The development of accurate chemical kinetic models capable of predicting the combustion of methane and dimethyl ether in common combustion environments such as compression ignition engines and gas turbines is important as it provides valuable data and understanding of these fuels under conditions that are difficult and expensive to study in the real combustors. In this work, both experimental and chemical kinetic model-predicted ignition delay time data are provided covering a range of conditions relevant to gas turbine environments (T = 600 − 1600 K, p = 7 − 41 atm, φ = 0.3, 0.5, 1.0, and 2.0 in 'air' mixtures). The detailed chemical kinetic model (Mech 56.54) is capable of accurately predicting this wide range of data, and it is the first mechanism to incorporate high-level rate constant measurements and calculations where available for the reactions of DME. This mechanism is also the first to apply a pressure-dependent treatment to the low-temperature reactions of DME. It has been validated using available literature data including flow reactor, jet-stirred reactor, shock-tube ignition delay times, shock-tube speciation, flame speed, and flame speciation data. New ignition delay time measurements are presented for methane, dimethyl ether, and their mixtures; these data were obtained using three different shock tubes and a rapid compression machine. In addition to the DME/CH 4 blends, high-pressure data for pure DME and pure methane were also obtained. Where possible, the new * address:

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The pyrolytic and oxidative behaviour of the biofuel 2,5-dimethylfuran (25DMF) has been studied in a range of experimental facilities in order to investigate the relatively unexplored combustion chemistry of the title species and to provide combustor relevant experimental data. The pyrolysis of 25DMF has been re-investigated in a shock tube using the single-pulse method for mixtures of 3% 25DMF in argon, at temperatures from 1200-1350 K, pressures from 2-2.5 atm and residence times of approximately 2 ms.Ignition delay times for mixtures of 0.75% 25DMF in argon have been measured at atmospheric pressure, temperatures of 1350-1800 K at equivalence ratios (ϕ) of 0.5, 1.0 and 2.0 along with auto-ignition measurements for stoichiometric fuel in air mixtures of 25DMF at 20 and 80 bar, from 820-1210 K. This is supplemented with an oxidative speciation study of 25DMF in a jet-stirred reactor (JSR) from 770-1220 K, at 10.0 atm, residence times of 0.7 s and at ϕ = 0.5, 1.0 and 2.0.Laminar burning velocities for 25DMF-air mixtures have been measured using the heat-flux method at unburnt gas temperatures of 298 and 358 K, at atmospheric pressure from ϕ = 0.6-1.6. * address: Combustion Chemistry Centre, National University of Ireland, Galway, University Road Galway, Ireland. Phone: +353-91-494087. k.somers1@nuigalway.ie, URL: http://c3.nuigalway.ie/ (Kieran P. Somers).. Electronic Supplementary Information Electronic supplementary information includes:• Tabulations of all new experimental data • Pressure-time profiles for high pressure shock tube experiments and volume-time profiles used for corresponding simulations• A description of the optimized group additivity rules for substituted furans •The chemkin format kinetic mechanism, thermodynamic and transport files• A list of species structures and names for interpretation of kinetic mechanism and sensitivity analysis diagrams These laminar burning velocity measurements highlight inconsistencies in the current literature data and provide a validation target for kinetic mechanisms.A detailed chemical kinetic mechanism containing 2768 reactions and 545 species has been simultaneously developed to describe the combustion of 25DMF under the experimental conditions described above. Numerical modelling results based on the mechanism can accurately reproduce the majority of experimental data. At high temperatures, a hydrogen atom transfer reaction is found to be the dominant unimolecular decomposition pathway of 25DMF. The reactions of hydrogen atom with the fuel are also found to be important in predicting pyrolysis and ignition delay time experiments.Numerous proposals are made on the mechanism and kinetics of the previously unexplored intermediate temperature combustion pathways of 25DMF. Hydroxyl radical addition to the furan ring is highlighted as an important fuel consuming reaction, leading to the formation of methyl vinyl ketone and acetyl radical. The chemically activated recombination of HȮ 2 or CH 3 Ȯ 2 with the 5-methyl-2-furanylmethyl radical, forming a 5-methy...

This paper describes our developing understanding of low-temperature oxidation kinetics. We have investigated the ignition of the three pentane isomers in a rapid compression machine over a wide range of temperatures and pressures, including conditions of negative temperature coefficient behavior. The pentane isomers are small alkanes, yet have structures that are complex enough to allow for the application of their kinetic and thermochemical rules to larger molecules. Updates to the thermochemistry of the species important in the low-temperature oxidation of hydrocarbons have been made based on a thorough literature review. An evaluation of recent quantum-chemically derived rate coefficients from the literature pertinent to important low-temperature oxidation reaction classes has been performed, and new rate rules are recommended for these classes. Several reaction classes have also been included to determine their importance with regard to simulation results, and we have found that they should be included when developing future chemical kinetic mechanisms. A comparison of the model simulations with pressure-time histories from experiments in a rapid compression machine shows very good agreement for both ignition delay time and pressure rise for both the firstand second-stage ignition events. We show that revisions to both the thermochemistry and the kinetics are required in order to replicate experiments well. A broader validation of the models with ignition delay times from shock tubes and a rapid compression machine is presented in an accompanying paper. The results of this study enhance our understanding of the combustion of straight-and branched-chained alkanes.

The theoretical atomization energies of some 45 CxHyOz molecules present in the Active Thermochemical Tables compilation and of particular interest to the combustion chemistry community have been computed using five composite model chemistries as titled. The species contain between 1-8 "heavy" atoms, and a few are conformationally diverse with up to nine conformers. The enthalpies of formation at 0 and 298.15 K are then derived via the atomization method and compared against the recommended values. In general, there is very good agreement between our averaged computed values and those in the ATcT; those for 1,3-cyclopentadiene exceptionally differ considerably, and we show from isodesmic reactions that the true value for 1,3-cyclopentadiene is closer to 134 kJ mol(-1) than the reported 101 kJ mol(-1). If one is restricted to using a single method, statistical measures indicate that the best methods are in the rank order G3 ≈ G4 > W1BD > CBS-APNO > CBS-QB3. The CBS-x methods do on average predict ΔfH(⊖)(298.15 K) within ≈5 kJ mol(-1) but are prone to occasional lapses. There are statistical advantages to be gained from using a number of methods in tandem, and all possible combinations have been tested. We find that the average formation enthalpy coming from using CBS-APNO/G4, CBS-APNO/G3, and G3/G4 show lower mean signed and mean unsigned errors, and lower standard and root-mean-squared deviations, than any of these methods in isolation. Combining these methods also leads to the added benefit of providing an uncertainty rooted in the chemical species under investigation. In general, CBS-APNO and W1BD tend to underestimate the formation enthalpies of target species, whereas CBS-QB3, G3, and G4 have a tendency to overestimate the same. Thus, combining CBS-APNO with a G3/G4 combination leads to an improvement in all statistical measures of accuracy and precision, predicting the ATcT values to within 0.14 ± 4.21 kJ mol(-1), thus rivalling "chemical accuracy" (±4.184 kJ mol(-1)) without the excessive cost associated with higher-level methods such as W1BD.

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...

An experimental ignition delay time study for the promising biofuel 2-methyl furan (2MF) was performed at equivalence ratios of 0.5, 1.0 and 2.0 for mixtures of 1% fuel in argon in the temperature range 1200-1800 K at atmospheric pressure. Laminar burning velocities were determined using the heat-flux method for mixtures of 2MF in air at equivalence ratios of 0.55-1.65, initial temperatures of 298-398 K and atmospheric pressure. A detailed chemical kinetic mechanism consisting of 2059 reactions and 391 species has been constructed to describe the oxidation of 2MF and is used to simulate experiment. Accurate reproduction of the experimental data has been obtained over all conditions with the developed mechanism. Rate of production and sensitivity analyses have been carried out to identify important consumption pathways of the fuel and key kinetic parameters under these conditions. The reactions of hydrogen atom with the fuel are highlighted as important under all experimental conditions studied, with abstraction by the hydrogen atom promoting reactivity and hydrogen atom addition to the furan ring inhibiting reactivity. This work, to the authors knowledge, is the first to combine theoretical and experimental work to describe the oxidation of any of the alkylated furans. The mechanism developed herein to describe 2MF combustion should also function as a sub-mechanism to describe the oxidation of 2,5-dimethyl furan whilst also providing key insights into the oxidation of this similar biofuel candidate.