Joe V. Michael Memorial Issue

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/167427/1/kin21481_am.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/167427/2/kin21481.pd

Pyrolysis of Cyclohexane and 1-Hexene at High Temperatures and Pressures—A Photoionization Mass Spectrometry Study

Cycloalkanes are important components of a wide range of fuels. However, there are few experimental data at simultaneously high temperatures and pressures similar to those found in practical systems. Such data are necessary for developing and testing chemical kinetic models. In this study, data relevant to cycloalkane pyrolysis were obtained from high repetition rate shock tube experiments coupled with synchrotron-based photoionization mass spectrometry diagnostics. The pyrolysis of cyclohexane was studied over 1270–1550 K and ~9 bar, while the more reactive primary decomposition product, 1-hexene, was studied at 1160–1470 K and ~5 bar. Insights into the decomposition of the parent molecules, the formation of primary products and the production of aromatic species were gained. Simulations were performed with models for cyclohexane and 1-hexene that were based on literature models. The results indicate that over several hundred microseconds reaction time at high pressures and temperatures the pyrolysis of cyclohexane is largely dominated by reactions initiated by cyclohexyl radicals. Furthermore, good agreement between the simulations and the experiments were observed for cyclohexane and 1-hexene with a modified version of the cyclohexane model. Conversely, the 1-hexene model did not reproduce the experimental observations

Shock Tube Investigation of CH₃ + CH₃OCH₃

The title reaction has been investigated in a diaphragmless shock tube by laser schlieren densitometry over the temperature range 1163–1629 K and pressures of 60, 120, and 240 Torr. Methyl radicals were produced by dissociation of 2,3-butanedione in the presence of an excess of dimethyl ether. Rate coefficients for CH₃ + CH₃OCH₃ were obtained from simulations of the experimental data yielding the following expression which is valid over the range 1100–1700 K: <i>k</i> = (10.19 ± 3.0)<i>T</i>^3.78 exp^(−4878/T) cm³ mol^–1s^–1. The experimental results are in good agreement with estimates by Curran and co-workers [Fischer, S. L.; Dryer, F. L.; Curran, H. J. <i>Int. J. Chem. Kinet.</i> <b>2000</b>, <i>32</i> (12), 713–740. Curran, H. J.; Fischer, S. L.; Dryer, F. L. <i>Int. J. Chem. Kinet.</i> <b>2000</b>, <i>32</i> (12), 741–759] but about a factor of 2.6 lower than those of Zhao et al. [Zhao, Z.; Chaos, M.; Kazakov, A.; Dryer, F. L. <i>Int. J. Chem. Kinet.</i> <b>2008</b>, <i>40</i> (1), 1–18]

Shock Tube Investigation of CH₃ + CH₃OCH₃

The title reaction has been investigated in a diaphragmless shock tube by laser schlieren densitometry over the temperature range 1163–1629 K and pressures of 60, 120, and 240 Torr. Methyl radicals were produced by dissociation of 2,3-butanedione in the presence of an excess of dimethyl ether. Rate coefficients for CH₃ + CH₃OCH₃ were obtained from simulations of the experimental data yielding the following expression which is valid over the range 1100–1700 K: <i>k</i> = (10.19 ± 3.0)<i>T</i>^3.78 exp^(−4878/T) cm³ mol^–1s^–1. The experimental results are in good agreement with estimates by Curran and co-workers [Fischer, S. L.; Dryer, F. L.; Curran, H. J. <i>Int. J. Chem. Kinet.</i> <b>2000</b>, <i>32</i> (12), 713–740. Curran, H. J.; Fischer, S. L.; Dryer, F. L. <i>Int. J. Chem. Kinet.</i> <b>2000</b>, <i>32</i> (12), 741–759] but about a factor of 2.6 lower than those of Zhao et al. [Zhao, Z.; Chaos, M.; Kazakov, A.; Dryer, F. L. <i>Int. J. Chem. Kinet.</i> <b>2008</b>, <i>40</i> (1), 1–18]

Shock Tube Investigation of CH₃ + CH₃OCH₃

The title reaction has been investigated in a diaphragmless shock tube by laser schlieren densitometry over the temperature range 1163–1629 K and pressures of 60, 120, and 240 Torr. Methyl radicals were produced by dissociation of 2,3-butanedione in the presence of an excess of dimethyl ether. Rate coefficients for CH₃ + CH₃OCH₃ were obtained from simulations of the experimental data yielding the following expression which is valid over the range 1100–1700 K: <i>k</i> = (10.19 ± 3.0)<i>T</i>^3.78 exp^(−4878/T) cm³ mol^–1s^–1. The experimental results are in good agreement with estimates by Curran and co-workers [Fischer, S. L.; Dryer, F. L.; Curran, H. J. <i>Int. J. Chem. Kinet.</i> <b>2000</b>, <i>32</i> (12), 713–740. Curran, H. J.; Fischer, S. L.; Dryer, F. L. <i>Int. J. Chem. Kinet.</i> <b>2000</b>, <i>32</i> (12), 741–759] but about a factor of 2.6 lower than those of Zhao et al. [Zhao, Z.; Chaos, M.; Kazakov, A.; Dryer, F. L. <i>Int. J. Chem. Kinet.</i> <b>2008</b>, <i>40</i> (1), 1–18]

Single Pulse Shock Tube Study of Allyl Radical Recombination

The recombination and disproportionation of allyl radicals has been studied in a single pulse shock tube with gas chromatographic measurements at 1–10 bar, 650–1300 K, and 1.4–2 ms reaction times. 1,5-Hexadiene and allyl iodide were used as precursors. Simulation of the results using derived rate expressions from a complementary diaphragmless shock tube/laser schlieren densitometry study provided excellent agreement with precursor consumption and formation of all major stable intermediates. No significant pressure dependence was observed at the present conditions. It was found that under the conditions of these experiments, reactions of allyl radicals in the cooling wave had to be accounted for to accurately simulate the experimental results, and this unusual situation is discussed. In the allyl iodide experiments, higher amounts of allene, propene, and benzene were found at lower temperatures than expected. Possible mechanisms are discussed and suggest that iodine containing species are responsible for the low temperature formation of allene, propene, and benzene

Probing Combustion Chemistry in a Miniature Shock Tube with Synchrotron VUV Photo Ionization Mass Spectrometry

Tunable synchrotron-sourced photoionization time-of-flight mass spectrometry (PI-TOF-MS) is an important technique in combustion chemistry, complementing lab-scale electron impact and laser photoionization studies for a wide variety of reactors, typically at low pressure. For high-temperature and high-pressure chemical kinetics studies, the shock tube is the reactor of choice. Extending the benefits of shock tube/TOF-MS research to include synchrotron sourced PI-TOF-MS required a radical reconception of the shock tube. An automated, miniature, high-repetition-rate shock tube was developed and can be used to study high-pressure reactive systems (<i>T</i> > 600 K, <i>P</i> < 100 bar) behind reflected shock waves. In this paper, we present results of a PI-TOF-MS study at the Advanced Light Source at Lawrence Berkeley National Laboratory. Dimethyl ether pyrolysis (2% CH₃OCH₃/Ar) was observed behind the reflected shock (1400 < <i>T</i>₅ < 1700 K, 3 < <i>P</i>₅ < 16 bar) with ionization energies between 10 and 13 eV. Individual experiments have extremely low signal levels. However, product species and radical intermediates are well-resolved when averaging over hundreds of shots, which is ordinarily impractical in conventional shock tube studies. The signal levels attained and data throughput rates with this technique are comparable to those with other synchrotron-based PI-TOF-MS reactors, and it is anticipated that this high pressure technique will greatly complement those lower pressure techniques

Measurements of structures and concentrations of carbon particle species in premixed flames by the use of in-situ wide angle X-ray scattering

In-situ wide-angle X-ray scattering (WAXS) measurements have been conducted on atmosphericpressure fuel-rich premixed freely propagating ethylene/oxygen flames with argon and nitrogen dilution. In this work, a novel analysis methodology able to provide quantitative information on soot/carbon particle species and concentrations was tested under heavy sooting conditions. The particle composition and concentrations were retrieved by fitting theoretical calculations of structural components from major molecular and nanometric species to the experimental WAXS data. The results show that argon dilution yields predominantly graphene-like components that are less stacked and amorphous carbon that is less structured than under nitrogen dilution. This finding was later confirmed by electron microscopy analysis on samples extracted from similar flames. In addition, the WAXS showed that most of the carbon present in the flames was bound as particles. These results constitute some of the first in-situ observations on structures and concentrations of carbon species in laboratory burner flames operating without stabilization plates. (C) 2015 Elsevier Ltd. All rights reserved