Eric L. Petersen

Kinetics modeling of shock-induced ignition in low-dilution CH4/O2 mixtures at high pressures and intermediate temperatures

Abstract 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 ≥30%), 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 + O 2 = O + CH 3 O and HO 2 + CH 3 = OH + 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 + CH 3 = CH 3 O + CH 3 O and H 2 O 2 + M = OH + OH + 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.

Fuel Flexibility Influences on Premixed Combustor Blowout, Flashback, Autoignition, and Stability

This paper addresses the impact of fuel composition on the operability of lean premixed gas turbine combustors. This is an issue of current importance due to variability in the composition of natural gas fuel supplies and interest in the use of syngas fuels. Of particular concern is the effect of fuel composition on combustor blowout, flashback, dynamic stability, and autoignition. This paper reviews available results and current understanding of the effects of fuel composition on the operability of lean premixed combustors. It summarizes the underlying processes that must be considered when evaluating how a given combustor’s operability will be affected as fuel composition is varied.Copyright

A facility for gas- and condensed-phase measurements behind shock waves

A shock-tube facility consisting of two, single-pulse shock tubes for the study of fundamental processes related to gas-phase chemical kinetics and the formation and reaction of solid and liquid aerosols at elevated temperatures is described. Recent upgrades and additions include a new high-vacuum system, a new gas-handling system, a new control system and electronics, an optimized velocity-detection scheme, a computer-based data acquisition system, several optical diagnostics, and new techniques and procedures for handling experiments involving gas/powder mixtures. Test times on the order of 3 ms are possible with reflected-shock pressures up to 100 atm and temperatures greater than 4000 K. Applications for the shock-tube facility include the study of ignition delay times of fuel/oxidizer mixtures, the measurement of chemical kinetic reaction rates, the study of fundamental particle formation from the gas phase, and solid-particle vaporization, among others. The diagnostic techniques include standard differential laser absorption, FM laser absorption spectroscopy, laser extinction for particle volume fraction and size, temporally and spectrally resolved emission from gas-phase species, and a scanning mobility particle sizer for particle size distributions. Details on the set-up and operation of the shock tube and diagnostics are given, the results of a detailed uncertainty analysis on the accuracy of the test temperature inferred from the incident-shock velocity are provided, and some recent results are presented.

REDUCED KINETICS MECHANISMS FOR RAM ACCELERATOR COMBUSTION

Two skeletal kinetics mechanisms for reactive CH 4/O2 and H2/O2 ram accelerator e owe elds are presented. Both models were derived from a 190-reaction, 38-species kinetics mechanism (RAMEC or RAM accelerator MEChanism) that successfully reproduces the high-pressure (>50 atm), low-dilution (<70%), fuel-rich chemistry of ram accelerator mixtures. The reduction procedure for the CH 4/O2 mechanism utilized a detailed-reduction technique with ignition delay time and heat release as the selection criteria. The methane-based mechanism (REDRAM or REDuced RAM accelerator mechanism ) contains 34 reactions and 22 species and predicts ignition times to better than 5% and postcombustion temperatures to within 10 K of the full mechanism for a representative range of ram accelerator mixtures and conditions. This CH 4/O2 mechanism is an improvement over existing reduced methane-oxidation mechanisms that arebased on lower-pressure, higher-temperature chemistry. An 18-step, 9-species mechanism is presented for hydrogen-based ram accelerator combustion that is based on the H2/O2 submechanism of the RAMEC/Gas Research Institute GRI-Mech 1.2 methane-oxidation mechanism. The H2/O2 kinetics model includes HO 2 and H2O2 chemistry near the second and third explosion limits, necessary for ignition at ram accelerator pressures but lacking in certain e nite rate chemistry models currently in use.

Ignition Delay Times of Ram Accelerator CH/O/Diluent Mixtures

An experimental study was performed to determine ignition delay times for CH4/O2/diluent mixtures and conditions relevant to forebody combustion on ram accelerator projectiles. All measurements were performed in the reflected-shock region of a high-pressure shock tube. Temperatures from 1040 to 1600 K and pressures between 35 and 260 atm were studied, and the CH4/O2/diluent mixtures had an equivalence ratio of 0.4, 3.0, or 6.0 with either N2, Ar, or He as the bath gas. Reaction progress was monitored primarily via piezoelectric pressure transducer and visible emission. For each mixture and condition, the ignition developed as a strong ignition front beginning at the endwall with little or no preignition deflagration. Ignition delay time (rigll) correlations were generated for each mixture and the entire data set; the latter correlation indicates that ignition delay is dependent only on the fuel and oxidizer concentrations and, therefore, not on the diluent species or concentration. At temperatures below approximately 1300 K for the fuel-rich mixtures, the Arrhenius temperature dependence of rign changes from an average activation energy of 32.7 kcal/mol, at higher temperatures, to approximately 19.0 kcal/mol, at lower temperatures. The transition occurs at higher temperatures as the pressure is increased, and is indicative of a shift in chain-branching kinetics between the highand intermediate-temperature regimes.

Laminar Flame Speed and Ignition Delay Time Data for the Kinetic Modeling of Hydrogen and Syngas Fuel Blends

Laminar flame speeds and ignition delay times have been measured for hydrogen and various compositions of H2/CO (syngas) at elevated pressures and elevated temperatures. Two constant-volume cylindrical vessels were used to visualize the spherical growth of the flame through the use of a schlieren optical setup to measure the laminar flame speed of the mixture. Hydrogen experiments were performed at initial pressures up to 10atm and initial temperatures up to 443K. A syngas composition of 50/50 by volume was chosen to demonstrate the effect of carbon monoxide on H2-O2 chemical kinetics at standard temperature and pressures up to 10atm. All atmospheric mixtures were diluted with standard air, while all elevated-pressure experiments were diluted with a He:O2 ratio of 7:1 to minimize instabilities. The laminar flame speed measurements of hydrogen and syngas are compared to available literature data over a wide range of equivalence ratios, where good agreement can be seen with several data sets. Additionally, an improved chemical kinetics model is shown for all conditions within the current study. The model and the data presented herein agree well, which demonstrates the continual, improved accuracy of the chemical kinetics model. A high-pressure shock tube was used to measure ignition delay times for several baseline compositions of syngas at three pressures across a wide range of temperatures. The compositions of syngas (H2/CO) by volume presented in this study included 80/20, 50/50, 40/60, 20/80, and 10/90, all of which are compared to previously published ignition delay times from a hydrogen-oxygen mixture to demonstrate the effect of carbon monoxide addition. Generally, an increase in carbon monoxide increases the ignition delay time, but there does seem to be a pressure dependency. At low temperatures and pressures higher than about 12atm, the ignition delay times appear to be indistinguishable with an increase in carbon monoxide. However, at high temperatures the relative composition of H2 and CO has a strong influence on ignition delay times. Model agreement is good across the range of the study, particularly at the elevated pressures. [DOI: 10.1115/1.4007737]

Ignition of Lean Methane-Based Fuel Blends at Gas Turbine Pressures

Shock-tube experiments and chemical kinetics modeling were performed to further understand the ignition and oxidation kinetics of lean methane-based fuel blends at gas turbine pressures. Such data are required because the likelihood of gas turbine engines operating on CH 4 -based fuel blends with significant (>10%) amounts of hydrogen, ethane, and other hydrocarbons is very high. Ignition delay times were obtained behind reflected shock waves for fuel mixtures consisting of CH 4 , CH 4 /H 2 , CH 4 /C 2 H 6 , and CH 4 /C 3 H 8 in ratios ranging from 90/10% to 60/40%. Lean fuel/air equivalence ratios (Φ=0.5) were utilized, and the test pressures ranged from 0.54 to 30.0 atm. The test temperatures were from 1090 K to 2001 K. Significant reductions in ignition delay time were seen with the fuel blends relative to the CH 4 -only mixtures at all conditions. However, the temperature dependence (i.e., activation energy) of the ignition times was little affected by the additives for the range of mixtures and temperatures of this study. In general, the activation energy of ignition for all mixtures except the CH 4 /C 3 H 8 one was smaller at temperatures below approximately 1300 K (∼27 kcal/mol) than at temperatures above this value (∼41 kcal/mol). A methane/hydrocarbon-oxidation chemical kinetics mechanism developed in a recent study was able to reproduce the high-pressure, fuel-lean data for the fuel/air mixtures. The results herein extend the ignition delay time database for lean methane blends to higher pressures (30 atm) and lower temperatures (1100 K) than considered previously and represent a major step toward understanding the oxidation chemistry of such mixtures at gas turbine pressures. Extrapolation of the results to gas turbine premixer conditions at temperatures less than 800 K should be avoided however because the temperature dependence of the ignition time may change dramatically from that obtained herein.

High-pressure methane oxidation behind reflected shock waves

Experiments on CH 4 oxidation behind reflected shock waves were conducted at elevated pressures, and the results compared to a detailed kinetics model. Mixtures of CH 4 and O 2 dilute in either argon or nitrogen were studied over a wide range of stoichiometry (=0.5–4.0), bath gas dilution (90.0–99.5%), pressure (9–480 atm), and temperature (1410–2040 K), corresponding to total concentrations from 5.6×10 −5 to 3.6×10 −3 mol/cm 3 . Reaction progress was monitored using narrow-line laser absorption of OH at 306 nm, infrared emission of CH 4 near 3.4 μm, and pressure measurements. The measured species time-histories and pressure traces were assembled into an extensive database of characteristic reaction times, peak OH mole fractions, and ignition delay times that can be used for comparisons with detailed kinetics mechanisms. The chemical kinetics model utilized in the present comparisons is the latest GRI mechanism, GRI-Mech 1.2. As a whole, agreement between the model predictions and the experimental measurements is good, particularly for ignition delay times. However, based on the results of certain CH 4 profiles, improvements in the model for high-pressure, fuel-rich conditions are needed. Sensitivity and species contribution analyses were used to identify the most important reactions at pressures up to 500 atm, some of which require more accurate rate coefficients. As pressure increases, the CH 3 removal pathways are altered, the CH 3 formation pathways remain the same, and reactions involving HO 2 become important.

Comparison of characteristic time diagnostics for ignition and oxidation of fuel/oxidizer mixtures behind reflected shock waves

ABSTRACT Various methods for determining characteristic times of shock-tube ignition and oxidation are compared. Onset and peak times were obtained from time histories for four different species (CH, CH*, OH, OH*) as predicted by a modern detailed kinetics mechanism. Appropriate submechanisms for CH* and OH* formation and quenching were added to the existing mechanism to differentiate the excited-state species from the ground-state molecules. The modeling focused on mixtures of acetylene or ethane with oxygen highly diluted in argon at high temperatures (1200–2050 K) and nearly atmospheric pressures. Using a detailed mechanism known to accurately simulate the shock-tube chemistry, emphasis was placed on cohesion of characteristic times among the species and the extent to which one may be used to predict another. Generally, ignition onset times were found to be more consistent than peak times, with OH peaking at times least typical of the group. Onset time versus inverse temperature curves based on any one species agree with those of the other three species to within 25% for the hydrocarbon mixtures and given mechanism utilized herein. Results suggest that ignition onset time should be used for greater consistency, and kinetics modeling of excited-state species such as OH* and CH* should be included if comparing to data obtained using chemiluminescence diagnostics.

Ignition and Oxidation of 50/50 Butane Isomer Blends

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 iso-butane, and the combustion characteristics of either isomer are not well known. Of particular interest to this work are mixtures of n-butane and iso-butane. 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 iso-butane. Ignition within the shock tube was determined from the sharp pressure rise measured at the endwall which is characteristic of such exothermic reactions. Both experimental and kinetics modeling results are presented for a wide range of stoichiometry (φ = 0.3–2.0), temperature (1056–1598 K), and pressure (1–21 atm). The results of this work serve as 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% iso-butane. The 50/50 mixture of n-butane and iso-butane was shown to be more readily ignitable than 100% iso-butane but reacts slower than 100% n-butane only for the richer mixtures. There was little difference in ignition time between the lean mixtures.Copyright