Direct measurements of intermediates of ignition are challenging experimental objectives, yet such measurements are critical for understanding fuel decomposition and oxidation pathways. This work presents experimental results, obtained using the University of Michigan Rapid Compression Facility, of ignition delay times and intermediates formed during the ignition of n-butanol. Ignition delay times for stoichiometric n-butanol/O(2) mixtures with an inert/O(2) ratio of 5.64 were measured over a temperature range of 920-1040 K and a pressure range of 2.86-3.35 atm and were compared to those predicted by the recent reaction mechanism developed by Black et al. (Combust. Flame 2010, 157, 363-373). There is excellent agreement between the experimental results and model predictions for ignition delay time, within 20% over the entire temperature range tested. Further, high-speed gas sampling and gas chromatography techniques were used to acquire and analyze gas samples of intermediate species during the ignition delay of stoichiometric n-butanol/O(2) (χ(n-but) = 0.025, χ(O(2)) = 0.147, χ(N(2)) = 0.541, χ(Ar) = 0.288) mixtures at P = 3.25 atm and T = 975 K. Quantitative measurements of mole fraction time histories of methane, carbon monoxide, ethene, propene, acetaldehyde, n-butyraldehyde, 1-butene and n-butanol were compared with model predictions using the Black et al. mechanism. In general, the predicted trends for species concentrations are consistent with measurements. Sensitivity analyses and rate of production analyses were used to identify reactions important for predicting ignition delay time and the intermediate species time histories. Modifications to the mechanism by Black et al. were explored based on recent contributions to the literature on the rate constant for the key reaction, n-butanol+OH. The results improve the model agreement with some species; however, the comparison also indicates some reaction pathways, particularly those important to ethene formation and removal, are not well captured.

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High-speed gas sampling experiments to measure the intermediate products formed during fuel decomposition remain challenging yet important experimental objectives. This article presents new speciation data on two important fuel reference compounds, n-heptane and n-butanol, at practical thermodynamic conditions of 700 K and 9 atm, for stoichiometric fuel-to-oxygen ratios and a dilution of 5.64 (molar ratio of inert gases to O(2)), and at two blend ratios, 80%-20% and 50%-50% by mole of n-heptane and n-butanol, respectively. When compared against 100% n-heptane ignition results, the experimental data show that n-butanol slows the reactivity of n-heptane. In addition, speciation results of n-butanol concentrations show that n-heptane causes n-butanol to react at temperatures where n-butanol in isolation would not be considered reactive. The chemical kinetic mechanism developed for this work accurately predicts the trends observed for species such as carbon monoxide, methane, propane, 1-butene, and others. However, the mechanism predicts a higher amount of n-heptane consumed at the first stage of ignition compared to the experimental data. Consequently, many of the species concentration predictions show a sharp rise at the first stage of ignition, a trend that is not observed experimentally. An important discovery is that the presence of n-butanol reduces the measured concentrations of the large linear alkenes, including heptenes, hexenes, and pentenes, showing that the addition of n-butanol affects the fundamental chemical pathways of n-heptane during ignition.

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This work presents the results of an experimental and computational study of methyl trans-3-hexenoate autoignition. Experimental autoignition studies were conducted using the University of Michigan rapid compression facility. Pressure time histories were used to determine ignition delay times as a function of test gas composition and experimental conditions. The fuel/oxygen equivalence ratio and dilution level were ϕ = 0.3 and inert/O 2 = 3.76 (mole basis). End of compression conditions targeted an average pressure of 10.5 atm and temperatures ranging from 884 to 1085 K. A correlation in Arrhenius form was developed by regression analysis of the experimental data, where the ignition delay time is τ ign (ms) = 1.4 × 10 −6 exp[30 100/(R̅ (cal mol −1 K −1 ) T)] with a R 2 value of 0.99. Gas-sampling experiments were also conducted to measure stable intermediates formed during autoignition. A detailed reaction mechanism was developed and model predictions were compared to the experimental data. While ignition delay time predictions are in excellent agreement with the experimental data, the speciation results highlight uncertainties in the reaction chemistry of unsaturated esters and small hydrocarbon intermediates.