Experimental and Modeling Investigation of the Low-Temperature Oxidation of Dimethyl Ether.

The oxidation of dimethyl ether (DME) was studied using a jet-stirred reactor over a wide range of conditions: temperatures from 500 to 1100 K; equivalence ratios of 0.25, 1, and 2; residence time of 2 s; pressure of 106.7 kPa (close to the atmospheric pressure); and an inlet fuel mole fraction of 0.02 (with high dilution in helium). Reaction products were quantified using two analysis methods: gas chromatography and continuous wave cavity ring-down spectroscopy (cw-CRDS). cw-CRDS enabled the quantification of formaldehyde, which is one of the major products from DME oxidation, as well as that of hydrogen peroxide, which is an important branching agent in low-temperature oxidation chemistry. Experimental data were compared with data computed using models from the literature with important deviations being observed for the reactivity at low-temperature. A new detailed kinetic model for the oxidation of DME was developed in this study. Kinetic parameters used in this model were taken from literature or calculated in the present work using quantum calculations. This new model enables a better prediction of the reactivity in the low-temperature region. Under the present JSR conditions, error bars on predictions were given. Simulations were also successfully compared with experimental flow reactor, jet-stirred reactor, shock tube, rapid compression machine, and flame data from literature. The kinetic analysis of the model enabled the highlighting of some specificities of the oxidation chemistry of DME: (1) the early reactivity which is observed at very low-temperature (e.g., compared to propane) is explained by the absence of inhibiting reaction of the radical directly obtained from the fuel (by H atom abstraction) with oxygen yielding an olefin + HO2·; (2) the low-temperature reactivity is driven by the relative importance of the second addition to O2 (promoting the reactivity through branching chain) and the competitive decomposition reactions with an inhibiting effect.

An experimental and modeling study of the low- and high-temperature oxidation of cyclohexane.

The experimental study of the oxidation of cyclohexane has been performed in a jet-stirred reactor at temperatures ranging from 500 to 1100 K (low- and intermediate temperature zones including the negative temperature-coefficient area), at a residence time of 2 s and for dilute mixtures with equivalence ratios of 0.5, 1, and 2. Experiments were carried out at quasi-atmospheric pressure (1.07 bar). The fuel and reaction product mole fractions were measured using online gas chromatography. A total of 34 reaction products have been detected and quantified in this study. Typical reaction products formed in the low-temperature oxidation of cyclohexane include cyclic ethers (1,2-epoxycyclohexane and 1,4-epoxycyclohexane), 5-hexenal (formed from the rapid decomposition of 1,3-epoxycyclohexane), cyclohexanone, and cyclohexene, as well as benzene and phenol. Cyclohexane displays high low-temperature reactivity with well-marked negative temperature-coefficient (NTC) behavior at equivalence ratios 0.5 and 1. The fuel-rich system (ϕ = 2) is much less reactive in the same region and exhibits no NTC. To the best of our knowledge, this is the first jet-stirred reactor study to report NTC in cyclohexane oxidation. Laminar burning velocities were also measured by the heated burner method at initial gas temperatures of 298, 358, and 398 K and at 1 atm. The laminar burning velocity values peak at ϕ = 1.1 and are measured as 40 and 63.1 cm/s for Ti = 298 and 398 K, respectively. An updated detailed chemical kinetic model including low-temperature pathways was used to simulate the present (jet-stirred reactor and laminar burning velocity) and literature experimental (laminar burning velocity, rapid compression machine, and shock tube ignition delay times) data. Reasonable agreement is observed with most of the products observed in our reactor, as well as the literature experimental data considered in this paper.

Experimental and modeling study of the oxidation of n-butane in a jet stirred reactor using cw-CRDS measurements.

The gas-phase oxidation of n-butane has been studied in an atmospheric jet-stirred reactor (JSR) at temperatures up to 950 K. For the first time, continuous wave cavity ring-down spectroscopy (cw-CRDS) in the near-infrared has been used, together with gas chromatography (GC), to analyze the products formed during its oxidation. In addition to the quantification of formaldehyde and water, which is always difficult by GC, cw-CRDS allowed as well the quantification of hydrogen peroxide (H2O2). A comparison of the obtained mole fraction temperature profiles with simulations using a detailed gas-phase mechanism shows a good agreement at temperatures below 750 K, but an overestimation of the overall reactivity above this temperature. Also, a strong overestimation was found for the H2O2 mole fraction at higher temperatures. In order to improve the agreement between model and experimental results, two modifications have been implemented to the model: (a) the rate constant for the decomposition of H2O2 (+M) ↔ 2OH (+M) has been updated to the value recently proposed by Troe (Combust. Flame, 2011, 158, 594-601) and (b) a temperature dependent heterogeneous destruction of H2O2 on the hot reactor walls with assumed rate parameters has been added. The improvement (a) slows down the overall reactivity at higher temperatures, but has a negligible impact on the maximal H2O2 mole fraction. Improvement (b) has also a small impact on the overall reactivity at higher temperatures, but a large effect on the maximal H2O2 mole fraction. Both modifications lead to an improved agreement between model and experiment for the oxidation of n-butane in a JSR at temperatures above 750 K.