Publications

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Autoignition chemistry is central to predictive modeling of many advanced engine designs that combine high efficiency and low inherent pollutant emissions. This chemistry, and especially its pressure dependence, is poorly known for fuels derived from heavy petroleum and for biofuels, both of which are becoming increasingly prominent in the nation's fuel stream. We have investigated the pressure dependence of key ignition reactions for a series of molecules representative of non-traditional and alternative fuels. These investigations combined experimental characterization of hydroxyl radical production in well-controlled photolytically initiated oxidation and a hybrid modeling strategy that linked detailed quantum chemistry and computational kinetics of critical reactions with rate-equation models of the global chemical system. Comprehensive mechanisms for autoignition generally ignore the pressure dependence of branching fractions in the important alkyl + O{sub 2} reaction systems; however we have demonstrated that pressure-dependent 'formally direct' pathways persist at in-cylinder pressures.

Reactions of α-hydroxyethyl (CH3CHOH) and β-hydroxyethyl (CH2CH2OH) radicals with oxygen are of key importance in ethanol combustion. High-level ab initio calculations of the potential energy surfaces of these two reactions were coupled with master equation methods to compute rate coefficients and product branching ratios for temperatures of 250-1000 K. The α-hydroxyethyl + O2 reaction is controlled by the barrierless entrance channel and shows negligible pressure dependence; in contrast, the reaction of the β isomer displays pronounced pressure dependence. The high pressure limit rate coefficients of both reactions are about the same at the temperatures investigated. Products of the reactions were monitored experimentally at 4 Torr and 300-600 K using tunable synchrotron photoionization mass spectrometry. Hydroxyethyl radicals were produced from the reaction of ethanol with chlorine atoms and the β isomer was also selectively produced by the addition reaction C2H4 + OH → CH2CH2OH. Formaldehyde, acetaldehyde, vinyl alcohol and H2O2 products were detected, in qualitative agreement with the theoretical predictions. © 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

The OH concentration in the Cl-initiated oxidation of cyclohexane has been measured between 6.5-20.3 bar and in the 586-828 K temperature range by a pulsed-laser photolytic initiation--laser-induced fluorescence method. The experimental OH profiles are modeled by using a master-equation-based kinetic model as well as a comprehensive literature mechanism. Below 700 K OH formation takes place on two distinct time-scales, one on the order of microseconds and the other over milliseconds. Detailed modeling demonstrates that formally direct chemical activation pathways are responsible for the OH formation on short timescales. These results establish that formally direct pathways are surprisingly important even for relatively large molecules at the pressures of practical combustors. It is also shown that remaining discrepancies between model and experiment are attributable to low-temperature chain branching from the addition of the second oxygen to hydroperoxycyclohexyl radicals.

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This is a pretty pretentious title, but I promised that I would write something autobiographical in this space, and I will do that. However, first I want to thank everyone who contributed to the Festschrift. When Stephen Klippenstein told me that he and Craig Taatjes had gotten approval for it (against my advice), I envisioned a situation where the issue had only two papers, both co-authored by Stephen. Luckily that turned out not to be the case. At the time of this writing there are forty-three manuscripts at various stages of review. I am extremely grateful to Craig and Stephen, to the editors of the journal, and to all the authors for the tribute. It is far and away the most flattering thing that anyone has ever done for me in my career.

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Flame-sampling photoionization mass spectrometry is used for measurements of the absolute molar composition of fuel-rich (φ = 1.8) low-pressure laminar flames of allene and propyne. The experiment combines molecular-beam mass spectrometry with photoionization by tunable vacuum-ultraviolet synchrotron radiation. This approach provides selective detection of individual isomers and unambiguous identifications of other flame species of near-equal mass by near threshold photoionization efficiency measurements. Mole fraction profiles for more than 30 flame species with ion masses ranging from 2 to 78 are presented. The isomeric composition is resolved for most intermediates, for example, mole fraction profiles are presented for both benzene and the fulvene isomer. The results are compared with predictions based on current kinetic models. The mole fractions of the major species are predicted quite accurately, however, some discrepancies are observed for minor species. © 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

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The potential energy surface for the reaction between OH and acetylene has been calculated using the RQCISD(T) method and extrapolated to the complete basis-set limit. Rate coefficients were determined for a wide range of temperatures and pressures, based on this surface and the solution of the one-dimensional and two-dimensional master equations. With a small adjustment to the association energy barrier (1.1 kcal/mol), agreement with experiments is good, considering the discrepancies in such data. The rate coefficient for direct hydrogen abstraction is significantly smaller than that commonly used in combustion models. Also in contrast to previous models, ketene + H is found to be the main product at normal combustion conditions. At low temperatures and high pressures, stabilization of the C{sub 2}H{sub 2}OH adduct is the dominant process. Rate coefficient expressions for use in modeling are provided.

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