A detailed chemical kinetic mechanism has been developed to study the oxidation of the straight-chain isomers of hexene over a wide range of operating conditions. The main features of this detailed kinetic mechanism, which includes both high and low temperature reaction pathways, are presented and discussed with special emphasis on the main classes of reactions involved in alkene oxidation. Simulation results have been compared with experimental data over a wide range of operating conditions including shock tube, jet stirred reactor and rapid compression machine. The different reactivities of the three isomers have been successfully predicted by the model. Isomerization reactions of the hexenyl radicals were found to play a significant role in the chemistry and interactions of the three n-hexene isomers. A comparative reaction flux analysis is used to verify and discuss the fundamental role of the double bond position in the isomerization reactions of alkenyl radicals, as well as the impact of the allylic site in the low and high temperature mechanism of fuel oxidation.3

The chemistry of oxidation and autoignition of 1-, 2-, and 3-hexene has been studied after rapid compression between 630 and 850 K for stoichiometric mixtures with ''air.'' The phenomenology of autoignition has been recognized, and intermediate products formed before autoignition have been identified and analyzed. They mainly comprise of hexadienes, O-heterocycles, and aldehydes. There are many common products, because some of the intermediate alkenyl or alkenylperoxy radicals are delocalized. Saturated O-heterocycles are specific products formed by addition of HO 2 to the double bond. Unsaturated O-heterocycles are products typical of the long alkenyl chain. Saturated and unsaturated lower aldehydes are the products of OH addition to the double bond of hexenes and hexadienes. The relative abundance of the intermediates enables a better insight into the competition between the reactivity of the double bond and the reactivity of the alkenyl chain. According to the position of the double bond, the behavior of 3-hexene is dominated by the properties of the double bond whereas the behavior of 1-hexene is dominated by the properties of the alkenyl chain. The reactivity of the alkenyl chain is related to the type and number of C-H bonds, the ability of stabilized radicals to react, and the cyclic strain of the transition state of isomerization reactions. Therefore, 1-hexene reacts much more with the typical features of alkanes like a two-stage ignition with a cool flame and a negative temperature coefficient. 3-Hexene does not have typical features and 2-hexene has an intermediate behavior.

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New kinetic mechanisms for the oxidation of 1-pentene and 1-hexene at low temperature have been developed that require important improvements of the kinetic rules used by the EXGAS system for the automatic generation of mechanisms. This paper details the changes or additions necessary for the definition of the specific generic reactions involving alkenes and their free radicals, as well as the correlations to estimate the related rate constants. Tests have been performed to verify that these improvements still allow good simulations in the case of propene. New mechanisms for the oxidation of 1-pentene and 1-hexene at low temperature have been thus generated and validated using experimental data obtained in a rapid compression machine between 600 and 900 K. The mechanism for the oxidation of 1-pentene has also been tested in a plug flow reactor between 654 and 716 K. Results reveal acceptable agreement between simulated and experimental data for autoignition delays and for the distribution of products. The analysis of mechanisms demonstrates the importance of new reaction pathways specific to long chain alkenes. This study confirms the significant role played in autoignition delays by the reaction of addition of hydroxyl radicals to the double bond and by the specific reactivity of the allylic radical. The important role played by the reactions of allylic and alkenyl radicals with O 2 to produce dienes is also emphasized and has allowed us to refine the kinetic value for these generic reactions.

, et al.. A model of tetrahydrofuran low-temperature oxidation based on theoretically calculated rate constants. Combustion and Flame, Elsevier, 2018, 191, pp.252-269. <10.1016/j.combustflame.2018 Published in Combustion and Flame (2018), 191, 252-269 AbstractThe first detailed kinetic model of the low-temperature oxidation of tetrahydrofuran has been developed. Thermochemical and kinetic data related to the most important elementary reactions have been derived from ab initio calculations at the CBS-QB3 level of theory. A comparison of the rate constants at 600 K, obtained from these calculations with values estimated using recently published rate rules for alkanes, sometimes show differences of several orders of magnitude for alkylperoxy radical isomerizations, HO2-eliminations, and oxirane formations. The new model satisfactorily reproduces previously published ignition delay times obtained in a rapid-compression machine and in a shock tube, as well as numerous product mole fractions measured in a jet-stirred reactor at low to intermediate temperatures and in a low-pressure laminar premixed flame. To highlight the most significant reaction pathways, flow-rate and sensitivity analyses have been performed with this new model.

Rapid compression machines (RCMs) are widely-used to acquire experimental insights into fuel autoignition and pollutant formation chemistry, especially at conditions relevant to current and future combustion technologies. RCM studies emphasize important experimental regimes, characterized by lowto intermediate-temperatures (600-1200 K) and moderate to high pressures (5-80 bar). At these conditions, which are directly relevant to modern combustion schemes including low temperature combustion (LTC) for internal combustion engines and dry low emissions (DLE) for gas turbine engines, combustion chemistry exhibits complex and experimentally challenging behaviors such as the chemistry attributed to cool flame behavior and the negative temperature coefficient regime. Challenges for studying this regime include that experimental observations can be more sensitive to coupled physicalchemical processes leading to phenomena such as mixed deflagrative/autoignitive combustion. Experimental strategies which leverage the strengths of RCMs have been developed in recent years to make RCMs particularly well suited for elucidating LTC and DLE chemistry, as well as convolved physicalchemical processes. Specifically, this work presents a review of experimental and computational efforts applying RCMs to study autoignition phenomena, and the insights gained through these efforts. A brief history of RCM development is presented towards the steady improvement in design, characterization, instrumentation and data analysis. Novel experimental approaches and measurement techniques, coordinated with computational methods are described which have expanded the utility of RCMs beyond empirical studies of explosion limits to increasingly detailed understanding of autoignition chemistry and the role of physical-chemical interactions. Fundamental insight into the autoignition chemistry of specific fuels is described, demonstrating the extent of knowledge of low-temperature chemistry derived from RCM studies, from simple hydrocarbons to multi-component blends and full-boiling range fuels. Emerging needs and further opportunities are suggested, including investigations of under-explored fuels and the implementation of increasingly higher fidelity diagnostics.

International audienceNew experimental results for the oxidation of n-butylbenzene, a component of diesel fuel, have been obtained using three different devices. A rapid compression machine has been used to measure autoignition delay times after compression at temperatures in the range 640-960 K, at pressures from 13 to 23 bar, and at equivalence ratios from 0.3 to 0.5. Results show low-temperature behavior, with the appearance of cool flames and a negative temperature coefficient (NTC) region for the richest mixtures. To investigate this reaction at higher temperatures, a shock tube has been used. The shock tube study was performed over a wide range of experimental temperatures, pressures, and equivalence ratios, with air used as the fuel diluent. The ignition temperatures were recorded over the range 980-1740 K, at reflected shock pressures of 1, 10, and 30 atm. Mixtures were investigated at equivalence ratios of 0.3, 0.5, 1.0 and 2.0 in order to determine the effects of fuel concentration on reactivity over the entire temperature range. Using a jet-stirred reactor, the formation of numerous reaction products has been followed at temperatures from 550 to 1100 K, at atmospheric pressure, and at equivalence ratios of 0.25, 1.0, and 2.0. Slight low-temperature reactivity (below 750 K) with a NTC region has been observed, especially for the leanest mixtures. A detailed chemical kinetic model has been written based on rules similar to those considered for alkanes by the system EXGAS developed at Nancy. Simulations using this model have been compared to the experimental results presented in this study, but also to results in the literature obtained in a jet-stirred reactor at 10 bar, in the same rapid compression machine for stoichiometric mixtures, in a plug flow reactor at 1069 K and atmospheric pressure, and in a low-pressure (0.066 bar) laminar premixed methane flame doped with n-butylbenzene. The observed agreement is globally better than that obtained with models from the literature. Flow rate and sensitivity analyses have revealed a preponderant role played by the addition to molecular oxygen of resonantly stabilized, 4-phenylbut-4-yl radicals

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