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ABSTRACT: Fuel decomposition and benzene formation processes in a premixed, laminar, low-pressure, fuel-rich flame of 1-hexene (C(6)H(12), CH(2)=CH-CH(2)-CH(2)-CH(2)-CH(3)) are investigated by comparing quantitative mole fraction profiles of flame species with kinetic modeling results. The premixed flame, which is stabilized on a flat-flame burner under a reduced pressure of 30 Torr (= 40 mbar), is analyzed by flame-sampling molecular-beam time-of-flight mass spectrometry which uses photoionization by tunable vacuum-ultraviolet synchrotron radiation. The temperature profile of the flame is measured by OH laser-induced fluorescence. The model calculations include the latest rate coefficients for 1-hexene decomposition (J. H. Kiefer et al., J. Phys. Chem. A, 2009, 113, 13570) and for the propargyl (C(3)H(3)) + allyl (a-C(3)H(5)) reaction (J. A. Miller et al., J. Phys. Chem. A, 2010, 114, 4881). The predicted mole fractions as a function of distance from the burner are acceptable and often even in very good agreement with the experimentally observed profiles, thus allowing an assessment of the importance of various fuel decomposition reactions and benzene formation routes. The results clearly indicate that in contrast to the normal reactions of fuel destruction by radical attack, 1-hexene is destroyed mainly by decomposition via unimolecular dissociation forming allyl (a-C(3)H(5)) and n-propyl (n-C(3)H(7)). Minor fuel-consumption pathways include H-abstraction reactions producing various isomeric C(6)H(11) radicals with subsequent β-scissions into C(2), C(3), and C(4) intermediates. The reaction path analysis also highlights a significant contribution through the propargyl (C(3)H(3)) + allyl (a-C(3)H(5)) reaction to the formation of benzene. In this flame, benzene is dominantly formed through H-assisted isomerization of fulvene, which itself is almost exclusively produced by the C(3)H(3) + a-C(3)H(5) reaction.
Physical Chemistry Chemical Physics 10/2010; 12(38):12112-22. · 3.57 Impact Factor
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ABSTRACT: Polyynic structures in fuel-rich low-pressure flames are observed using VUV photoionization molecular-beam mass spectrometry. High-level ab initio calculations of ionization energies for C2nH2 (n=1-5) and partially hydrogenated CnH4 (n=7-8) polyynes are compared with photoionization efficiency measurements in flames fuelled by allene, propyne, and cyclopentene. C2nH2 (n=1-5) intermediates are unambiguously identified, while HC[triple bond, length as m-dash]C-C[triple bond, length as m-dash]C-CH=C=CH2, HC[triple bond, length as m-dash]C-C[triple bond, length as m-dash]C-C[triple bond, length as m-dash]C-CH=CH2 (vinyltriacetylene) and HC[triple bond, length as m-dash]C-C[triple bond, length as m-dash]C-CH[double bond, length as m-dash]CH-C[triple bond, length as m-dash]CH are likely to contribute to the C7H4 and C8H4 signals. Mole fraction profiles as a function of distance from the burner are presented. C7H4 and C8H4 isomers are likely to be formed by reactions of C2H and C4H radicals but other plausible formation pathways are also discussed. Heats of formation and ionization energies of several combustion intermediates have been determined for the first time.
Physical Chemistry Chemical Physics 02/2008; 10(3):366-74. · 3.57 Impact Factor
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ABSTRACT: A fuel-rich, nonsooting, premixed laminar cyclopentene flame (phi = 2.0) at 37.6 Torr (50 mbar) is investigated by flame-sampling photoionization molecular-beam mass spectrometry utilizing vacuum-ultraviolet synchrotron radiation. Mole fractions as a function of distance from the burner are measured for 49 intermediates with ion masses ranging from 2 (H2) to 106 (C8H10), providing a broad database for flame modeling studies. The isomeric composition is resolved for most species, and the identification of several C4Hx, C7H6, and C7H8 isomers is discussed in detail. The presence of C5H5CCH/C5H4CCH2 and cycloheptatriene is revealed by comparisons between flame-sampled photoionization efficiency data and theoretical simulations, based on calculated ionization energies and Franck-Condon factors. This insight suggests a new potential molecular- weight growth mechanism that is characterized by C5-C7 ring enlargement reactions.
The Journal of Physical Chemistry A 06/2007; 111(19):4081-92. · 2.95 Impact Factor
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ABSTRACT: Molecular-beam mass spectrometry (MBMS) has proven to be a powerful tool for the general analysis of flame structure, providing
concentrations of radical and stable species for low-pressure flat flames since the work of Homann and Wagner in the 1960’s.
In this paper, we will describe complementary measurements using electron-impact ionization with a high-mass-resolution quadrupole
mass spectrometer and vacuum-ultraviolet photoionization in a time-of-flight mass spectrometer. Isomers are resolved that
have not been separately detectable before in MBMS studies of flames, including C3H2, C3H4, C4H3, C4H4, C4H5, C6H6, and C2H4O. The qualitative and quantitative results of MBMS have led to advances in modeling and applying flame chemistry.
Combustion Explosion and Shock Waves 01/2006; 42(6):672-677. · 0.54 Impact Factor
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C.K. Westbrook,
W.J. Pitz,
P.R. Westmoreland,
F.L. Dryer,
M. Chaos,
P. Osswald,
K. Kohse-Höinghaus, T.A. Cool,
J. Wang,
B. Yang,
N. Hansen,
T. Kasper
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ABSTRACT: A detailed chemical kinetic reaction mechanism has been developed for a group of four small alkyl ester fuels, consisting of methyl formate, methyl acetate, ethyl formate, and ethyl acetate. This mechanism is validated by comparisons between computed results and recently measured intermediate species mole fractions in fuel-rich, low-pressure, premixed laminar flames. The model development employs a principle of similarity of functional groups in constraining the H atom abstraction and unimolecular decomposition reactions for each of these fuels. As a result, the reaction mechanism and formalism for mechanism development are suitable for extension to larger oxygenated hydrocarbon fuels, together with an improved kinetic understanding of the structure and chemical kinetics of alkyl ester fuels that can be extended to biodiesel fuels. Variations in concentrations of intermediate species levels in these flames are traced to differences in the molecular structure of the fuel molecules.
Proceedings of the Combustion Institute.
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ABSTRACT: The combustion of 1-propanol and 2-propanol was studied in low-pressure, premixed flat flames using two independent molecular-beam mass spectrometry (MBMS) techniques. For each alcohol, a set of three flames with different stoichiometries was measured, providing an extensive data base with in total twelve conditions. Profiles of stable and intermediate species, including several radicals, were measured as a function of height above the burner. The major-species mole fraction profiles in the 1-propanol flames and the 2-propanol flames of corresponding stoichiometry are nearly identical, and only small quantitative variations in the intermediate species pool could be detected. Differences between flames of the isomeric fuels are most pronounced for oxygenated intermediates that can be formed directly from the fuel during the oxidation process. The analysis of the species pool in the set of flames was greatly facilitated by using two complementary MBMS techniques. One apparatus employs electron ionization (EI) and the other uses VUV light for single-photon ionization (VUV-PI). The photoionization technique offers a much higher energy resolution than electron ionization and as a consequence, near-threshold photoionization-efficiency measurements provide selective detection of individual isomers. The EI data are recorded with a higher mass resolution than the PI spectra, thus enabling separation of mass overlaps of species with similar ionization energies that may be difficult to distinguish in the photoionization data. The quantitative agreement between the EI- and PI-datasets is good. In addition, the information in the EI- and PI-datasets is complementary, aiding in the assessment of the quality of individual burner profiles. The species profiles are supplemented by flame temperature profiles. The considerable experimental efforts to unambiguously assign intermediate species and to provide reliable quantitative concentrations are thought to be valuable for improving the mechanisms for higher alcohol combustion.
Combustion and Flame. 156(6):1181-1201.
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ABSTRACT: The oxidation of methyl formate (CH3OCHO), the simplest methyl ester, is studied in a series of burner-stabilized laminar flames at pressures of 22–30 Torr and equivalence ratios (Φ) from 1.0 to 1.8 for flame conditions of 25–35% fuel. Flame structures are determined by quantitative measurements of species mole fractions with flame-sampling molecular-beam synchrotron photoionization mass spectrometry (PIMS). Methyl formate is observed to be converted to methanol, formaldehyde and methane as major intermediate species of mechanistic relevance. Smaller amounts of ethylene and acetylene are also formed from methyl formate oxidation. Reactant, product and major intermediate species profiles are in good agreement with the computations of a recently developed kinetic model for methyl formate oxidation [S. Dooley, M.P. Burke, M. Chaos, Y. Stein, F.L. Dryer, V.P. Zhukov, O. Finch, J.M. Simmie, H.J. Curran, Int. J. Chem. Kinet. 42 (2010) 527–529] which shows that hydrogen abstraction reactions dominate fuel consumption under the tested flame conditions. Radical–radical reactions are shown to be significant in the formation of a number of small concentration intermediates, including the production of ethyl formate (C2H5OCHO), the subsequent decomposition of which is the major source of observed ethylene concentrations. The good agreement of model computations with this set of experimental data provides a further test of the predictive capabilities of the proposed mechanism of methyl formate oxidation. Other salient issues in the development of this model are discussed, including recent controversy regarding the methyl formate decomposition mechanism, and uncertainties in the experimental measurement and modeling of low-pressure flame-sampling experiments. Kinetic model computations show that worst-case disturbances to the measured temperature field, which may be caused by the insertion of the sampling cone into the flame, do not alter mechanistic conclusions provided by the kinetic model. However, such perturbations are shown to be responsible for disparities in species location between measurement and computation.
Combustion and Flame. 158(4):732-741.
