Shock tube measurements of branched alkane ignition delay times (original) (raw)
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Shock tube measurements of iso-octane ignition times and OH concentration time histories
Proceedings of the Combustion Institute, 2002
Ignition times and OH radical concentration time histories were measured behind reflected shock waves in iso-octane/O 2 /Ar mixtures. Initial reflected shock conditions were in the ranges 1177 to 2009 K and 1.18 to 8.17 atm, with fuel concentrations of 100 ppm to 1% and equivalence ratios from 0.25 to 2. Ignition times were measured using endwall emission of CH and sidewall pressure. OH concentrations were measured using narrow-linewidth ring-dye laser absorption of the R 1 (5) line of the OH A-X (0,0) band at 306.5 nm. The ignition time data and OH concentration time-history measurements were compared with model predictions of four current iso-octane oxidation mechanisms, and the implications of these comparisons are discussed. To our knowledge, these data provide the first extensive measurements of low-fuelconcentration ignition times and OH concentration time histories for iso-octane autoignition, and hence provide a critical contribution to the database needed for validation of a detailed mechanism for this primary reference fuel.
Ethane ignition and oxidation behind reflected shock waves
Combustion and Flame, 2007
Several diluted C 2 H 6 /O 2 /Ar mixtures of varying concentrations and equivalence ratios (0.5 < φ < 2.0) were studied at temperatures between 1218 and 1860 K and at pressures between 0.57 and 3.0 atm using a shock tube. The argon dilution ranged from 91 to 98% by volume. Reaction progress was monitored using chemiluminescence emission from OH * and CH * at 307 and 431 nm, respectively. The dependence of ignition delay time on temperature, activation energy, and reactant concentrations is given in a master correlation of all the experimental data. The overall activation energy was found to be 39.6 kcal/mol over the range of conditions studied. For the first time in a shock-tube C 2 H 6 oxidation study, detailed species profile data and quantitative OH * time histories were documented, in addition to ignition delay times, and compared against modern detailed mechanisms. Because of the comprehensive scope of the present study and the high precision of the experimental data, several conclusions can be drawn that could not have been reached from earlier studies. Although there is some discrepancy among previous ethane oxidation data, the present work clearly shows the convergence of ignition delay time measurements to those herein and the remarkable accuracy of current kinetics models over most of the parameter space explored, despite the variation in the literature data. However, two areas shown to still need more measurements and better modeling are those of higher pressures and fuel-rich ethane-air mixtures. After appropriate OH * and CH * submechanisms are added, two modern chemical kinetics mechanisms containing high-temperature ethane chemistry are compared to the data to gauge the current state of C 2 H 6 oxidation modeling over the conditions of this study. The reproduction of the OH * and CH * profiles, together with τ ign predictions by these models, are compared against the profiles and ignition times found in the experimental data. The models are then used to identify some key reactions in ethane oxidation and CH formation under the conditions of this study.
Shock tube measurements of the rate constants for seven large alkanes+OH
Proceedings of the Combustion Institute, 2015
Reaction rate constants for seven large alkanes + hydroxyl (OH) radicals were measured behind reflected shock waves using OH laser absorption. The alkanes, n-hexane, 2-methyl-pentane, 3-methyl-pentane, 2,2-dimethyl-butane, 2,3-dimethyl-butane, 2-methyl-heptane, and 4-methyl-heptane, were selected to investigate the rates of site-specific H-abstraction by OH at secondary and tertiary carbons. Hydroxyl radicals were monitored using narrow-line-width ring-dye laser absorption of the R 1 (5) transition of the OH spectrum near 306.7 nm. The high sensitivity of the diagnostic enabled the use of low reactant concentrations and pseudo-first-order kinetics. Rate constants were measured at temperatures ranging from 880 K to 1440 K and pressures near 1.5 atm. High-temperature measurements of the rate constants for OH + n-hexane and OH + 2,2-dimethyl-butane are in agreement with earlier studies, and the rate constants of the five other alkanes with OH, we believe, are the first direct measurements at combustion temperatures. Using these measurements and the site-specific H-abstraction measurements of , general expressions for three secondary and two tertiary abstraction rates were determined as follows (the subscripts indicate the number of carbon atoms bonded to the next-nearest-neighbor carbon):
Combustion and Flame, 1999
An analytical study was conducted to supplement recent high-pressure shock tube measurements of CH 4 /O 2 ignition at elevated pressures (40 -260 atm), low dilution levels (fuel plus oxidizer Ն30%), intermediate temperatures (1040 -1500 K), and equivalence ratios as high as 6. A 38-species, 190-reaction kinetics model, based on the Gas Research Institute's GRI-Mech 1.2 mechanism, was developed using additional reactions that are important in methane oxidation at lower temperatures. The detailed-model calculations agree well with the measured ignition delay times and reproduce the accelerated ignition trends seen in the data at higher pressures and lower temperatures. Although the expanded mechanism provides a large improvement relative to the original model over most of the conditions of this study, further improvement is still required at the highest CH 4 concentrations and lowest temperatures. Sensitivity and species flux analyses were used to identify the primary reactions and kinetics pathways for the conditions studied. In general, reactions involving HO 2 , CH 3 O 2 , and H 2 O 2 have increased importance at the conditions of this work relative to previous studies at lower pressures and higher temperatures. At a temperature of 1400 K and pressure of 100 atm, the primary ignition promoters are CH 3 ϩ O 2 ϭ O ϩ CH 3 O and HO 2 ϩ CH 3 ϭ OH ϩ CH 3 O. Methyl recombination to ethane is a primary termination reaction and is the major sink for CH 3 radicals. At 1100 K, 100 atm, the dominant chain-branching reactions become CH 3 O 2 ϩ CH 3 ϭ CH 3 O ϩ CH 3 O and H 2 O 2 ϩ M ϭ OH ϩ OH ϩ M. These two reactions enhance the formation of H and OH radicals, explaining the accelerated ignition delay time characteristics at lower temperatures (19.0 kcal/mol activation energy at 1100 K versus 32.7 kcal/mol at 1400 K). A literature review indicated few measurements exist for many of the most influential rate coefficients, suggesting the need for further study in this area. This paper represents a first step toward understanding the kinetics of CH 4 ignition and oxidation at the extreme conditions of the shock tube experiments.
Proceedings of the Combustion Institute, 2020
We report the first shock tube measurements of formaldehyde (CH 2 O) during the first stage ignition of n-heptane, 2-methylhexane and 3,3-dimethylpentane, in highly diluted fuel/oxygen mixtures in the pressure range of 7-10 atm and temperature range of 700-880 K. Combined time histories of all carbonyl (-C = O) species, CO and fuel were also measured simultaneously in an effort to study the impact of fuel structure on the concentration and the rate of evolution of first stage ignition products. Of the three isomers studied in this work, n-heptane was found to be the fastest, while 3,3-dimethylpentane was found to be the slowest. The differences in the time scale of formation, and plateau concentration of the intermediates between the isomers across the entire range of test conditions suggests a strong dependency of the measured time histories to fuel structure. These species therefore act as markers of the Negative Temperature Coefficient (NTC) behavior of fuels and can be used as targets for developing semi-empirical, hybrid chemistry models of complex, multicomponent petroleum derived gasoline and jet fuels. The time histories reported in this work should prove very useful in the refinement of detailed kinetic models of n-heptane, and development of rate rules for branched alkane isomers.
Ignition delay times of methyl oleate and methyl linoleate behind reflected shock waves
Proceedings of the Combustion Institute, 2013
Ignition delay times for methyl oleate (C 19 H 36 O 2 , CAS: 112-62-9) and methyl linoleate (C 19 H 34 O 2 , CAS: 112-63-0) were measured for the first time behind reflected shock waves, using an aerosol shock tube. The aerosol shock tube enabled study of these very-low-vapor-pressure fuels by introducing a spatiallyuniform fuel aerosol/4% oxygen/argon mixture into the shock tube and employing the incident shock wave to produce complete fuel evaporation, diffusion, and mixing. Reflected shock conditions covered temperatures from 1100 to 1400 K, pressures of 3.5 and 7.0 atm, and equivalence ratios from 0.6 to 2.4. Ignition delay times for both fuels were found to be similar over a wide range of conditions. The most notable trend in the observed ignition delay times was that the pressure and equivalence ratio scaling were a strong function of temperature, and exhibited cross-over temperatures at which there was no sensitivity to either parameter. Data were also compared to the biodiesel kinetic mechanism of Westbrook et al. [10], which underpredicts ignition delay times by about 50%. Differences between experimental and computed ignition delay times were strongly related to existing errors and uncertainties in the thermochemistry of the large methyl ester species, and when these were corrected, the kinetic simulations agreed significantly better with the experimental measurements.
Methyl concentration time-histories during iso-octane and n-heptane oxidation and pyrolysis
Proceedings of the Combustion Institute, 2007
Methyl radical concentration time-histories were measured during the oxidation and pyrolysis of iso-octane and n-heptane behind reflected shock waves. Initial reflected shock conditions covered temperatures of 1100-1560 K, pressures of 1.6-2.0 atm and initial fuel concentrations of 100-500 ppm. Methyl radicals were detected using cw UV laser absorption near 216 nm; three wavelengths were used to compensate for time-and wavelength-dependent interference absorption. Methyl time-histories were compared to the predictions of several current oxidation models. While some agreement was found between modeling and measurement in the early rise, peak and plateau values of methyl, and in the ignition time, none of the current mechanisms accurately recover all of these features. Sensitivity analysis of the ignition times for both iso-octane and n-heptane showed a strong dependence on the reaction C 3 H 5 + H = C 3 H 4 + H 2 , and a recommended rate was found for this reaction. Sensitivity analysis of the initial rate of CH 3 production during pyrolysis indicated that for both iso-octane and n-heptane, reaction rates for the initial decomposition channels are well isolated, and overall values for these rates were obtained. The present concentration time-history data provide strong constraints on the reaction mechanisms of both iso-octane and n-heptane oxidation, and in conjunction with OH concentration time-histories and ignition delay times, recently measured in our laboratory, should provide a self-consistent set of kinetic targets for the validation and refinement of iso-octane and n-heptane reaction mechanisms.
JOURNAL GEOLOGICAL SOCIETY OF INDIA, 2019
Shock tube facility in the Propulsion and High Enthalpy laboratory, Aerospace Engineering Department, has been extensively used to study the ignition characteristics of fuels by measuring the ignition delay times for various hydrocarbon fuels at high temperatures. Initially a systematic method has been followed to calibrate the shock tube for ignition delay time measurements by measuring the delay times of C2H6 – O2 gas mixture diluted with argon. The results show good agreement with earlier reported works of Ethane ignition. Ignition times of low molecular weight liquefied petroleum gas, a fuel used in many industrial and household applications has been studied in the temperature range of 1250-1880 K and in the pressure range of 6- 11 atm at equivalence ratios (F = 0.5 & 1). The ignition delay was measured in the reflected shock region by recording the ignitioninduced pressure rise and emission from CH radical simultaneously. From the present study it is noted that the ignition delay time for liquefied petroleum gas reduces with increase in temperature and its activation energy lies in the range of 40 kcal/mol.