescence (LII) is a versatile technique for quantitative soot measurements in flames and exhausts. When used for particle sizing, the time-resolved signals are analysed as these will show a decay rate dependent on the soot particle size. Such an analysis has traditionally been based on the assumption of isolated primary particles. However, soot particles in flames and exhausts are usually aggregated, which implies loss of surface area, less heat conduction and hence errors in estimated particle sizes. In this work we present an experimental investigation aiming to quantify this effect. A soot generator, based on a propane diffusion flame, was used to produce a stable soot stream and the soot was characterised by transmission electron microscopy (TEM), a scanning mobility particle sizer (SMPS) and an aerosol particle mass analyzer coupled in series after a differential mobility analyzer (DMA-APM). Despite nearly identical primary particle size distributions for three selected operating conditions, LII measurements resulted in signal decays with significant differences in decay rate. However, the three cases were found to have quite different levels of aggregation as shown both in TEM images and mobility size distributions, and the results agree qualitatively with the expected effect of diminished heat conduction from aggregated particles resulting in longer LII signal decays. In an attempt to explain the differences quantitatively, the LII signal dependence on aggregation was modelled using a heat and mass transfer model for LII given the primary particle and aggregate size distribution data as input. Quantitative agreement was not reached and reasons for this discrepancy are discussed.     

Laser-induced incandescence technique to identify soot nucleation and very small particles in low-pressure methane flames

This paper presents the study we carried out on the formation of soot particles in low-pressure premixed CH₄/O₂/N₂ flames by using Laser-Induced Incandescence (LII). Flames were stabilised at 26.6 kPa (200 torr). Four different equivalence ratios were tested (Φ = 1.95, 205, 2.15 and 2.32), Φ = 1.95 corresponding to the equivalence ratio for which LII signals begin to be measurable along the flame. The evolution of the LII signals with laser fluence (fluence curve), time (temporal decay) and emission wavelength is reported at different heights above the burner. We specifically took advantage of the low-pressure conditions to probe with a good spatial resolution the soot inception zone of the flames. Significant different behaviours of the fluence curves are observed according to the probed region of the flames and Φ. In addition, while the surface growth process is accompanied by an increase in the LII decay-times (indicator of the primary particle diameter) at higher Φ, decay-times become increasingly short at lower Φ reaching a constant value along the flame at Φ = 1.95. These behaviours are consistent with the detection of the smallest incandescent particles in the investigated flames, these particles having experienced very weak surface growth. Flame modelling including soot formation has been implemented in flames Φ = 2.05 and 2.32. Experimental quantitative soot volume fraction profiles were satisfactorily reproduced by adjusting the fraction of reactive soot surface available for reactions. The qualitative variation of the computed soot particle diameter and the relative weight of surface growth versus nucleation were consistent with the experimental observations.     

Transported JPDF modelling and measurements of soot at elevated pressures

Accurate measurements and modelling of soot formation in turbulent flames at elevated pressures form a crucial step towards design methods that can support the development of practical combustion devices. A mass and number density preserving sectional model is here combined with a transported joint-scalar probability density function (JDPF) method that enables a fully coupled scalar space of soot, gas-phase species and enthalpy. The approach is extended to the KAUST turbulent non-premixed ethylene-nitrogen flames at pressures from 1 to 5 bar via an updated global bimolecular (second order) nucleation step from acetylene to pyrene. The latter accounts for pressure-induced density effects with the rate fitted using comparisons with full detailed chemistry up to 20 bar pressure and with experimental data from a WSR/PFR configuration and laminar premixed flames. Soot surface growth is treated via a PAH analogy and soot oxidation is considered via O, OH and O₂ using a Hertz-Knudsen approach. The impact of differential diffusion between soot and gas-phase particles is included by a gradual decline of diffusivity among soot sections. Comparisons with normalised experimental OH-PLIF and PAH-PLIF signals suggest good predictions of the evolution of the flame structure. Good agreement was also found for predicted soot volume statistics at all pressures. The importance of differential diffusion between soot and gas-phase species intensifies with pressure with the impact on PSDs more evident for larger particles which tend to be transported towards the fuel rich centreline leading to reduced soot oxidation.     

Effect of fuel/air ratio and aromaticity on the molecular weight distribution of soot in premixed n-heptane flames

Soot growth from inception to mass-loading is studied in a wide range of molecular weights (MW) from 10⁵ to 10¹⁰u by means of size exclusion chromatography (SEC) coupled with on-line UV-visible spectroscopy. The evolution of MW distributions of soot is also numerically predicted by using a detailed kinetic model coupled with a discrete-sectional approach for the modeling of the gas-to-particle process. Two premixed flames burning n-heptane in slightly sooting and heavily sooting conditions are studied. The effect of aromatic addition to the fuel is studied by adding n-propylbenzene (10% by volume) to n-heptane in the heavily sooting condition. A progressive reduction of the MW distribution from multimodal to unimodal is observed along the flames testifying the occurrence of particle growth and agglomeration. These processes occur earlier in the aromatic-doped n-heptane flame due to the overriding role of benzene on soot formation which results in bigger young soot particles. Modeled MW distributions are in reasonable agreement with experimental data although the model predicts a slower coagulation process particularly in the slightly sooting n-heptane flame. Given the good agreement between model predictions and experiments, the model is used to explore the role of fuel chemistry on MW distributions. Two flames of n-heptane and n-heptane/n-propylbenzene in heavily sooting conditions with the same temperature profile and inert dilution are modeled. The formation of larger soot particles is still evident in the n-heptane/n-propylbenzene flame with respect to the n-heptane flame in the same operating conditions of temperature and dilution. In addition the model predicts a larger formation of molecular particles in the flame containing n-propylbenzene and shows that soot inception occurs in correspondence of their maximum formation thus indicating the importance of molecular growth in soot inception.     

Modelling soot formation in a premixed flame using an aromatic-site soot model and an improved oxidation rate

An updated rate of O₂ oxidation of one to four ring polyaromatic hydrocarbons in premixed flames is presented based on density function theory simulations of oxygen attack at different radical sites on various PAHs. The rate is in agreement with other rates found in the literature; however, it is several orders of magnitude lower than the currently accepted oxidation rate of multi-ring aromatic species, including soot. Simulations are presented of a premixed flame using this improved rate and a new advanced soot particle model, which is developed in this paper. This model includes unprecedented detail of the particles in the ensemble, including the aromatic content, C/H composition and primary-particle aggregate structure. The O₂ oxidation rate calculated in this paper is shown to give a better prediction of particle number density and soot volume fraction for a premixed flame. The predicted particle size distributions are shown also to describe better the experimental data. Predicted C/H ratio and PAH size distributions are shown for the flame. Computed TEM-style images are compared to experimental TEM images, which show that the aggregate structure of the particles is well predicted.     

Nano organic carbon and soot in turbulent non-premixed ethylene flames

Spectral optical techniques are combined to characterise the distribution of large-molecule soot precursors, nanoparticles of organic carbon, and soot in two turbulent non-premixed ethylene flames with differing residence times. Laser-induced fluorescence, laser-induced incandescence and light scattering are used to define distributions across the particle size distribution. From the scattering and laser-induced emission measurements it appears that two classes of particles are formed. The first ones are preferentially formed in the fuel-rich region of the flame closer to the nozzle, have sizes of the order of few nanometers but are not fully solid particles, because the constituent molecules still maintain their individual identity exhibiting strong broadband fluorescence in the UV. The second class of particles constituted by solid particles, with sizes of the order of tens of nanometers are able to absorb a sufficient number of photons to be heated to incandescent temperatures. These larger particles are formed at larger residence times in the flame since they are the result of slow growth processes such as coagulation or carbonization. The flames are also modeled in order to produce mixture fraction maps. A new discovery is that nanoparticles of organic carbon concentration, unlike soot, does correlate well with mixture fraction, independent of position in the flame. This is likely to be a significant benefit to future modelling of soot inception processes in turbulent non-premixed flames.     

碳黑颗粒生长模型的初步研究

本文利用反应类(Reaction Class)概念和矩(Moment)方法,研究了层流预混甲烷火焰中碳黑颗粒的成核与长大过程。模型综合考虑了颗粒的成核、颗粒间由于碰撞的聚合、以及气态组分在颗粒表面的生长。通过数值计算预报了碳黑颗粒平均粒径、总表面积、体积分数和数密度,以及萘(A4)和乙炔(C2H2)在颗粒表面的增长速率。     

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, Y_S, 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 Y_S ≥ 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.     

Soot formation characteristics in a lab-scale turbulent pulverized coal flame with simultaneous planar measurements of laser induced incandescence of soot and Mie scattering of pulverized coal

Soot formation characteristics of a lab-scale pulverized coal flame were investigated by performing carefully controlled laser diagnostics. The spatial distributions of soot volume fraction and the pulverized coal particles were measured simultaneously by laser induced incandescence (LII) and Mie scattering imaging, respectively. In addition, the radial distributions of the soot volume fraction were compared with the OH radical fluorescence, gas temperature and oxygen concentration obtained in our previous studies [1], [2]. The results indicated that the laser pulse fluence used for LII measurement should be carefully controlled to measure the soot volume fraction in pulverized coal flames. To precisely measure the soot volume fraction in pulverized coal flames using LII, it is necessary to adjust the laser pulse fluence so that it is sufficiently high to heat up all the soot particles to the sublimation temperature but also sufficiently low to avoid including a too large of a change in the morphology of the soot particles and the superposition of the LII signal from the pulverized coal particles on that from the soot particles. It was also found that the radial position of the peak LII signal intensity was located between the positions of the peak Mie scattering signal intensity and peak OH radical signal intensity. The region, in which LII signal, OH radical fluorescence and Mie scattering coexisted, expanded with increasing height above the burner port. It was also found that the soot formation in pulverized coal flames was enhanced at locations where the conditions of high temperature, low oxygen concentration and the existence of pulverized coal particles were satisfied simultaneously.     

Experimental determination of distributions of soot particle diameter and number density by emission and scattering techniques

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-504 C 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 SOC701 V 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.     

Influence of polydisperse distributions of both primary particle and aggregate size on soot temperature in low-fluence LII

An improved aggregate-based low-fluence laser-induced incandescence (LII) model has been developed. The shielding effect in heat conduction between aggregated soot particles and the surrounding gas was modeled using the concept of the equivalent heat transfer sphere. The diameter of such an equivalent sphere was determined from direct simulation Monte Carlo calculations in the free molecular regime as functions of the aggregate size and the thermal accommodation coefficient of soot. Both the primary soot particle diameter and the aggregate size distributions are assumed to be lognormal. The effective temperature of a soot particle ensemble containing different primary particle diameters and aggregate sizes 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. The effect of primary particle diameter polydispersity is in general important and should be considered. The effect of aggregate size polydispersity is relatively unimportant when the heat conduction between the primary particles and the surrounding gas takes place in the free-molecular regime; however, it starts to become important when the heat conduction process occurs in the near transition regime. The model developed in this study was also applied to the re-determination of the thermal accommodation coefficient of soot in an atmospheric pressure laminar ethylene diffusion flame. PACS 44.05.+e; 61.46.Df; 65.80.+n     

Analysis of the impact of agglomeration and surface chemistry models on soot formation and oxidation

Models for soot aggregation that account for the influence of soot surface chemistry on mass growth and oxidation are still at the formative stage. Past studies have considered techniques ranging from the method of moments to stochastic approaches and significantly different sensitivities to chemical processes such as mass growth and oxidation have been reported. The method of moments is computationally efficient and can yield encouraging results for laminar flames as well as for turbulent flames when combined with transported probability density function (PDF) methods. However, an assessment of the sensitivity to constituent model assumptions is not trivial and information regarding the soot size distribution is incomplete. In the current work, the ability of a sectional method to reproduce population dynamics data has been evaluated along with the sensitivity of predictions to closure elements associated with soot nucleation, agglomeration, surface growth and oxidation. A detailed chemistry model with 285 chemical species and 1520 reactions was used for the gas phase. It is shown that the approach to the fuel lean sooting limit can be reproduced with reasonable accuracy and that the inclusion of fractal aggregates and surface chemistry effects improve agreement with experimental data.     

Modeling soot formation in flames and reactors: Recent progress and current challenges

The study of soot has long been motivated by its adverse impacts on health and the environment. However, this combustion knowledge is also relevant to the production of carbon black and hydrogen via methane pyrolysis which are important commodities. Over the last decade, steady progress has been made in the development of detailed continuum models of soot formation in flames and reactors. Developing more comprehensive models has often been motivated by the need for predicting soot formation over a wider range of conditions (e.g., temperature, pressure, fuels). Measurements with novel experimental techniques have given us new insights into the chemistry, particle dynamics and optical properties of soot particles and even molecules and radicals forming them. Also, multi-scale modeling has enabled us to translate the detailed mechanisms of soot processes based on first principles into computationally efficient but accurate continuum models of soot formation in flames and reactors. However, important questions remain including (1) what is the mechanism of soot inception and surface growth, (2) which gas-phase species are involved in soot inception and surface growth (3) how surface growth and oxidation are affected by soot surface properties. Proposed models need to be evaluated against experimental data over a wide range of conditions to determine their predictive strength. These questions are critical for the accurate prediction of soot formation in flames and its emissions from engines. However, this knowledge can also be used to develop predictive process design and optimization tools for carbon black and other nanocarbon formation in reactors.     

Investigation of soot sensitivity to strain rate in ethylene counterflow soot formation oxidation flames

Soot sensitivity to strain rate is mainly responsible for soot formation intermittence in practical combustion devices. This work provides a fundamental study on soot formation in Soot Formation Oxidation (SFO) counterflow flames at varying strain rates. While the problem has been extensively studied in Soot Formation (SF) configurations, where the dominant process is nucleation, investigations remain scarce in the corresponding SFO cases. In the latter, the high temperatures and strong oxidative environments make the surface reactions prevail over nucleation. The work provides a new dataset for ethylene SFO flames in a wide range of strain rates and sheds light on the main processes concurring in determining soot strain rate sensitivity in such conditions. In particular, the peak of soot volume fraction (SVF) is primarily controlled by surface growth and oxidation. The latter becomes progressively more dominant on the side of the SVF distribution toward the oxidizer nozzle, where the presence of oxidizing agents is significant. The soot mechanism adopted predicts a SVF distribution and sensitivity to strain rate in agreement with experimental data. The latter is found similar to corresponding SF cases, although soot loads in the two configurations differ by almost an order magnitude, and the SVF sensitivity is known to be more accentuated for lower soot loads. A deeper investigation revealed that the nucleation process through dimerizations primarily controls the SVF sensitivity, providing the onset of soot necessary for further growth. Then, the latter tends to reduce SVF sensitivity depending on its impact. PAH sensitivities mostly agree with theoretical observation even though further validations on the kinetic mechanism are needed to improve its predictions in lean conditions. The simplistic yet effective model based on the hybrid method of moments and the employment of a reduced kinetic mechanism makes the approach amenable for turbulent computational fluid dynamic (CFD) simulations.     

A computational study of soot formation and flame structure of coflow laminar methane/air diffusion flames under microgravity and normal gravity

A numerical study is conducted of methane–air coflow diffusion flames at microgravity (μg) and normal gravity (1g), and comparisons are made with experimental data in the literature. The model employed uses a detailed gas phase chemical kinetic mechanism that includes PAH formation and growth, and is coupled to a sectional soot particle dynamics model. The model is able to accurately predict the trends observed experimentally with reduction of gravity without any tuning of the model for different flames. The microgravity sooting flames were found to have lower temperatures and higher volume fraction than their normal gravity counterparts. In the absence of gravity, the flame radii increase due to elimination of buoyance forces and reduction of flow velocity, which is consistent with experimental observations. Soot formation along the wings is seen to be surface growth dominated, while PAH condensation plays a more major role on centreline soot formation. Surface growth and PAH growth increase in microgravity primarily due to increases in the residence time inside the flame. The rate of increase of surface growth is more significant compared to PAH growth, which causes soot distribution to shift from the centreline of the flame to the wings in microgravity.     

The effect of particle aggregation on the absorption and emission properties of mono- and polydisperse soot aggregates

This study concerns the effect of soot-particle aggregation on the soot temperature derived from the signal ratio in two-color laser-induced incandescence measurements. The emissivity of aggregated fractal soot particles was calculated using both the commonly used Rayleigh–Debye–Gans fractal-aggregate theory and the generalized Mie-solution method in conjunction with numerically generated fractal aggregates of specified fractal parameters typical of flame-generated soot. The effect of aggregation on soot temperature was first evaluated for monodisperse aggregates of different sizes and for a lognormally distributed aggregate ensemble at given signal ratios between the two wavelengths. Numerical calculations were also conducted to account for the effect of aggregation on both laser heating and thermal emission at the two wavelengths for determining the effective soot temperature of polydisperse soot aggregates. The results show that the effect of aggregation on laser energy absorption is important at low fluences. The effect of aggregation on soot emissivity is relatively unimportant in LII applications to typical laminar diffusion flames at atmospheric pressure, but it can become more important in flames at high pressures due to larger primary particles and wider aggregate distributions associated with enhanced soot loading.     

On existence of nanoparticles below the sooting threshold

Nanometer-sized particles were studied by photoionisation mass spectrometry and scanning mobility particle sizer in laminar premixed ethylene flames above and below the critical sooting threshold. For sooting flames, both techniques detected a large number of particles with masses between 1 and 50 ku or diameter around a few nanometers. Neither method detected an appreciable number of particles below the sooting threshold in flames similar to those studied earlier for UV absorption and scattering of transparent soot. The absence of particle signals in both experimental techniques raises the question about the origin of UV absorption under nonsooting conditions.