Over the last two decades, our understanding of soot formation has evolved from an empirical, phenomenological description to an age of quantitative modeling for at least small fuel compounds. In this paper, we review the current state of knowledge of the fundamental sooting processes, including the chemistry of soot precursors, particle nucleation and mass/size growth. The discussion shows that though much progress has been made, critical gaps remain in many areas of our knowledge. We propose the roles of certain aromatic radicals resulting from localized π electron structures in particle nucleation and subsequent mass growth. The existence of these free radicals provides a rational explanation for the strong binding forces needed for forming initial clusters of polycyclic aromatic hydrocarbons. They may also explain a range of currently unexplained sooting phenomena, including the large amount of aliphatics observed in nascent soot formed in laminar premixed flames and the mass growth of soot in the absence of gas-phase H atoms. While the above suggestions are inspired, to an extent, by recent theoretical findings from the materials research community, this paper also demonstrates that the knowledge garnered through our longstanding interest in soot formation may well be carried over to flame synthesis of functional nanomaterials for clean and renewable energy applications. In particular, work on flame-synthesized thin films of nanocrystalline titania illustrates how our combustion knowledge might be useful for developing advanced yet inexpensive thin-film solar cells and chemical sensors for detecting gaseous air pollutants.

Formation of nascent soot and other condensed-phase materials in flames

Hierarchical and comparative kinetic modeling of laminar flame speeds of hydrocarbon and oxygenated fuels

Alcohol combustion chemistry

Recent progress in the development of diesel surrogate fuels

Kinetics of elementary reactions in low-temperature autoignition chemistry

Propagation and extinction of premixed C5–C12 n-alkane flames

Detailed flame simulations require the use of both chemical kinetics and molecular transport. Compared to kinetics, notably less work has been done to assess the validity of transport properties and their effect on the results of flame simulations. In the present study, transport parameters were proposed for n -alkanes up to tetradecane and 1 -alkene up to dodecene, based on the correlations of corresponding states. The effect of molecular diffusion was illustrated by considering the phenomena of propagation and extinction for flames of n -dodecane, a main component of surrogates of practical jet fuels. Sensitivity analyses were performed on the reaction rates and binary diffusion coefficients and the relative importance of chemical kinetics and transport properties on flame propagation and extinction was quantified. The effect of carbon number on the extinction of non-premixed n -alkane flames was addressed also based on transport effects.

Sensitivity of propagation and extinction of large hydrocarbon flames to fuel diffusion

Laminar flame speeds of cyclohexane/air, methylcyclohexane/air, ethylcyclohexane/air, n-propylcyclohexane/air, and n-butylcyclohexane/air mixtures were determined in the counterflow configuration at atmospheric pressure, unburned mixture temperature of 353 K, and for a wide range of equivalence ratios. The results indicate that cyclohexane/air flames propagate somewhat faster than mono-alkylated cyclohexane/air flames. Flames of mono-alkylated cyclohexane compounds were found to have similar laminar flame speeds, from methylcyclohexane to n-butylcyclohexane, suggesting that the different alkyl groups have a secondary effect on flame propagation. The experiments were modeled using JetSurF (version 1.1) – a detailed kinetic model for the combustion of cyclohexane and its derivatives. Both experiment and model show satisfactory agreement with each other. Based on the analysis of the model results, the somewhat lower rates of mono-alkylated cyclohexane flame propagation are attributed to the greater production of propene and allyl and the increased H-atom scavenging by these C3 intermediates. Though these fuel-specific reaction kinetic features do not limit the overall oxidation rates, the distribution of the cracked products do exert influences on flame propagation, leading to the subtle differences in the laminar flame speeds observed experimentally.

An experimental and modeling study of the propagation of cyclohexane and mono-alkylated cyclohexane flames

The gas-phase reaction of benzene with O(3P) is of considerable interest for modeling of aromatic oxidation, and also because there exist fundamental questions concerning the prominence of intersystem crossing in the reaction. While its overall rate constant has been studied extensively, there are still significant uncertainties in the product distribution. The reaction proceeds mainly through the addition of the O atom to benzene, forming an initial triplet diradical adduct, which can either dissociate to form the phenoxy radical and H atom, or undergo intersystem crossing onto a singlet surface, followed by a multiplicity of internal isomerizations, leading to several possible reaction products. In this work, we examined the product branching ratios of the reaction between benzene and O(3P) over the temperature range of 300 to 1000 K and pressure range of 1 to 10 Torr. The reactions were initiated by pulsed-laser photolysis of NO2 in the presence of benzene and helium buffer in a slow-flow reactor, and reaction products were identified by using the multiplexed chemical kinetics photoionization mass spectrometer operating at the Advanced Light Source (ALS) of Lawrence Berkeley National Laboratory. Phenol and phenoxy radical were detected and quantified. Cyclopentadiene and cyclopentadienyl radical were directly identified for the first time. Finally, ab initio calculations and master equation/RRKM modeling were used to reproduce the experimental branching ratios, yielding pressure-dependent rate expressions for the reaction channels, including phenoxy + H, phenol, cyclopentadiene + CO, which are proposed for kinetic modeling of benzene oxidation.

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Enoch E. Dames

Papers

Propagation and extinction of premixed C5–C12 n-alkane flames

Sensitivity of propagation and extinction of large hydrocarbon flames to fuel diffusion

An experimental and modeling study of the propagation of cyclohexane and mono-alkylated cyclohexane flames

Products of the Benzene + O(3P) Reaction

Propagation and extinction of benzene and alkylated benzene flames