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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 O 2 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. [ABSTRACT FROM AUTHOR]

Prenucleation reaction chemistry of polycyclic aromatic hydrocarbons was examined in detailed, site-specific kinetic Monte-Carlo simulations, tracking a single aromatic structure evolving from naphthalene in soot environments of two laminar premixed flames. The numerical results offer a possible reconciliation of a recent flame-based suggestion that one- or two-ring aromatics are the precursors to soot with the otherwise accumulating evidence that soot nucleation begins with mid-size aromatics. The resolution rests in the numerical observation of rapid destruction of the two-ring aromatic molecule, naphthalene, and its replacement with cyclopenta-added derivative structures. The analysis of the reaction events identified important contributions to aromatics evolution from methyl addition to edge five-membered rings. The computed curvature of the growing aromatics exhibited a similar dependence on the molecular size in two substantially different flame environments, thus raising a possibility of universality in aromatic-structure evolution. [ABSTRACT FROM AUTHOR]

Establishing the mechanism for soot nucleation will require in situ experimental characterization of the identity and intermolecular interactions of the initial precursors, and electronic spectroscopy methods have the potential to do both. However, the optical response of polycyclic aromatic hydrocarbon (PAH) dimers and complexes differs significantly from that of the constituent monomers, and studies of soot precursor complexes have largely been limited to PAHs containing only six-membered aromatic rings. Hydrocarbons containing unsaturated five-membered rings are also present in high concentration in flames, and the photoresponse of complexes containing five-membered rings has not yet been examined. In this work, we elucidate the spectroscopic properties of small hydrocarbon complexes containing cyclopentadienyl groups or cyclopenta-fused groups, calculating ground- and excited-state binding energies, optimal excited-state geometries, repulsion energies, and fluorescence emission wavelengths. We show that excited-state distortion of cyclopentadienyl groups allows strong binding and low-energy fluorescence emission compared to similar-sized PAHs, and that a cyclopenta-fused group dramatically lowers the absorption and emission energies for acenaphthylene, dominating excited-state noncovalent interactions—findings that could shed light on the complex electronic properties of flames. [ABSTRACT FROM AUTHOR]

Some aspects of soot formation from gas phase molecules at high temperatures are still unclear. Aromatic π-radicals may be key elements for a change in the model of carbon clustering and particle formation. However, experimental investigations are still needed on the radical nature of molecules and particles in flames and on their roles in the transition from molecules to incipient molecular clusters and their further evolution to mature soot. In this paper, we present electron paramagnetic resonance (EPR) and Raman spectroscopy measurements of particles collected in an ethylene-rich premixed flame at various residence times during nucleation. The experimental results, combined with measurement of the particle size distribution by a differential mobility analyzer, are used to investigate the role of radicals in particle nucleation and rearrangement of aromatic molecules in just-nucleated particles. For all the sampled particles, an EPR signal typical of persistent carbon-centered aromatic radicals is measured. An abrupt change in the EPR signal intensity, which is characteristic of stronger supramolecular interactions, is observed when the size distribution changes from monomodal to bimodal, as confirmed by Raman spectroscopy. Our experimental results indicate strong involvement of π-radicals during particle nucleation and growth. The paramagnetic/radical nature of the sampled particles is discussed on the basis of recent studies on the role of resonantly stabilized radicals in soot nucleation. The presence of localized π-electrons on the edge of aromatic soot constituents and their role in carbon clustering are also discussed. [ABSTRACT FROM AUTHOR]

Pyrene dimerisation was successfully used to model the beginning of soot nucleation in some simulation models. However, the quantum mechanics (QM) calculations proved that the binding energy of a PAH dimer with three six-member rings was similar to that of a pyrene dimer. Meanwhile, the high concentration of phenanthrene at flame conditions indicated high probability of collisions among them. The small difference of the binding energy and high concentration indicated that PAHs structurally smaller than pyrene also could be involved in soot inception. Hence, binary collisions of phenanthrene were simulated to find out whether phenanthrene dimers can serve as soot primary nuclei or not by using non-equilibrium molecular dynamics (MD). Three temperatures, six collision orientations and 155 initial translational velocities (ITVs) were considered. The results indicated that the number of dimers with lifetime over 10 ps which can serve as soot nuclei decreased from 52 at 1000 K to 17 at 1600 K, and further to 6 at 2400 K, which means that low temperature was more favourable for phenanthrene to form soot nuclei. Meanwhile, no soot nuclei were formed at the high velocity region (HVR), compared to 43 and 9 at low and middle velocity regions (LVR and MVR), respectively, when temperature was 1000 K. Also, no soot nuclei were formed at HVR when the temperature was raised to 1600 K and 2400 K. This indicated that HVR was unfavourable for phenanthrene to form soot nuclei. The results computationally further illustrated that small PAHs such as phenanthrene could serve as soot primary nuclei, since they have similar mole fractions in some flames. This may be useful for future soot simulation models. [ABSTRACT FROM PUBLISHER]

The soot formation process was investigated in two low pressure premixed methane flames: a reference sooting flame with equivalence ratio 2.32 and a so called nucleation flame with equivalence ratio 1.95. In the nucleation flame no measureable soot growth after soot inception occurs, and the soot mass increases only by nucleation from the gas phase. Several experimental techniques were used to gain insights on the soot formation and growth processes, including (1) in situ laser induced incandescence (LII) to measure the soot volume fraction and the primary particle size, (2) online jet cooled laser induced fluorescence (JCLIF) to measure the mole fraction of pyrene and fluoranthene and (3) ex-situ secondary ion mass spectrometry (ToF-SIMS) to investigate the chemical composition of the gas phase. Particular attention is devoted to soot gaseous precursors like polycyclic aromatic hydrocarbons (PAHs). ToF-SIMS analyses provide evidences that large hydrocarbons are formed in both flames via hydrogen abstraction/acetylene addition (HACA) mechanism. However, significant differences in the soot volume fraction and concentration profiles of PAHs are observed in the nucleation flame with respect to the reference sooting flame. In particular, in the nucleation flame the average soot particle diameter and the concentration of the larger measured gaseous soot precursors show weak variation all along the flame reaction coordinate. Kinetic modeling was used to help understanding this steady state behavior which is well reproduced in simulations and is found to be related to nearly balanced soot mass growth and loss phenomena currently modeled only by acetylene addition and soot oxidation. [ABSTRACT FROM AUTHOR]

• Yield aromatic index is used to quantify the formation tendency of aromatic products. • The correlation between 1-methylnaphthalene content and sooting tendency is linear. • Acenaphthylene is a key coupling point of two pyrene formation pathways. • 1-methylnaphthalene can stride over the rate limiting step of soot formation process. 1-Methylnaphthalene (1-MN) is a typical diaromatic component in petroleum-based fuels and has been widely chosen to represent aromatic fractions in surrogate fuels of diesel and jet fuels. In this study, experimental and modeling studies were performed to investigate the formation of typical polycyclic aromatic hydrocarbons (PAHs) and soot in coflow methane/air diffusion flames doped with 1-methylnaphthalene/n-dodecane mixtures, and explore chemical effects of 1-methylnaphthalen on key soot formation steps. The laser-induced incandescence (LII) and laser-induced fluorescence (LIF) techniques were used to obtained the soot volume fraction and relative concentration of typical PAHs along the flame centerline, respectively. A novel apparatus-independent indicator, yield aromatic index (YAI), was first proposed to quantify the formation tendency of a specific aromatic product in a flame. It can be found that the addition of 1-methylnaphthalene in the fuel mixtures has a significantly promoting effect on soot formation processes. The correlation between the mass fraction of 1-methylnaphthalene and sooting tendency shows a strong linear relationship. As to the PAH formation tendency, YAI A2R5 have the smaller deviations from corresponding YSIs than YAI A2&A3 and YAI A4 , which indicating that A2R5 can be a type of important species leading to soot inception and have strong contribution to soot formation. Through the chemical kinetic analysis, A2R5 is proved to be a key coupling point of two A4 formation pathways, which start from n-dodecane and 1-methylnaphthalene, respectively. As a small amount of 1-methylnaphthalene is introduced into the fuel, the dominant approach to A2R5 rapidly shifts to a more efficient pathway, and is continually enhanced with increasing 1-methylnaphthalene at a moderate trend. The results reveal that 1-methylnaphthalene in the fuel can stride over the rate limiting step of the soot formation process of non-aromatic fuels, eventually leading to an obviously increasing sooting tendency. This study aims to provide useful information for the development of soot nucleation mechanisms and the control of soot formation processes. [ABSTRACT FROM AUTHOR]

• Sensitivity of optical diagnostics to soot onset studied in counterflow flames. • Sooting limits quantified by different optical diagnostics. • Emission/absorption behavior of nascent soot in near-sooting flames provided. Sooting limit representing the critical flame condition where soot starts to appear is a useful quantitative index to assess the sooting tendency of a particular fuel. Experimental determination of sooting limit is closely related to the establishment of a well-defined "nucleation" flame which facilitates further scrutinization of the soot formation process. The present study was devoted to probing the earliest soot appearance in typical counterflow diffusion flames. The sooting limit was defined as the critical fuel (or oxygen) mole fraction in the nitrogen-balanced fuel (or oxidizer) stream. Multiple optical diagnostic techniques including flame luminosity, light extinction and scattering, spectral soot emission (SSE) as well as laser-induced incandescence (LII) were used to determine the sooting limits. The results showed that the value of sooting limit can be quantitatively different among the used detection techniques. Flame luminosity and SSE were found to be most sensitive to the presence of nascent soot, followed by LII and laser light scattering, while light extinction is the least sensitive technique. The effects of the wavelength of probing light on sooting limits were also assessed for the laser extinction and scattering technique. The contribution of the present work can be summarized into the following two aspects: (1) a quantitative database of sooting limits as determined from different soot detection measurements is provided to facilitate the choice of nascent soot-probing technique; (2) the first appearance of soot particles in nucleation flames are fully characterized and interpreted through a cross-comparison among multiple diagnostics techniques. It is also the authors' expectation that the present results may provide new insights into the understanding of soot nucleation process and the interaction between light and nascent soot. [ABSTRACT FROM AUTHOR]