Accurate chemical kinetic combustion models of lightly branched alkanes (e.g., 2-methylalkanes) are important to investigate the combustion behavior of real fuels. Improving the fidelity of existing kinetic models is a necessity, as new experiments and advanced theories show inaccuracies in certain portions of the models. This study focuses on updating thermodynamic data and the kinetic reaction mechanism for a gasoline surrogate component, 2-methylhexane, 2 based on recently published thermodynamic group values and rate rules derived from quantum calculations and experiments. Alternative pathways for the isomerization of peroxyalkylhydroperoxide (OOQOOH) radicals are also investigated. The effects of these updates are compared against new high-pressure shock tube and rapid compression machine ignition delay measurements. It is shown that rate constant modifications are required to improve agreement between kinetic modeling simulations and experimental data. We further demonstrate the ability to optimize the kinetic model using both manual and automated techniques for rate parameter tunings to improve agreement with the measured ignition delay time data. Finally, additional low temperature chain branching reaction pathways are shown to improve the model's performance.The present approach to model development provides better performance across extended operating conditions while also strengthening the fundamental basis of the model.

CitationAl Rashidi MJ, Mehl M, Pitz WJ, Mohamed S, Sarathy SM (2017) Cyclopentane combustion chemistry. Part I: Mechanism development and computational kinetics. Combustion and Flame 183: 358-371. Available: http://dx. AbstractCycloalkanes are significant constituents of conventional fossil fuels, in which they are one of the main contributors to soot formation, but also significantly influence the ignition characteristics below ~900 K. This paper discusses the development of a detailed high-and low-temperature oxidation mechanism for cyclopentane, which is an important archetypical cycloalkane. The differences between cyclic and non-cyclic alkane chemistry, and thus the inapplicability of acyclic alkane analogies, required the detailed theoretical investigation of the kinetics of important cyclopentane oxidation reactions as part of the mechanism development. The cyclopentyl + O2 reaction was investigated at the UCCSD(T)-F12a/cc-pVTZ-F12//M06-2X/6-311++G(d,p) level of theory in a timedependent master equation framework. Comparisons with analogous cyclohexane or noncyclic alkane reactions are presented. Our study suggests that beyond accurate quantum chemistry the inclusion of pressure dependence and especially that of formally direct kinetics is crucial even at pressures relevant for practical application. KeywordsCyclopentane, detailed mechanism, computational kinetics, pressure-dependent rate constants IntroductionCycloalkanes are important constituents of petroleum-derived liquid fuels. They make up ~40 wt% of diesel [1,2], ~20 wt% of kerosene [3,4], and ~10 to 15 wt% of gasoline [5]. Some studies have shown that at high temperatures, cycloalkanes may contribute to the production of soot by means of de-hydrogenation reactions [6]. Generally, cycloalkanes exhibit less low-temperature reactivity than their non-cyclic counterparts due to the conformational inhibition of the alkylperoxyhydroperoxyalkyl isomerization, an important low-temperature chain branching pathway. Yang et al. [7,8] have shown that in the case of cyclohexane, the suppression of low-temperature isomerization renders the HO2-elimination pathway more important. This leads to higher concentrations of olefins, which reduces reactivity, delays ignition and also promotes soot formation [7]. The ring strain energy changes the oxidation kinetics, particularly for the ring-opening reactions, which also involve significant change in entropy [8].Furthermore, unlike in n-alkanes, methyl substitution in cycloalkanes increases lowtemperature reactivity [9] for reasons that are not well known on the molecular level.Therefore, more detailed kinetic research is needed to better explain the observed trends, and to enable accurate predictive modeling of cycloalkane-containing fuels.Due to their simplicity and abundance, particularly in shale-and oil sand-derived fuels [10], cyclohexane and cyclopentane are often used to represent the naphthenic fraction in surrogate fuels. While models for cyclohexane [11][12][13][14] cover a wide temperature range, the cyclop...

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.

Hydroperoxyalkylperoxy (OOQOOH) radical isomerization is an important low-temperature chain branching reaction within the mechanism of hydrocarbon oxidation. This isomerization may proceed via the migration of the α-hydrogen to the hydroperoxide group. In this work, a combination of high level composite methods-CBS-QB3, G3, and G4-is used to determine the high-pressure-limit rate parameters for the title reaction. Rate rules for H-migration reactions proceeding through 5-, 6-, 7-, and 8-membered ring transitions states are determined. Migrations from primary, secondary and tertiary carbon sites to the peroxy group are considered. Chirality is also investigated by considering two diastereomers for reactants and transition states with two chiral centers. This is important since chirality may influence the energy barrier of the reaction as well as the rotational energy barriers of hindered rotors in chemical species and transition states. The effect of chirality and hydrogen bonding interactions in the investigated energies and rate constants is studied. The results show that while the energy difference between two diastereomers ranges from 0.1-3.2 kcal/mol, chirality hardly affects the kinetics, except at low temperatures (atmospheric conditions) or when two chiral centers are present in the reactant. Regarding the effect of the H-migration ring size, it is found that in most cases, the 1,5 and 1,6 H-migration reactions have similar rates at low temperatures (below ∼830 K) since the 1,6 H-migration proceeds via a cyclohexane-like transition state similar to that of the 1,5 H-migration.