OH Concentration Time-histories Research Articles

Shock tube measurements of OH concentration time-histories in benzene, toluene, ethylbenzene and xylene oxidation

Microsecond-resolved measurements of OH concentrations time-histories were performed during the oxidation of four common monocyclic aromatic hydrocarbons, namely benzene, toluene, ethylbenzene and o-xylene, behind reflected shock waves. Ar-diluted mixtures of 200 ppm aromatics in stoichiometric O2 were shock-heated to temperatures of 1513–1877 K and pressures of about 1.2 atm, where quantitative and time-resolved concentration time-histories of OH were measured by narrow-linewidth laser absorption using a novel solid-state laser near 308.6 nm, at the peak of the Q1(5) transition of the OH A-X (0,0) rovibronic band. The use of the Q1(5) transition improved the detection sensitivity of OH by a factor of 2.2, as compared to the R1(5) transition utilized in most previous OH absorption measurements. The resulting OH time-history data accurately resolved the fine details in multiple stages of oxidation of these aromatics, e.g. the initial decomposition of fuel, the subsequent H-abstraction and radical scavenging, and the final radical surge and onset of ignition, thereby providing multiple valuable kinetics targets for validation and improvement of modern reaction models. Three comprehensive reaction models of aromatics combustion from recent studies were evaluated based on the current experimental results. Key reactions governing the OH time-histories were identified, and further studies on selected reaction rate constants were suggested to improve these models. To our knowledge, the current work presents the first quantitative measurements of OH time-histories in the oxidation of benzene, ethylbenzene and o-xylene, and hence provides a critical and unique contribution to the database needed for development and optimization of modern reaction kinetic models regarding aromatics combustion.

Rate Constant Measurements for the Overall Reaction of OH + 1-Butanol → Products from 900 to 1200 K

The rate constant for the overall reaction OH + 1-butanol → products was determined in the temperature range 900 to 1200 K from measurements of OH concentration time histories in reflected shock wave experiments of tert-butyl hydroperoxide (TBHP) as a fast source of OH radicals with 1-butanol in excess. Narrow-linewidth laser absorption was employed for the quantitative OH concentration measurement. A detailed kinetic mechanism was constructed that includes updated rate constants for 1-butanol and TBHP kinetics that influence the near-first-order OH concentration decay under the present experimental conditions, and this mechanism was used to facilitate the rate constant determination. The current work improves upon previous experimental studies of the title rate constant by utilizing a rigorously generated kinetic model to describe secondary reactions. Additionally, the current work extends the temperature range of experimental data in the literature for the title reaction under combustion-relevant conditions, presenting the first measurements from 900 to 1000 K. Over the entire temperature range studied, the overall rate constant can be expressed in Arrhenius form as 3.24 × 10(-10) exp(-2505/T [K]) cm(3) molecule(-1) s(-1). The influence of secondary reactions on the overall OH decay rate is discussed, and a detailed uncertainty analysis is performed yielding an overall uncertainty in the measured rate constant of ±20% at 1197 K and ±23% at 925 K. The results are compared with previous experimental and theoretical studies on the rate constant for the title reaction and reasonable agreement is found when the earlier experimental data were reinterpreted.

OH and C2H4 species time-histories during hexadecane and diesel ignition behind reflected shock waves

The first simultaneous multi-species laser absorption time-history measurements for OH and C2H4 were acquired during the oxidation of n-hexadecane and commercial diesel fuel (DF-2). The experiments were performed behind reflected shock waves in a new second-generation aerosol shock tube over a temperature range of 1120K to 1373K and a pressure range of 4–7atm. Initial fuel concentrations varied between 150 and 1800ppm with equivalence ratios between 0.4 and 2, and were determined using 3.39μm He–Ne laser absorption. OH concentration time-histories were measured using absorption of frequency-doubled ring-dye laser radiation near 306.7nm. Ethylene time-histories were measured using absorption of CO2 gas-laser radiation near 10.5μm. Comparisons are given of these species concentration time-histories with two current large n-alkane mechanisms: the LLNL-C-16 mechanism of Westbrook et al. [13] and the JetSurF C-12 mechanism of Sirjean et al. [14]. Fair agreement between model and experiment is seen in the peak ethylene yields for both fuels; however, modeled early time-histories of OH, an important chain-branching species, differ significantly from current measurements.

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).

OH time-histories during oxidation of n-heptane and methylcyclohexane at high pressures and temperatures

OH concentration time-histories during n-heptane and methylcyclohexane (MCH) oxidation were measured behind reflected shock waves in a heated, high-pressure shock tube. Experimental conditions covered temperatures of 1121 to 1332 K, pressures near 15 atm, and initial fuel concentrations of 750 and 1000 ppm (by volume), and an equivalence ratio of 0.5 with O 2 as the oxidizer and argon as the bath gas. OH concentrations were measured using narrow-linewidth ring-dye laser absorption near the R-branchhead of the OH A – X ( 0 , 0 ) system at 306.47 nm. These current measurements together with our recent results for n-dodecane oxidation [S.S. Vasu, D.F. Davidson, Z. Hong, V. Vasudevan, R.K. Hanson, Proc. Combust. Inst. 32 (2009), doi:10.1016/j.proci.2008.05.006] provide critically needed validation targets for jet fuel surrogate kinetic mechanisms and further improve understanding of high-pressure, high-temperature oxidation chemistry. Detailed comparisons of these OH time-histories with the predictions of various kinetic mechanisms were made. Sensitivity and pathway analyses for these reference fuel components were performed, leading to reaction rate recommendations with improved model performance. Current results are the first quantitative measurements of OH time-histories during high-pressure oxidation of these fuels, and hence are a critical step toward development of accurate reaction models for jet fuel surrogates.

Shock tube measurements of ignition delay times and OH time-histories in dimethyl ether oxidation

Ignition delay times and OH concentration time-histories were measured in DME/O2/Ar mixtures behind reflected shock waves. Initial reflected shock conditions covered temperatures (T5) from 1175 to 1900K, pressures (P5) from 1.6 to 6.6bar, and equivalence ratios (ϕ) from 0.5 to 3.0. Ignition delay times were measured by collecting OH∗ emission near 307nm, while OH time-histories were measured using laser absorption of the R1(5) line of the A–X(0,0) transition at 306.7nm. The ignition delay times extended the available experimental database of DME to a greater range of equivalence ratios and pressures. Measured ignition delay times were compared to simulations based on DME oxidation mechanisms by Fischer et al. [7] and Zhao et al. [9]. Both mechanisms predict the magnitude of ignition delay times well. OH time-histories were also compared to simulations based on both mechanisms. Despite predicting ignition delay times well, neither mechanism agrees with the measured OH time-histories. OH Sensitivity analysis was applied and the reactions DME ↔ CH3O+CH3 and H+O2 ↔ OH+O were found to be most important. Previous measurements of DME ↔ CH3O+CH3 are not available above 1220K, so the rate was directly measured in this work using the OH diagnostic. The rate expression k[1/s]= 1.61×1079T−18.4 exp(−58600/T), valid at pressures near 1.5bar, was inferred based on previous pyrolysis measurements and the current study. This rate accurately describes a broad range of experimental work at temperatures from 680 to 1750K, but is most accurate near the temperature range of the study, 1350–1750K. When this rate is used in both the Fischer et al. and Zhao et al. mechanisms, agreement between measured OH and the model predictions is significantly improved at all temperatures.

N-Dodecane oxidation at high-pressures: Measurements of ignition delay times and OH concentration time-histories

Ignition delay times and OH concentration time-histories were measured during n-dodecane oxidation behind reflected shocks waves using a heated, high-pressure shock tube. Measurements were made over temperatures of 727–1422K, pressures of 15–34atm, and equivalence ratios of 0.5 and 1.0. Ignition delay times were measured using side-wall pressure and OH∗ emission diagnostics, and OH concentration time-histories were measured using narrow-linewidth ring-dye laser absorption near the R-branchhead of the OH A–X (0,0) system at 306.47nm. Shock tube measurements were compared to model predictions of four current n-dodecane oxidation detailed mechanisms, and the differences, particularly in the low-temperature negative-temperature-coefficient (NTC) region where the influence of non-ideal facility effects can be significant, are discussed. To our knowledge, the current measurements provide the first gas-phase shock tube ignition delay times (at pressures above 13 atm) and quantitative OH concentration time-histories for n-dodecane oxidation under practical engine conditions, and hence provide benchmark validation targets for refinement of jet fuel detailed kinetic modeling, since n-dodecane is widely used as the principal representative for n-alkanes in jet fuel surrogates.

Methyl concentration time-histories during iso-octane and n-heptane oxidation and pyrolysis

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 3H 5 + H = C 3H 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.

High-Temperature Measurements of the Reactions of OH with Toluene and Acetone

The reaction of hydroxyl [OH] radicals with toluene [C6H5CH3] was studied at temperatures between 911 and 1389 K behind reflected shock waves at pressures of approximately 2.25 atm. OH radicals were generated by rapid thermal decomposition of shock-heated tert-butyl hydroperoxide [(CH3)3-CO-OH], and monitored by narrow-line width ring dye laser absorption of the well-characterized R1(5) line of the OH A-X (0,0) band near 306.7 nm. OH time histories were modeled by using a comprehensive toluene oxidation mechanism. Rate constants for the reaction of C6H5CH3 with OH were extracted by matching modeled and measured OH concentration time histories in the reflected shock region. Detailed error analyses yielded an uncertainty estimate of +/-30% at 1115 K for the rate coefficient of this reaction. The current high-temperature data were fit with the lower temperature measurements of Tully et al. [J. Phys. Chem. 1981, 85, 2262-2269] to the following two-parameter form, applicable over 570-1389 K: k3 = (1.62 x 10(13)) exp(-1394/T [K]) [cm3 mol(-1) s(-1)]. The reaction between OH radicals and acetone [CH3COCH3] was one of the secondary reactions encountered in the toluene + OH experiments. Direct high-temperature measurements of this reaction were carried out at temperatures ranging from 982 to 1300 K in reflected shock wave experiments at an average total pressure of 1.65 atm. Uncertainty limits were estimated to be +/-25% at 1159 K. A two-parameter fit of the current data yields the following rate expression: k6 = (2.95 x 10(13)) exp(-2297/T [K]) [cm3 mol(-1) s(-1)].

Shock tube measurements of toluene ignition times and OH concentration time histories

Ignition delay times and OH radical concentration profiles were measured in toluene/O 2/Ar mixtures behind reflected shock waves. Initial reflected shock conditions spanned 1400–2000 K and 1.5–5.0 atm, with equivalence ratios of 0.5–1.875 and toluene concentrations of 0.025–0.5%. OH time histories were monitored using narrow-linewidth ring dye laser absorption of the well-characterized R 1 (5) line of the OH A–X (0, 0) band at 306.7 nm. Ignition time data were extracted from the OH traces and were found to compare very well with measurements using sidewall pressure. These new data are in agreement with the recent measurements of Burcat et al. [NASA TM-87312, 1986], but not with the measurements of Pitz et al. [Second Joint Meeting, US Sections of the Combustion Institute, Paper 253, 2001] or Burcat et al. [Combust. Flame 36 (1979)]. The results of this study were compared to three detailed kinetic models: Pitz et al. [Second Joint Meeting, US Sections of the Combustion Institute, Paper 253, 2001], Dagaut et al.[Phys. Chem. Chem. Phys. 4 (2002)], and Lindstedt and Maurice [Combust. Sci. Technol. 120 (1996)]. The ability of the mechanisms to predict the measured ignition time data and OH concentration profiles was analyzed. Suggestions to improve model performance have been made, and key reactions that need to be studied further have been identified. This work has yielded the first quantitative measurements of OH time histories during toluene oxidation, and hence provides a critical data set useful for evaluating and refining comprehensive mechanisms on toluene oxidation.

Shock tube measurements of branched alkane ignition times and OH concentration time histories

AbstractIgnition times and hydroxyl (OH) radical concentration time histories were measured behind reflected shock waves during the oxidation of three branched alkanes: iso‐butane (2‐methylpropane), iso‐pentane (2‐methylbutane), and iso‐octane (2,2,4‐trimethylpentane). Initial reflected shock conditions ranged from 1177 to 2009 K and 1.10 to 12.58 atm with dilute fuel/O2/Ar mixtures varying in fuel concentration from 100 ppm to 1.25% and in equivalence ratio from 0.25 to 2. Ignition times were measured using endwall CH emission and OH concentrations were measured using narrow‐linewidth ring‐dye laser absorption of the R1(5) line of the OH A‐X (0,0) band at 306.7 nm. The ignition times and OH concentration time histories were compared to modeled predictions of seven branched alkane oxidation mechanisms currently available in the literature and the implications of these comparisons are discussed. These data provide a unique database for the validation of detailed hydrocarbon oxidation mechanisms of propulsion related fuels. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 36: 67–78 2004

Shock tube measurements of iso-octane ignition times and OH concentration time histories

Ignition times and OH radical concentration time histories were measured behind reflected shock waves in iso-octane/O2/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 R1(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 mechnisms, and the implications of these comparisons are discussed. To our knowledge, these data provide the first extensive measurements of low-fuel-concentration 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.

Shock tube measurements of JP-10 ignition

Ignition times and OH concentration time histories for JP-10/O2/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 C2 chemistry in JP-10 decomposition.

High temperature determination of the rate coefficient for the reaction H2O + CN → HCN + OH

AbstractThe rate coefficient, k, of the reaction has been determined in the temperature range 2460–2840 K using a shock tube technique. C2N2H2OAr mixtures were heated behind incident shock waves and the CN and OH concentration time histories were monitored simultaneously using broad‐band absorption near 388 nm (CN) and narrow‐line laser absorption at 306.67 nm (OH). The rate coefficient expression providing the best fit to the data was with uncertainty limits of about ±45% in the temperature range 2460–2840 K. The rate coefficient of the reverse reaction was calculated using detailed balancing, and its extrapolation to lower temperatures was compared with previously published results.