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New Haven, CT, United States

Carbone F.,Yale Center for Combustion Studies | Gomez A.,Yale Center for Combustion Studies
Combustion and Flame | Year: 2012

The structure of gaseous counterflow diffusion flames perturbed with the addition of hundreds of ppm of prevaporized toluene is studied in two distinct flame environments: a blue methane flame stabilized on the fuel side of the gas stagnation plane and an incipiently sooting ethylene flame stabilized on the oxidizer side. The goal is to provide a well-defined testbed in terms of temperature-time history, major species and part of the radical pool, for the examination of reference fuels that are critical components of practical fuel blends. Gas samples are extracted from the flame with fused silica microprobes for subsequent GC/MS analysis and thermocouples and thin filament pyrometry are used to characterize the temperature field. Profiles of critical toluene pyrolysis products and stable soot precursors are compared with computational models using two semi-detailed chemical mechanisms. Results show that in the methane flame some oxygen containing radicals like O and OH are contributing early on to the toluene destruction path. In the incipiently sooting ethylene flame, the primary attack is from H alone. This finding confirms the different challenges that such flames pose to the validation of a chemical kinetic mechanism. The onset of toluene decay in these flames begins at relatively modest temperatures, on the order of 800. K. This reactivity is captured reasonably well by both chemical mechanisms in the methane flame, in the absence of reactants larger than C2, but not so in the ethylene flame, in the presence of a richer, more complex mixture. The aromatic ring opening mechanisms are not adequately modeled in either case. This discrepancy has implications for the modeling of practically relevant fuel blends with both aliphatic and aromatic compounds. The dominant species larger than toluene in the doped methane flame is ethylbenzene, which at least one of the mechanisms reproduces quite well. The largest measured species in the incipiently sooting flame is indene, whose concentration increase due to toluene addition is properly captured by one of the models. The experimental dataset reported here may help identifying future improvements to chemical kinetic mechanisms and complement other reactor datasets lacking the coupling of kinetics and transport of flame environments. © 2012 The Combustion Institute. Source


Carbone F.,Yale Center for Combustion Studies | Gomez A.,Yale Center for Combustion Studies
Proceedings of the Combustion Institute | Year: 2013

The structure of gaseous counterflow diffusion flames perturbed with the addition of hundreds of ppm of prevaporized 1,2,4-trimethyl benzene (TMB) is studied in two distinct flame environments: a blue methane flame and an incipiently sooting ethylene flame. The two flames provide well defined temperature-time histories and chemical environments to investigate the behavior of complex fuels and complement other reacting environments lacking the coupling of kinetics and transport that is typical of flame environments. Profiles of critical pyrolysis products and of some stable soot precursors are determined from GC/MS analysis of gas samples extracted from the flames and compared with results from the OPPDIFF model using a semidetailed chemical mechanism. Experimentally, because of the presence of aliphatic fragments, TMB reactivity is enhanced in these flames with the onset of TMB decay beginning at relatively modest temperatures, on the order of 800 K. The dominant path to stable species is driven by H radical attack. It leads in sequence to xylenes, toluene (through benzyl radical) and benzene formation. This enhanced reactivity is captured reasonably well by the model in the methane flame, but not in the ethylene flame, in the presence of a richer, more complex mixture. The model does not reproduce accurately the pathway yielding C3 and some C 4 species from TMB cracking. Aromatic ring opening is the bottleneck in the TMB cracking process in the methane flame but not in the ethylene one. Indene, an important soot precursor for monoaromatic fuels since the second aromatic ring formation is considered to be a bottleneck in the process, is measured in the ethylene flame in poor agreement with the model predictions. The dataset presented here and available supplemental data online may help identifying improvements to the chemical kinetic mechanism of this reference fuel. © 2012 The Combustion Institute. Source


Carbone F.,Yale Center for Combustion Studies | Carbone F.,University of Southern California | Gomez A.,Yale Center for Combustion Studies
Combustion and Flame | Year: 2014

The destruction of n-decane is investigated with a perturbative approach by adding hundreds of ppm to the fuel stream of two gaseous counterflow diffusion flames at atmospheric pressure: a blue methane flame and an incipiently sooting ethylene flame that offer distinct reacting environments. The detailed chemical structure of the flames including the products of n-decane consumption is determined using a microprobe gas sampling technique followed by GC/MS analysis. Experimentally, principal products of n-decane destruction are C2-C9 linear alpha-olefins that are found at ever increasing concentrations with decreasing carbon number, starting with 1-nonene all the way to propene and ethylene, the most abundant products. Successive fragmentation steps of the n-decane primary products lead to the formation of C2-C5 dienes and other hydrocarbons with multiple unsaturated bonds. The consumption rate of n-decane is more abrupt in the methane flame as compared to the gentler decay observed in the ethylene flame. The addition of n-decane in the ethylene flame does not contribute to the formation of soot precursors such as aromatic compounds because the pool of C2-C4 fragments of the baseline flame, playing a key role in aromatic growth, is only marginally affected by n-decane addition. The comprehensive database of stable species of the experimental component of the study is tested by a comparison with the results of modeling the flames using two semi-detailed chemical kinetic mechanisms, Ranzi-mech and JetSurF. Shortcomings of these mechanisms are highlighted for different classes of compounds by comparison of the model results with the experimental data leaving room for future improvements in their formulation. © 2013 The Combustion Institute. Source

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