David F. Davidson

Study of the High-Temperature Autoignition of n-Alkane/O/Ar Mixtures

Ignition time measurements of propane, n-butane, n-heptane, and n-decane have been studied behind reflected shock waves over the temperature range of 1300-1700 K and pressure range of 1-6 atm. The test mixture compositionvaried from approximately 2-20% O 2 , and the equivalence ratio ranged from 0.5 to 2.0. To determine more precisely the fuel mole fraction of the test mixture, a new technique has been employed in which a 3.39-μm HeNe laser and multiple-pass setup is utilized to measure the fuel in situ by absorption. Ignition delay times were measured at the shock tube endwall by a CH emission diagnostic (431 nm) that viewed the shock-heated mixture through a window in the endwall. This enabled the ignition time at the unperturbed endwall conditions to be determined accurately, thereby avoiding problems inherent in measuring ignition times from the shock tube sidewall. A parametric study of the experimental data reveals marked similarity of the ignition delay time characteristics among these four n-alkanes, and a unique correlation is presented in which the stoichiometric ignition time data for all four n-alkanes has been correlated into a single expression with an R 2 value of 0.992: Τ=9.4×10 - 1 2 P - 0 . 5 5 X O 2 - 0 . 6 3 C - 0 . 5 0 exp(46,550/RT) where the ignition time is in seconds, pressure in atmospheres, the activation energy in calories per mole, X O 2 is the mole fraction of oxygen in the test mixture, and C is the number of carbons atoms in the n-alkane. Comparisons to past ignition time studies and detailed kinetic mechanisms further validate the correlations presented here.

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.

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.

Shock tube measurements of JP-10 ignition

Ignition times and OH concentration time histories for JP-10/O 2 /Ar mixtures have been measured behind reflected shock waves. Experiments were performed over the temperature range of 1200–1700 K, pressure range of 1–9 atm, fuel concentrations of 0.2% and 0.4%, and stoichiometries of Φ=0.5, 1.0, and 2.0. Fuel concentrations were measured in the shock tube using laser absorption at 3.39 μm, ignition times were determined using CH emission, and OH concentration histories were inferred from narrow-linewidth cw laser absorption measurements near 306 nm. The laser measurements also revealed evidence for a long-lived JP-10 decomposition product with strong absorption near 306 nm. A kinetic model for JP-10 oxidation was developed using global decomposition reactions proposed by F. Williams in conjunction with the larger alkane mechanism of Lindstedt and Maurice. This modeling gave good agreement with the ignition times at higher pressures, and sensitivity studies using this model indicate the possible important role of C 2 chemistry in JP-10 decomposition.

Quantitative detection of HCO behind shock waves: The thermal decomposition of HCO

Using FM spectroscopy formyl radicals were detected for the first time behind shock waves. HCO radicals have been generated by 308 nm photolysis of mixtures of formaldehyde in argon. The HCO spectrum of the (A2A″ ← 2A′) (0900 ← 0010) transition was measured at room temperature with high resolution and the predissociative linewidths Γ of the individual rotational lines were fitted to Γ = X + ZN′2(N′ + 1)2, where X = 0.22 cm−1 and Z = 1.0 × 10−5 cm−1. Since FM spectroscopy is very sensitive to small line shape variations the spin splitting in the Q-branch could be resolved. Time resolved measurements of HCO profiles at temperatures below 820 K provided the temperature independent rates of reaction (4), H + HCO → H2 + CO, and reaction (5), HCO + HCO → CH2O + CO, k4 = 1.1 × 1014 cm3 mol−1 s−1k5 = 2.7 × 1013 cm3 mol−1 s−1and the low pressure room temperature absorption cross section of the Q(9)P(2) line at 614.872 nm, αc = (1.5 ± 0.4) × 106 cm2 mol−1 (base e). Measurements of the unimolecular decomposition of HCO, reaction (3) HCO + M → H + CO + M, were performed at temperatures from 835 to 1230 K and at total densities from 3.3 × 10−6 to 2.5 × 10−5 mol cm−3. They can be represented by the following Arrhenius expression. k3 = 4.0 × 1013·exp(−65 kJ mol−1/RT) cm3 mol−1 s−1 (Δ log k3 = ±0.23)The corresponding RRKM fit, 4.8 × 1017·(T/K)−1.2·exp(−74.2 kJ mol−1/RT) cm3 mol−1 s−1 (600 < T/K < 2500), supports the lower range of previously reported high temperature rate expressions.

Ultraviolet absorption spectra of shock-heated carbon dioxide and water between 900 and 3050 K

Abstract Spectrally resolved UV absorption cross-sections between 190 and 320 nm were measured in shock-heated CO 2 between 880 and 3050 K and H 2 O between 1230 and 2860 K. Absorption spectra were acquired with 10 μs time resolution using a unique kinetic spectrograph, thereby enabling comparisons with time-dependent chemical kinetic modeling of post-shock thermal decomposition and chemical reactions. Although room temperature CO 2 is transparent (σ −22 cm 2 ) at wavelengths longer than 200 nm, hot CO 2 has significant absorption (σ>10 −20 cm 2 ) extending to wavelengths longer than 300 nm. The temperature dependence of CO 2 absorption strongly suggests sharply increased transition probabilities from excited vibrational levels.

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.

Impact of UV absorption by CO2 and H2O on no lif inhigh-pressure combustion applications

The influence of UV light absorption by hot CO2 and H2O is evaluated for laser-induced fluorescence(LIF) measurements of NO in high-pressure combustors. UV lasers are ubiquitously used to measure LIF from species like NO, OH, HCO, and O2, as well as Raman and Rayleigh scattering in combusting environments. However, attenuation of the laser probe and/or signal by optical absorption from major combustion species is seldom considered. In this paper, we show that neglecting UV attenuation by major product species like CO2 may lead to large errors in combustion measurements. Absorption cross sections between 190 and 320 nm are measured in shock-heated CO2 and H2O at temperatures ranging from 900 to 3050 K. The absorption cross section of CO2 has strong temperature dependence and increases by 4 orders of magnitude at 193 nm between 300 and 2000 K. The measured temperature-dependen tabsorption spectra for CO2 and H2O are fit to an empirical function to provide a tool for facile assessment of potential errors and quantitative corrections for UV combustion diagnostics. LIF measurements of NO in a high-pressure burner and an internal combustion engine are adjusted for CO2 and H2O absorption to demonstrate the importance of these corrections.

A cw laser absorption diagnostic for methyl radicals

Abstract Absorption of narrow-line laser radiation by methyl radicals produced at high temperatures was studied in the Herzberg β 1 band near 216nm. cw radiation for these measurements was generated in a ring dye laser system using intracavity BBO frequency doubling. Methyl radicals were produced by the shock wave heating of five selected source compounds: azomethane, methyl iodide, tetramethyl tin, tetramethyl silane and ethane. The variation with wavelength of the CH 3 absorption coefficient between 210 and 225 nm was measured at 1625 K and 1.25 atm using ethane/Ar and methyl iodide/Ar gas mixtures. The variation of the absorption coefficient with temperature from 1350 to 2450 K was measured at 216.615 nm using four source compounds. Using this wavelength for CH 3 measurements resulted in a typical detectivity limit of 2ppm at 1650 K, 1 atm, L = 10 cm, with a SNR of unity and a detection bandwidth of 500 kHz.

Experimental Study of the Rate of OH + HO2 → H2O + O2 at High Temperatures Using the Reverse Reaction

The rate constant of the reaction OH + HO(2) --> H(2)O + O(2) (1) can be inferred at high temperatures from measurements of the rate of its reverse reaction H(2)O + O(2) --> OH + HO(2) (-1). In this work, we used laser absorption of both H(2)O and OH to study the reverse reaction in shock-heated H(2)O/O(2)/Ar mixtures over the temperature range 1600-2200 K. Initial H(2)O concentrations were determined using tunable diode laser absorption near 2.5 microm, and OH concentration time-histories were measured using UV ring dye laser absorption near 306.7 nm. Detailed kinetic analysis of the OH time-history profiles yielded a value for the rate constant k(1) of (3.3 +/- 0.9) x 10(13) [cm(3) mol(-1) s(-1)] between 1600 and 2200 K. The results of this study agree well with those reported by Srinivasan et al. (Srinivasan, N.K.; Su, M.-C.; Sutherland, J.W.; Michael, J.V.; Ruscic, B. J. Phys. Chem. A 2006, 110, 6602-6607) in the temperature regime between 1200 and 1700 K. The combination of the two studies suggests only a weak temperature dependence of k(1) above 1200 K. Data from the current study and that of Keyser (Keyser, L.F. J. Phys. Chem. 1988, 92, 1193-1200) at lower temperatures can be described by the k(1) expression proposed by Baulch et al. (Baulch, D.L.; Cobos, C.J.; Cox, R.A.; Esser, C.; Frank, P.; Just, Th.; Kerr, J.A.; Pilling, M.J.; Troe, J.; Walker, R.W.; Warnatz, J. J. Phys. Chem. Ref. Data 1992, 21, 411), k(1) = 2.89 x 10(13) exp(252/T) [cm(3) mol(-1) s(-1)]. However, it should be noted that some previous studies suggest a k(1) minimum around 1250 K (Hippler, H.; Neunaber, H.; Troe, J. J. Chem. Phys. 1995, 103, 3510-3516) or 1000 K (Kappel, C.; Luther, K.; Troe, J. Phys. Chem. Chem. Phys. 2002, 4, 4392-4398).