The oxidation of methane/ethane/propane mixtures, for blends containing 90/6.6/3.3, 70/15/15 and 70/20/10 percent by volume of each fuel respectively in 'air,' has been studied over the temperature range 770-1580 K, at compressed gas pressures of approximately 1, 10, 20, 30, 40 and 50 atm, and at equivalence ratios of 0.5, 1.0 and 2.0 using both a high-pressure shock tube and a rapid compression machine. The present work represents the most comprehensive set of methane/ethane/propane ignition delay time measurements available in a single study which extends the composition envelope over an industrially relevant pressure range. It is also the first such study to present ignition delay times at significantly overlapping conditions from both a rapid compression machine and a shock tube. The data were simulated using a detailed chemical kinetic model comprised of 289 species and 1580 reactions. It was found that qualitatively, the model reproduces correctly the effect of change in equivalence ratio and pressure, predicting that fuel-rich, high-pressure mixtures ignite fastest while fuel-lean, low-pressure mixtures ignite slowest. Moreover, the reactivity as a function of temperature is well captured with the model predicting negative temperature coefficient behavior similar to the experiments. Quantitatively the model is in general excellent agreement with the experimental results but is faster than experiment for the fuel-rich (φ = 2.0) mixture containing the highest quantity of propane (70/15/15 mixture) at the lowest temperatures (770-900 K).
High-pressure experiments and chemical kinetics modeling were performed to generate a database and a chemical kinetic model that can characterize the combustion chemistry of methane-based fuel blends containing significant levels of heavy hydrocarbons (up to 37.5% by volume). Ignition delay times were measured in two different shock tubes and in a rapid compression machine at pressures up to 34 atm and temperatures from 740 to 1660 K. Laminar flame speeds were also measured at pressures up to 4 atm using a high-pressure vessel with optical access. Two different fuel blends containing ethane, propane, n-butane, and n-pentane added to methane were studied at equivalence ratios varying from lean (0.3) to rich (2.0). This paper represents the most comprehensive set of experimental ignition and laminar flame speed data available in the open literature for CH4/C2H6/C3H8/C4H10/C5H12 fuel blends with significant levels of C2+ hydrocarbons. Using these data, a detailed chemical kinetics model, based on current and recent work by the authors, was compiled and refined. The predictions of the model are very good over the entire range of ignition delay times, considering the fact that the data set is so thorough. Nonetheless, some improvements to the model can still be made with respect to ignition times at the lowest temperatures and for the laminar flame speeds at pressures above 1 atm and rich conditions.
One of the alkanes found within gaseous fuel blends of interest to gas turbine applications is butane. There are two structural isomers of butane, normal butane and isobutane, and the combustion characteristics of either isomer are not well known. Of particular interest to this work are mixtures of n-butane and isobutane. A shock-tube experiment was performed to produce important ignition-delay-time data for these binary butane isomer mixtures, which are not currently well studied, with emphasis on 50-50 blends of the two isomers. These data represent the most extensive shock-tube results to date for mixtures of n-butane and isobutane. Ignition within the shock tube was determined from the sharp pressure rise measured at the end wall, which is characteristic of such exothermic reactions. Both experimental and kinetics modeling results are presented for a wide range of stoichiometries (ϕ=0.3−2.0), temperatures (1056–1598 K), and pressures (1–21 atm). The results of this work serve as a validation for the current chemical kinetics model. Correlations in the form of Arrhenius-type expressions are presented, which agree well with both the experimental results and the kinetics modeling. The results of an ignition-delay-time sensitivity analysis are provided, and key reactions are identified. The data from this study are compared with the modeling results of 100% normal butane and 100% isobutane. The 50/50 mixture of n-butane and isobutane was shown to be more readily ignitable than 100% isobutane but reacts slower than 100% n-butane only for the richer mixtures. There was little difference in ignition time between the lean mixtures.
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