Isobutene is an important intermediate in the pyrolysis and oxidation of higher-order branched alkanes, and it is also a component of commercial gasolines. To better understand its combustion characteristics, a series of ignition delay time (IDT) and laminar flame speed (LFS) measurements have been performed. In addition, flow reactor speciation data recorded for the pyrolysis and oxidation of isobutene is also reported. Predictions of an updated kinetic model described herein are compared with each of these data sets, as well as with existing jet-stirred reactor (JSR) species measurements.IDTs of isobutene oxidation were measured in four different shock tubes and in two rapid compression machines (RCMs) under conditions of relevance to practical combustors. The combination of shock tube and RCM data greatly expands the range of available validation data for isobutene oxidation models to pressures of 50 atm and temperatures in the range 666-1715 K. Isobutene flame speeds were measured experimentally at 1 atm and at unburned gas temperatures of 298-398 K over a wide range of equivalence ratios. For the flame speed results, there was good agreement between different facilities and the current model in the fuel-rich region.Ab initio chemical kinetics calculations were carried out to calculate rate constants for important reactions such as H-atom abstraction by hydroxyl and hydroperoxyl radicals and the decomposition of 2-methylallyl radicals.A comprehensive chemical kinetic mechanism has been developed to describe the combustion of isobutene and is validated by comparison to the presently considered experimental measurements. Important reactions, highlighted via flux and sensitivity analyses, include: (a) hydrogen atom abstraction from isobutene by hydroxyl and hydroperoxyl radicals, and molecular oxygen; (b) radical-radical recombination reactions, including 2-methylallyl radical self-recombination, the recombination of 2-methylallyl radicals with hydroperoxyl radicals; and the recombination of 2-methylallyl radicals with methyl radicals; (c) addition reactions, including hydrogen atom and 2 hydroxyl radical addition to isobutene; and (d) 2-methylallyl radical decomposition reactions. The current mechanism accurately predicts the IDT and LFS measurements presented in this study, as well as the JSR and flow reactor speciation data already available in the literature.The differences in low-temperature chemistry between alkanes and alkenes are also highlighted in this work. In normal alkanes, the fuel radical Ṙ adds to molecular oxygen forming alkylperoxyl (RȮ 2 ) radicals followed by isomerization and chain branching reactions which promote low-temperature fuel reactivity. However, in alkenes, because of the relatively shallow well (~20 kcal mol -1 ) for RȮ 2 formation compared to ~35 kcal mol -1 in alkanes, the Ṙ + O 2 ⇌ RȮ 2 equilibrium lies more to the left favoring Ṙ + O 2 rather than RȮ 2 radical stabilization. Based on this work, and related studies of allylic systems, it is apparent that reactivity fo...

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This study is concerned with the identification and quantification of species generated during the combustion of cyclopentane in a jet stirred reactor (JSR). Experiments were carried out for temperatures between 740 and 1250 K, equivalence ratios from 0.5 to 3.0, and at an operating pressure of 10 atm. The fuel concentration was kept at 0.1% and the residence time of the fuel/O 2 /N 2 mixture was maintained at 0.7 s. The reactant, product, and intermediate species concentration profiles were measured using gas chromatography and Fourier transform infrared spectroscopy. The concentration profiles of cyclopentane indicate inhibition of reactivity between 850-1000 K for φ=2.0 and φ=3.0. This behavior is interesting, as it has not been observed previously for other fuel molecules, cyclic or non-cyclic. A kinetic model including both low-and high-temperature reaction pathways was developed and used to simulate the JSR experiments. The pressure-dependent rate coefficients of all relevant reactions lying on the PES of cyclopentyl + O 2 , as well as the C-C and C-H scission reactions of the cyclopentyl radical were calculated at the UCCSD(T)-F12b/cc-pVTZ-F12//M06-2X/6-311++G(d,p) level of theory. The simulations reproduced the unique reactivity trend of cyclopentane and the measured concentration profiles of intermediate and product species. Sensitivity and reaction path analyses indicate that this reactivity trend may be attributed to differences in the reactivity of allyl radical at different conditions, and it is highly sensitive to the C-C/C-H scission branching ratio of the cyclopentyl radical decomposition.

Cyclopentanone oxidation was studied in a jet-stirred reactor at 1 and 10 atm and over the temperature range of 730–1280 K for fuel-lean (φ = 0.5), stoichiometric, and fuel-rich (φ = 2) mixtures. A total of 16 reaction intermediates and products were identified and quantified using online Fourier transform infrared spectrometry and offline gas chromatography. A kinetic submodel was developed, supported by theoretical calculations for the rate constants of hydrogen abstraction reactions by H atoms and OH and CH3 radicals at the MP2/aug-cc-pVDZ level of theory. The resulting model consisting of 343 species involved in 2065 reactions was used to simulate the present experiments and showed good agreement with the data. The main oxygenated intermediates are aldehydes, and cyclopentenone was also found to be an important species for cyclopentanone oxidation. The rate of production analyses showed that cyclopentanone is mainly consumed by a sequence of reactions producing CO and the but-1-en-4-yl radical. Unimolecular reactions reported in the literature were found to have a very low contribution to the fuel consumption in our experimental conditions. It was finally highlighted that some of the discrepancies observed between the simulation and experiments arise from the chemistry of cyclopentenone that would need to be more detailed.

Among various categories of potential biofuel molecules, ketones are of significant interest. Cyclopentanone is a cyclic ketone that can be produced from biomass, and its combustion is still unknown. Moreover, its cyclic configuration makes it an interesting feedstock for the further production of high-density fuels such as bi(cyclopentane). This study reports the first computational kinetic investigation of the unimolecular decomposition pathways of cyclopentanone by using the compound G3B3 method. The rate constants were calculated using Rice-Ramsperger-Kassel-Marcus theory in the temperature range of 800-2000 K. The results presented here can be used in a future kinetic combustion mechanism. C 2015 Wiley Periodicals, Inc. Int J Chem Kinet 47: [439][440][441][442][443][444][445][446] 2015

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