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Abstract: HO2 and CH3 are major intermediate species presented during the oxidation of natural gas at intermediate temperatures and high pressures. Previous theoretical calculations have identified several product channels for HO2 and CH3 reactions, with CH3 +HO2 →CH3O+OH and CH3 +HO2 →CH4 +O2 being the dominant reaction pathways. Both reaction pathways play an important role in the kinetics of CH4 oxidation as CH3 +HO2 →CH3O+OH is a chain-branching reaction whereas CH3 +HO2 →CH4 +O2 a chain termination reaction. H2O2/CH4/Ar mixtures were shock-heated to a temperature between 1054 and 1249K near 3.5atm to initiate the reaction. OH radicals yielded from H2O2 thermal decomposition react with H2O2 and CH4 respectively to produce HO2 and CH3 in the reacting system. Using laser absorption spectroscopy, time-histories of H2O, OH and HO2 were measured behind reflected shock waves. The rate constant of reaction CH3 +HO2 →CH3O+OH was determined to be 6.8×1012 cm3 mol−1 s−1 with an uncertainty factor of 1.4. The rate constant of the competing CH3 +HO2 →CH4 +O2 reaction was determined to be 4.4×1012 cm3 mol−1 s−1, with an uncertainty factor of 2.1. In addition, the rate constants of two other major reactions of the reacting system, H2O2 (+M)→2OH (+M) and OH+CH4 →CH3O+OH, were found to have excellent agreement with values recommended in literature. [Copyright &y& Elsevier]

Abstract: Ignition delay times were measured behind reflected shock waves for cyclohexane, methylcyclohexane, and n-butylcyclohexane at 1.5 and 3atm, equivalence ratios near 1 and 0.5, and temperatures between 1280 and 1480K. The observed ignition delay times can be summarized as follows: methylcyclohexane> n-butylcyclohexane≈cyclohexane. Several reasons are suggested to explain the ordering of the ignition delay times for these three naphthenes. We believe that this work provides the first set of ignition delay time data for n-butylcyclohexane. In addition, H2O and OH time-histories were recorded during the oxidation of cyclohexane, methylcyclohexane, n-butylcyclohexane, iso-octane and n-heptane under similar test conditions. OH time-histories near time zero are distinctive for each type of fuel studied, and these early-time OH profiles provides critical insight into the influence of molecular structures on ignition behavior, particularly in the case of the cycloalkanes. Comparisons of measured time-histories with simulations from recent cycloalkane oxidation mechanisms are also presented. [Copyright &y& Elsevier]

Abstract: An updated H2/O2 reaction mechanism is presented that incorporates recent reaction rate determinations in shock tubes from our laboratory. These experiments used UV and IR laser absorption to monitor species time-histories and have resulted in improved high-temperature rate constants for the following reactions: The updated mechanism also takes advantage of the results of other recent rate coefficient studies, and incorporates the most current thermochemical data for OH and HO2. The mechanism is tested (and its performance compared to that of other H2/O2 mechanisms) against recently reported OH and H2O concentration time-histories in various H2/O2 systems, such as H2 oxidation, H2O2 decomposition, and shock-heated H2O/O2 mixtures. In addition, the mechanism is validated against a wide range of standard H2/O2 kinetic targets, including ignition delay times, flow reactor species time-histories, laminar flame speeds, and burner-stabilized flame structures. This validation indicates that the updated mechanism should perform reliably over a range of reactant concentrations, stoichiometries, pressures, and temperatures from 950 to greater than 3000K. [Copyright &y& Elsevier]

Abstract: High-temperature 3-pentanone pyrolysis and oxidation studies were performed behind reflected shock waves using laser-based species time-history measurements (3-pentanone, CH3, CO, C2H4, OH and H2O) and ignition delay time measurements. The overall 3-pentanone decomposition rate coefficient was inferred from the measured 3-pentanone and CH3 time-histories during pyrolysis at temperatures of 1070–1530K and a pressure of 1.6atm., and yielded a mathematical expression for k tot =4.383×1049 T−10 exp(−44,780/T)s−1 with an uncertainty of ±35% over 1070–1330K. The measured species time-histories and ignition delay times were also compared to simulations from a detailed kinetic mechanism of Serinyel et al. (2010) [14]. The measured k tot was approximately 3.5 times faster than the value used by Serinyel et al. Additionally, the absence of a methyl ketene decomposition reaction was identified as the cause of a deficiency in the O-atom balance of the measured 3-pentanone and CO time-histories. Using the revised overall 3-pentanone decomposition rate coefficient and an additional methyl ketene decomposition pathway, the modified mechanism was able to successfully simulate all six species time-histories, and showed a significant improvement in the predictions of ignition delay times. Finally, a comparison of ignition delay times and OH species time-histories during 3-pentanone, 2-pentanone and acetone oxidation found that 3-pentanone was the most reactive of the three ketones. [Copyright &y& Elsevier]