Uncertainty in measuring laminar burning velocity from expanding methane-air flames at low pressures

International audienceThe experimental determination of laminar burning velocity remains essential to evaluate the combustion potential of any fuels but also to validate kinetic mechanisms. Recently, researchers are making great efforts to improve the accuracy of the different setups and techniques to determine this parameter. This work proposes an attempt to summarize the different factors contributing to the uncertainty of the expanding spherical flame method. In particular, the validity of two hypothesis (adiabatic flame propagation and thin flame front) is discussed in the case of stoichiometric methane-air flames in low-pressure environment (from 0.2 to 2 bar). Last, the effect of spark electrode diameters was also considered (0.2, 0.5 and 1 mm)

Low temperature oxidation of n-hexane in a flow reactor

The risk of igniting a flammable mixture in fuel tank vapor space is a major concern in aviation safety. In order to analyze the hazards and develop mitigation strategies, it is necessary to characterize the explosive properties of kerosene vapor–air mixtures over wide ranges of initial conditions. n-Hexane has been extensively used in our laboratory as a single component surrogate of kerosene. In the present study, hexane oxidation by oxygen was studied in a flow reactor at equivalence ratios of 0.7, 1 and 1.5 for mixtures diluted at 90% with nitrogen. Residence time was set at 2 s and the pressure at 100 kPa. The evolution of the gas phase composition at the reactor exit was studied over the range 450–1000 K. Laser-based diagnostics and gas chromatography analysis were used to characterize the exit mixture composition. The chemical species measurements revealed three distinct regimes of oxidation, namely (i) the cool flame region from 600 to 650 K, (ii) the NTC region between 675 and 775 K, and (iii) the high temperature oxidation regime from 800 K. The modeling study demonstrated the capability of reproducing most of the trends observed experimentally

EXPERIMENTAL AND KINETIC MODELLING STUDY OF THE OXIDATION OF CYCLOPENTANE AND METHYLCYCLOPENTANE AT ATMOSPHERIC PRESSURE

International audienceCyclopentane and methylcyclopentane oxidation was investigated in a jet-stirred reactor at 1 atm, over temperatures ranging from 900 K to 1250 K, for fuel-lean, stoichiometric, and fuel-rich mixtures at a constant residence time of 70 ms. The initial mole fraction of both fuels was kept constant at 1000 ppm. The reactants were highly diluted by a flow of nitrogen to ensure thermal homogeneity. Samples of the reacting mixture were analyzed on-line or off-line by Fourier transform infrared spectroscopy and gas chromatography. A detailed kinetic mechanism consisting of 590 species involved in 3469 reversible reactions was developed and validated against these new experimental results and previously reported ignition delays. Reaction pathways analysis as well as sensitivity analyses were performed to get insights into the differences observed during the oxidation process of cyclopentane and methylcyclopentane

Hydrogen-enriched natural gas blend oxidation under high-pressure conditions: Experimental and detailed chemical kinetic modeling

International audienceFor the first time, experimental results were obtained for the kinetics of oxidation of hydrogen/natural gas mixtures in a fused silica jet-stirred reactor operating at 10 atm, over the temperature range 900–1200K for equivalence ratios of 0.3, 0.6, and 1. The concentration profiles of the reactants, stable intermediates and the final products were measured by probe sampling followed by on-line FTIR analyses and off-line GC-TCD/FID analyses. The addition of hydrogen in variable concentrations significantly increases the reactivity of the natural gas blend used (methane–ethane 10:1), particularly under fuel-lean conditions. The present experiments were modeled by means of a detailed chemical kinetic reaction mechanism (97 species involved in 732 reversible reactions). We obtained an overall good agreement between the present data and the modeling. Accordingto the proposed kinetic scheme, the enhanced oxidation of methane by hydrogen proceeds through an increased production of OH. The increase in hydrogen initial concentration boosts the formation of HO2 radicals at low temperature, yielding higher concentrations of H2O2 and OH. The oxidation of hydrogen, methane and ethane mostly proceeds via reaction with OH. The following sequence of reactions summarizes the mechanism yielding the observed reactivity increase in hydrogenenriched mixtures: CH3 + H2 ⇒ CH4 + H; H + O2 ⇒ HO2; 2HO2 ⇒ H2O2 + O2; H2O2 ⇒ 2 OH; HO2 + H ⇒ 2 OH; OH+H2 ⇒ H2O+H followed by reactions of hydrocarbons with OH. The proposed kinetic scheme was also used to simulatethe burning velocities of methane–hydrogen–air mixtures over the pressure range 1–5 atm

Oxidation of Natural Gas, Natural Gas/Syngas Mixtures, and Effect of Burnt Gas Recirculation: Experimental and Detailed Kinetic Modeling

International audienceThe oxidation of methane-based fuels was studied experimentally in a fused-silica jet-stirred reactor (JSR) operating at 1–10atm, over the temperature range of 900–1450K, from fuel-lean to fuel-rich conditions. Similar experiments were performed in the presence of carbon dioxide or syngas (CO∕H2). A previously proposed kinetic reaction mechanism updated for modeling the oxidation of hydrogen, CO, methane, methanol, formaldehyde, and natural gas over a wide range of conditions including JSR, flame, shock tube, and plug flow reactor was used. A detailed chemical kinetic modeling of the present experiments was performed yielding a good agreement between the modeling, the present data and literature burning velocities, and ignition data. Reaction path analyses were used to delineate the important reactions influencing the kinetic of oxidation of the fuels in the presence of variable amounts of CO2. The kinetic reaction scheme proposed helps understand the effect of the additives on the oxidation of methane

Chemical Kinetic Study of the Oxidation of a Biodiesel−Bioethanol Surrogate Fuel: Methyl Octanoate−Ethanol Mixtures †

International audienceThere is a growing interest for using bioethanol−biodiesel fuel blends in diesel engines but no kinetic data and model for their combustion were available. Therefore, the kinetics of oxidation of a biodiesel−bioethanol surrogate fuel (methyl octanoate−ethanol) was studied experimentally in a jet-stirred reactor at 10 atm and constant residence time, over the temperature range 560−1160 K, and for several equivalence ratios (0.5−2). Concentration profiles of reactants, stable intermediates, and final products were obtained by probe sampling followed by online FTIR, and off-line gas chromatography analyses. The oxidation of this fuel in these conditions was modeled using a detailed chemical kinetic reaction mechanism consisting of 4592 reversible reactions and 1087 species. The proposed kinetic reaction mechanism yielded a good representation of the kinetics of oxidation of this biodiesel−bioethanol surrogate under the JSR conditions. The modeling was used to delineate the reactions triggering the low-temperature oxidation of ethanol important for diesel engine applications

Highly oxygenates molecules formed by oxidation of terpenes in a jet-stirred reactor

International audienceWith the growing interest for biomass-derived fuels the understanding of the combustion chemistry of terpenes becomes of major scientific importance. Terpenes have been proposed as biofuels for aviation because of their high energy density. They usually develop cool flames below 800 K. Very complex processes occur there, with the formation of peroxides intermediates such as ketohydroperoxides and highly oxidized molecules (HOMs) containing both hydroperoxy and carbonyl groups. Such chemicals are relatively unstable and difficult to analyze.We studied the low-temperature oxidation of alpha-pinene, beta-pinene, and limonene (C10H16) in a jet-stirred reactor. The experimental conditions were selected to maximize the production of ketohydroperoxides. We oxidized 5000 ppm of these three terpenes at 1 bar, T = 590 K, an equivalence ratio of 0.5, and at a residence time of 1 s. High-resolution mass spectrometry analyses were performed on solubilized products of terpenes oxidation in cooled acetonitrile. The samples were analyzed using soft HESI electrospray ionization (+/-) and an Orbitrap® mass spectrometer (resolution: 140,000, mass accuracy RO2 QOOH; QOOH + O2 OOQOOH HOOQ’OOH followed by HOOQ’OOH + O2 (HOO)2Q’OO (i) (HOO)2POOH → OH + (HOO)2P=O (i.e., C10H14O5) and (ii) (HOO)2POOH + O2 → (HOO)3POO (HOO)3P’OOH → OH + (HOO)3P=O (i.e., C10H14O7). Fourth oxygen addition yielding C10H14O9 was also observed in the present work. Hydrogen–Deuterium exchange reactions using D2O were used to confirm the presence of –OH groups in the products