Reflected Shock Tube Research Articles

Collisional broadening and pressure shift coefficients for the potassium D1 and D2 transitions in oxygen and carbon dioxide at high temperatures

Collisional broadening and pressure shift parameters for the potassium resonance doublet, near 770 nm, are reported for collisions with molecular oxygen and carbon dioxide. Experiments were conducted in a reflected shock tube from 1200–2200 K and used potassium chloride (KCl) salt as the atomic potassium source. The measured absorption lineshapes were fit with Voigt profiles to infer the collisional broadening and pressure shifts. Power-law correlations were then developed to describe the pressure-normalized results as functions of temperature. Generally, the collisional broadening coefficients in oxygen agree well with theoretical predictions and are similar to those in nitrogen. Conversely, the pressure shift coefficients in oxygen differ from those in nitrogen by up to 15%. Broadening coefficients in carbon dioxide disagree with theoretical predictions by 20% or more over the range of temperatures explored in this work. These results expand the existing database of potassium lineshape coefficients, and they are expected to be useful for further development of potassium sensing diagnostics in terrestrial, Martian, and Venusian atmospheric flight studies, and in combustion systems. Other anticipated applications include interpretation of astrophysical spectroscopic observations.

Two-Temperature Modeling of Nonequilibrium Relaxation and Dissociation in Shock-Heated Oxygen

Two-temperature models for coupled vibrational relaxation and dissociation in shock-heated oxygen are assessed using low-uncertainty measured data from reflected shock tube experiments. A computationally efficient multistep technique is developed to model the unsteady dynamics of shock reflection in a relaxing and dissociating gas. The developed technique is then benchmarked through comparison with unsteady computational fluid dynamic simulations. Results from the benchmarking effort demonstrate that the adopted multistep modeling procedure accurately captures the dominant gas dynamic effects influencing the state of the test gas at the measurement location. A parametric study is then performed to assess several combinations of possible two-temperature modeling approaches for nonequilibrium oxygen dissociation. The current assessment demonstrates that the widely adopted Park model is inconsistent with the measured data, while the recently developed modified Marrone and Treanor (MMT) model demonstrates promising agreement with the data. The results of the present study clearly indicate that the MMT model is more appropriate for two-temperature modeling of nonequilibrium oxygen dissociation than the legacy Park model. Patterns in the parametric comparison also suggest that the approximate treatment of non-Boltzmann vibrational state distributions within the MMT model may require improvement.

Combustion properties of n-heptane/hydrogen mixtures

The possibility to operate current diesel engines in dual-fuel mode with the addition of hydrogen can be limited by the variation in the combustion properties of the fuel mixture. In the present work, n-heptane was selected as a representative fuel to test the effects of hydrogen addition on the laminar flame speeds and ignition delay times. The spherical bomb technique was used to derive the laminar flame speeds of (n-heptane + hydrogen)/air mixtures (0%, 25%, and 50% hydrogen in the fuel) for an initial temperature of 294 K, pressure of 1 bar, and for equivalence ratios between 0.8 and 1.35. The results showed that average increases of 3% and 10% in the flame speeds were obtained with 25% and 50% hydrogen-enrichment, respectively, while a slight decrease of the Markstein length was obtained. Similar laminar flame speed results were predicted numerically with two kinetic models available in the literature with remarkable accuracy, especially for the Cai and Pitsch model [Cai L, Pitsch H. Combust Flame 2015; 162:1623–37]. The kinetic model was subsequently used to perform additional sensitivity and reaction pathway analyses that showed how the chemistry of n-heptane is not substantially influenced by the presence of hydrogen; while the increase in the flame speed is mainly due to the higher concentrations of radical intermediates. The ignition delay times were measured using the reflected shock tube technique for equivalence ratios equal to 0.832, 1.000, and 1.248, initial nominal pressure of 20 bar, temperatures between 730 K and 1200 K, and for different percentages of hydrogen in the fuel (20%, 50%, and 75%). The Cai and Pitsch model once again did a good job of reproducing the experimental data, indicating how at high temperatures the addition of hydrogen does not significantly affect the ignition delay; and in the NTC region (810 K–920 K) the mixtures composed of (50% n-heptane + 50% hydrogen) and (25% n-heptane + 75% hydrogen) are considerably slower than the reference n-heptane case. This is linked to the concentration of the alkane component and the related low temperature chemistry.

Impact of non-ideal behavior on ignition delay and chemical kinetics in high-pressure shock tube reactors

Here, we study real gas effects on high-pressure combustion by comparing simulated and experimentally-measured shock tube ignition delay measurements for n-dodecane/O2/N2 mixtures. Experiments and simulations occur at conditions relevant to diesel engines: 40–80 atm, 774–1163 K, equivalence ratios of 1.0 and 2.0, and O2 concentrations of 13–21%. At these conditions the fuel, oxidizer and intermediate species may exist in a supercritical state during combustion, requiring a real gas equation of state to incorporate non-ideal effects on thermodynamics, chemical kinetics, and the resulting ignition characteristics. A constant-volume, adiabatic reactor model is developed to simulate the reflected shock tube experiments, and simulations compare results for real and ideal gas equations of state, with different expressions for reacting species’ activity concentrations. This paper focuses particularly on the cubic Redlich–Kwong equation of state and thermodynamically consistent chemical kinetic rate calculations based on it. Results demonstrate that the equation of state can have considerable influence on ignition delay times with increasing pressure, particularly in the negative temperature coefficient region. Additionally, the results establish important practices for incorporating real gas effects, namely that (i) the compressibilities of key species (i.e. those participating in rate-limiting reactions) are the appropriate way to screen for real gas effects, rather than the average mixture compressibility; and (ii) incorporating a real gas equation of state without also incorporating thermodynamically consistent chemical kinetics significantly under-predicts the magnitude of real gas effects.

Reflected shock tube experiments on aeroacoustic signature of hot jets

We used a reflected shock tube to investigate the acoustic signature of a hot jet at the far-field. Experiments were performed at Mach = 1.4 and a total temperature of T t = 950 Kelvin. Far-field acoustic signatures of the hot jet at six polar angles θ = 15˚ to θ = 90˚ from the jet axis) were measured and imaged by the means of continuous wavelet transform in scalograms. The results were compared with experiments from a steady test facility at similar test conditions. Further, the primary characteristics of noise events in jet far-field were compared with analytical models considering wavepacket as the main source of jet noise. The results indicate that higher frequency events at θ = 90˚ occur with a small time shift relative to the low azimuthal mode base event. This phenomenon might be the main reason why the acoustic signal at the side angles behaves more noisily than at low polar angles.

Development of a skeletal mechanism for biodiesel blend surrogates with varying fatty acid methyl esters proportion

A skeletal four-component biodiesel combustion mechanism comprising methyl decenoate, methyl-5-decenoate, n-decane and methyl linoleate is proposed in the present study. The motivation of this work is to develop a mechanism for accurate modeling on the effect of varying fatty acid methyl ester proportion on biodiesel ignition and combustion. The skeletal mechanism was generated by coupling an existed methyl decenoate, methyl-5-decenoate, n-decane skeletal mechanism with a new developed skeletal methyl linoleate mechanism, which was reduced from a penta-component detailed mechanism consisting of 5025 species and 20,163 reactions. The employed reduction strategies are directed relation graph error propagation and sensitivity analysis (DRGEPSA), isomer lumping, computational singular perturbation and reaction rate adjustment. The skeletal methyl linoleate mechanism consists of 92 species and 249 reactions, and the coupled biodiesel blend surrogates mechanism consists of 106 species and 263 reactions. In order to validate the feasibility of the new developed skeletal mechanism, zero-dimension ignition delay testing, reflected shock tube experiment validation and 3-dimensional engine testing about cylinder pressures, heat release rates and NOx emission trends were conducted at different conditions. And the new developed skeletal mechanism was compared with a biodiesel surrogates skeletal mechanism without double-bond species in the 3-dimensional engine testing. The results indicate that the new developed skeletal mechanism is suitable to predict ignition behavior, combustion characteristics, NOx emission trends of biodiesel with varying fatty acid methyl ester, and it is more accurate than a biodiesel surrogates skeletal mechanism without double-bond species in the 3-dimensional engine testing.

Direct Measurements of Rate Constants for the Reactions of CH3 Radicals with C2H6, C2H4, and C2H2 at High Temperatures

The shock tube technique has been used to study the reactions CH3 + C2H6 → C2H4 + CH4 + H (1), CH3 + C2H4 → Products + H (2), and CH3 + C2H2 → Products + H (3). Biacetyl, (CH3CO)2, was used as a clean high temperature thermal source for CH3-radicals for all the three reactions studied in this work. For reaction 1, the experiments span a T-range of 1153 K ≤ T ≤ 1297 K, at P ~ 0.4 bar. The experiments on reaction 2 cover a T-range of 1176 K ≤ T ≤ 1366 K, at P ~ 1.0 bar, and those on reaction 3 a T-range of 1127 K ≤ T ≤ 1346 K, at P ~ 1.0 bar. Reflected shock tube experiments performed on reactions 1-3, monitored the formation of H-atoms with H-atom Atomic Resonance Absorption Spectrometric (ARAS). Fits to the H-atom temporal profiles using an assembled kinetics model were used to make determinations for k1, k2, and k3. In the case of C2H6, the measurements of [H]-atoms were used to derive direct high-temperature rate constants, k1, that can be represented by the Arrhenius equation k1(T) = 5.41 × 10(-12) exp(-6043 K/T) cm(3) molecules(-1) s(-1) (1153 K ≤ T ≤ 1297 K) for the only bimolecular process that occurs, H-atom abstraction. TST calculations based on ab initio properties calculated at the CCSD(T)/CBS//M06-2X/cc-pVTZ level of theory show excellent agreement, within ±20%, of the measured rate constants. For the reaction of CH3 with C2H4, the present rate constant results, k2', refer to the sum of rate constants, k(2b) + k(2c), from two competing processes, addition-elimination, and the direct abstraction CH3 + C2H4 → C3H6 + H (2b) and CH3 + C2H4 → C2H2 + H + CH4 (2c). Experimental rate constants for k2' can be represented by the Arrhenius equation k2'(T) = 2.18 × 10(-10) exp(-11830 K/T) cm(3) molecules(-1) s(-1) (1176 K ≤ T ≤ 1366 K). The present results are in excellent agreement with recent theoretical predictions. The present study provides the only direct measurement for the high-temperature rate constants for these channels. Lastly, measurements of H-atoms from the reaction of CH3 with C2H2 provided direct unambiguous determinations of the rate constant for the dominant process under the present experimental conditions, the addition-elimination, CH3 + C2H2 → p-C3H4 + H (3b). Experimental rate constants for k(3b) can be represented by the Arrhenius equation k(3b)(T) = 5.16 × 10(-13) exp(-3852 K/T) cm(3) molecules(-1) s(-1) (1127 K ≤ T ≤ 1346 K). The present determinations for k(3b) represent the only direct measurements for this reaction and are also in good agreement with recent theoretical predictions. The present experimental k(3b) values were also used to derive rate constants, k(-3b), for the more extensively studied back-process, the reaction of H-atoms with propyne. The best fit Arrhenius equation, combining the presently derived k(-3b) values with a recent experimental determination for k(-3b), can be represented by k(-3b)(T) = 3.87 × 10(-11) exp(-1313 K/T) cm(3) molecules(-1) s(-1) (870 K ≤ T ≤ 1346 K). The present studies represent a novel implementation of the sensitive H-ARAS technique to measure rate constants for poorly characterized and difficult to isolate "slow" CH3-radical reactions with stable C2 hydrocarbons.

High-Temperature Shock Tube and Modeling Studies on the Reactions of Methanol with D-Atoms and CH3-Radicals

The shock tube technique has been used to study the hydrogen abstraction reactions D + CH3OH → CH2O + H + HD (A) and CH3 + CH3OH → CH2O + H + CH4 (B). For reaction A, the experiments span a T-range of 1016 K ≤ T ≤ 1325 K, at pressures 0.25 bar ≤ P ≤ 0.46 bar. The experiments on reaction B, CH3 + CH3OH, cover a T-range of 1138 K ≤ T ≤ 1270 K, at pressures around 0.40 bar. Reflected shock tube experiments, monitoring the depletion of D-atoms by applying D-atom atomic resonance absorption spectrometry (ARAS), were performed on reaction A using gas mixtures of C2D5I and CH3OH in Kr bath gas. C2D5I was used as precursor for D-atoms. For reaction B, reflected shock tube experiments monitoring H-atom formation with H-ARAS, were carried out using gas mixtures of diacetyl ((CH3CO)2) and CH3OH in Kr bath gas. (CH3CO)2 was used as the source of CH3-radicals. Detailed reaction models were assembled to fit the D-atom and H-atom time profiles in order to obtain experimental rate constants for reactions A and B. Total rate constants from the present experiments on D + CH3OH and CH3 + CH3OH can be represented by the Arrhenius equations kA(T) = 1.51 × 10(-10) exp(-3843 K/T) cm(3) molecules(-1) s(-1) (1016 K ≤ T ≤ 1325 K) and kB(T) = 9.62 × 10(-12) exp(-7477 K/T) cm(3) molecules(-1) s(-1) (1138 K ≤ T ≤ 1270 K). The experimentally obtained rate constants were compared with available rate data from the literature. The results from quantum chemical studies on reaction A were found to be in good agreement with the present results. The present work represents the first direct experimental study on these bimolecular reactions at combustion temperatures and is important to the high-temperature oxidation of CH3OH.

High Temperature Shock Tube Studies on the Thermal Decomposition of O3and the Reaction of Dimethyl Carbonate with O-Atoms

The shock tube technique was used to study the thermal decomposition of ozone, O3, with a view to using this as a thermal precursor of O-atoms at high temperatures. The formation of O-atoms was measured behind reflected shock waves by using atomic resonance absorption spectrometry (ARAS). The experiments span a T-range, 819 K ≤ T ≤ 1166 K, at pressures 0.13 bar ≤ P ≤ 0.6 bar. Unimolecular rate theory provides an excellent representation of the falloff characteristics from the present and literature data on ozone decomposition at high temperatures. The present decomposition study on ozone permits its usage as a thermal source for O-atoms allowing measurements for, O + CH3OC(O)OCH3 → OH + CH3OC(O)OCH2 [A]. Reflected shock tube experiments monitoring the formation and decay of O-atoms were performed on reaction A using mixtures of O3 and CH3OC(O)OCH3, (DMC), in Kr bath gas over the T-range, 862 K ≤ T ≤ 1167 K, and pressure range, 0.15 bar ≤ P ≤ 0.33 bar. A detailed model was used to fit the O-atom temporal profile to obtain experimental rate constants for reaction A. Rate constants from the present experiments for O + DMC can be represented by the Arrhenius expression: kA(T) = 2.70 × 10(-11) exp(-2725 K/T) cm(3) molecule(-1) s(-1) (862-1167 K). Transition state theory calculations employing CCSD(T)/cc-pv∞z//M06-2X/cc-pvtz energetics and molecular properties for reaction A are in good agreement with the experimental rate constants. The theoretical rate constants can be well represented (to within ±10%) over the 500-2000 K temperature range by: kA(T) = 1.87 × 10(-20)T(2.924) exp(-2338 K/T) cm(3) molecule(-1) s(-1). The present study represents the first experimental measurement and theoretical study on this bimolecular reaction which is of relevance to the high temperature oxidation of DMC.

Roaming radicals in the thermal decomposition of dimethyl ether: Experiment and theory

The thermal dissociation of dimethyl ether has been studied with a combination of reflected shock tube experiments and ab initio dynamics simulations coupled with transition state theory based master equation calculations. The experiments use the extraordinary sensitivity provided by H-atom ARAS detection with an unreversed light source to measure both the total decomposition rate and the branching to radical products versus molecular products, with the molecular products arising predominantly through roaming according to the theoretical analysis. The experimental observations also provide a measure of the rate coefficient for H + CH 3OCH 3. An evaluation of the available experimental results for H + CH 3OCH 3 can be expressed by a three parameter Arrhenius expression as, k = 6.54 × 10 - 24 T 4.13 exp ( - 896 / T ) cm 3 molecule - 1 s - 1 ( 273 - 1465 K ) The potential energy surface is explored with high level ab initio electronic structure theory. The dynamics of roaming versus radical formation is studied with a reduced dimensional trajectory approach. The requisite potential energy surface is obtained from an interpolative moving least squares fit to wide-ranging ab initio data for the long-range interactions between methyl and methoxy. The predicted roaming and radical micro-canonical fluxes are incorporated in a master equation treatment of the temperature and pressure dependence of the dissociation process. The tight (i.e., non-roaming) transition states leading to a variety of additional molecular fragments are also included in the master equation analysis, but are predicted to have a negligible contribution to product formation. The final theoretical results reliably reproduce the measured dissociation rate to radical products reported here and are well reproduced over the 500–2000 K temperature range and the 0.01–300 bar pressure range by the following modified Arrhenius parameters for the Troe falloff format: k 1 , ∞ ( T ) = 2.33 × 10 19 T - 0.661 exp ( - 42345 / T ) s - 1 k 1 , 0 ( T ) = 2.86 × 10 35 T - 11.4 exp ( - 46953 / T ) cm 3 molecule - 1 s - 1 F cent ( T ) = exp ( - T / 880 ) The experimentally observed branching ratio of 0.19 ± 0.07 provides a direct measure of the contribution from the roaming radical mechanism. The theoretical analysis predicts a much smaller roaming contribution of 0.02.

H- and D-atom formation from the pyrolysis of C6H5CH2Br and C6H5CD2Br: Implications for high-temperature benzyl decomposition

The thermal decompositions of benzyl (C6H5CH2) and benzyl,α,ά-d2 (C6H5CD2) have been studied at high temperatures with the reflected shock tube technique using H- and D-atom Atomic Resonance Absorption Spectroscopy (ARAS). The experiments were performed at high-T (1437–1801K) at nominal pressures ≈ 0.3–1.3atm.The present experiments utilizing the ultra-sensitive H- and D-atom ARAS technique reveal that there are three unique channels that contribute to benzyl decomposition: H-atom removal from the ring, H/D-atom removal from the side-chain, and a non-atom producing process (postulated to be cyclopentadienyl + acetylene). The rate constants for the three channels can be represented in s−1 by, kH,ring=1.14×1012exp(-32180K/T)(1475–1801K)kH/D,side-chain=4.50×1013exp(-40650K/T)(1487–1789K)knon-atom=1.55×109exp(-23470K/T)(1437–1627K) The experimental data was also used to obtain rate constants for total benzyl decomposition and an Arrhenius fit gives, ktotal=1.05×1012exp(-31450K/T)(1437–1801K) The present study gives reliable determinations of rate constants for the three channels that, when coupled with the postulated mechanistic interpretations for these processes, should be useful in future theoretical and experimental characterizations of benzyl decomposition.

Rate Constants for the Thermal Decomposition of Ethanol and Its Bimolecular Reactions with OH and D: Reflected Shock Tube and Theoretical Studies

The thermal decomposition of ethanol and its reactions with OH and D have been studied with both shock tube experiments and ab initio transition state theory-based master equation calculations. Dissociation rate constants for ethanol have been measured at high T in reflected shock waves using OH optical absorption and high-sensitivity H-atom ARAS detection. The three dissociation processes that are dominant at high T are C2H5OH--> C2H4+H2O (A) -->CH3+CH2OH (B) -->C2H5+OH (C).The rate coefficient for reaction C was measured directly with high sensitivity at 308 nm using a multipass optical White cell. Meanwhile, H-atom ARAS measurements yield the overall rate coefficient and that for the sum of reactions B and C , since H-atoms are instantaneously formed from the decompositions of CH(2)OH and C(2)H(5) into CH(2)O + H and C(2)H(4) + H, respectively. By difference, rate constants for reaction 1 could be obtained. One potential complication is the scavenging of OH by unreacted ethanol in the OH experiments, and therefore, rate constants for OH+C2H5OH-->products (D)were measured using tert-butyl hydroperoxide (tBH) as the thermal source for OH. The present experiments can be represented by the Arrhenius expression k=(2.5+/-0.43) x 10(-11) exp(-911+/-191 K/T) cm3 molecule(-1) s(-1) over the T range 857-1297 K. For completeness, we have also measured the rate coefficient for the reaction of D atoms with ethanol D+C2H5OH-->products (E) whose H analogue is another key reaction in the combustion of ethanol. Over the T range 1054-1359 K, the rate constants from the present experiments can be represented by the Arrhenius expression, k=(3.98+/-0.76) x10(-10) exp(-4494+/-235 K/T) cm3 molecule(-1) s(-1). The high-pressure rate coefficients for reactions B and C were studied with variable reaction coordinate transition state theory employing directly determined CASPT2/cc-pvdz interaction energies. Reactions A , D , and E were studied with conventional transition state theory employing QCISD(T)/CBS energies. For the saddle point in reaction A , additional high-level corrections are evaluated. The predicted reaction exo- and endothermicities are in good agreement with the current Active Thermochemical Tables values. The transition state theory predictions for the microcanonical rate coefficients in ethanol decomposition are incorporated in master equation calculations to yield predictions for the temperature and pressure dependences of reactions A - C . With modest adjustments (<1 kcal/mol) to a few key barrier heights, the present experimental and adjusted theoretical results yield a consistent description of both the decomposition (1-3) and abstraction kinetics (4 and 5). The present results are compared with earlier experimental and theoretical work.

Direct Observation of Roaming Radicals in the Thermal Decomposition of Acetaldehyde

The thermal dissociation of acetaldehyde has been studied with the reflected shock tube technique using H(D)-atom atomic resonance absorption spectrometry detection. The use of an unreversed light source yields extraordinarily sensitive H atom detection. As a result, we are able to measure both the total decomposition rate and the branching to radical versus molecular channels. This branching provides a direct measure of the contribution from the roaming radical mechanism since the contributions from the usual tight transition states are predicted by theory to be negligible. The experimental observations also provide a measure of the rate coefficient for H + CH(3)CHO. Another set of experiments employing C(2)H(5)I as an H-atom source provides additional data for this rate coefficient that extends to lower temperature. An evaluation of the available experimental results for H + CH(3)CHO can be expressed by a three-parameter Arrhenius expression as k = 7.66 x 10(-20)T(2.75) exp((-486 K)/T) cm(3) molecule(-1) s(-1) (298-1415 K). Analogous experiments employing C(2)D(5)I as a D-atom source allow for the study of the isotopically substituted reaction. The present experiments are the only direct measure for this reaction rate constant, and the results can be expressed by an Arrhenius expression as k = 5.20 x 10(-10) exp((-4430 K)/T) cm(3) molecule(-1) s(-1) (1151-1354 K). The H/D + CH(3)CHO reactions are also studied with ab initio transition-state theory, and the results are in remarkably good agreement with the current experimental data.

Thermal Decomposition of NH2OH and Subsequent Reactions: Ab Initio Transition State Theory and Reflected Shock Tube Experiments

Primary and secondary reactions involved in the thermal decomposition of NH2OH are studied with a combination of shock tube experiments and transition state theory based theoretical kinetics. This coupled theory and experiment study demonstrates the utility of NH2OH as a high temperature source of OH radicals. The reflected shock technique is employed in the determination of OH radical time profiles via multipass electronic absorption spectrometry. O-atoms are searched for with atomic resonance absorption spectrometry. The experiments provide a direct measurement of the rate coefficient, k1, for the thermal decomposition of NH2OH. Secondary rate measurements are obtained for the NH2 + OH (5a) and NH2OH + OH (6a) abstraction reactions. The experimental data are obtained for temperatures in the range from 1355 to 1889 K and are well represented by the respective rate expressions: log[k/(cm3 molecule(-1) s(-1))] = (-10.12 +/- 0.20) + (-6793 +/- 317 K/T) (k1); log[k/(cm3 molecule(-1) s(-1))] = (-10.00 +/- 0.06) + (-879 +/- 101 K/T) (k5a); log[k/(cm3 molecule(-1) s(-1))] = (-9.75 +/- 0.08) + (-1248 +/- 123 K/T) (k6a). Theoretical predictions are made for these rate coefficients as well for the reactions of NH2OH + NH2, NH2OH + NH, NH + OH, NH2 + NH2, NH2 + NH, and NH + NH, each of which could be of secondary importance in NH2OH thermal decomposition. The theoretical analyses employ a combination of ab initio transition state theory and master equation simulations. Comparisons between theory and experiment are made where possible. Modest adjustments of predicted barrier heights (i.e., by 2 kcal/mol or less) generally yield good agreement between theory and experiment. The rate coefficients obtained here should be of utility in modeling NOx in various combustion environments.

Shock tube measurements of high temperature rate constants for OH with cycloalkanes and methylcycloalkanes

High temperature experiments were performed with the reflected shock tube technique using multi-pass absorption spectrometric detection of OH radicals at 308 nm. The present experiments span a wide T-range, 801–1347 K, and represent the first direct measurements of the title rate constants at T > 500 K for cyclopentane and cyclohexane and the only high temperature measurements for the corresponding methyl derivatives. The present work utilized 48 optical passes corresponding to a total path length ∼4.2 m. As a result of this increased path length, the high [OH] detection sensitivity permitted unambiguous analyses for measuring the title rate constants. The experimental rate constants in units, cm 3 molecule −1 s −1, can be expressed in Arrhenius form as k OH + Cyclopentane = ( 1.90 ± 0.30 ) × 10 − 10 exp ( − 1705 ± 156 K / T ) ( 813–1341 K ) , k OH + Cyclohexane = ( 1.86 ± 0.24 ) × 10 − 10 exp ( − 1513 ± 123 K / T ) ( 801–1347 K ) , k OH + Methylcyclopentane = ( 2.02 ± 0.19 ) × 10 − 10 exp ( − 1799 ± 96 K / T ) ( 859–1344 K ) , k OH + Methylcyclohexane = ( 2.55 ± 0.30 ) × 10 − 10 exp ( − 1824 ± 114 K / T ) ( 836–1273 K ) . These results and lower- T experimental data were used to obtain three parameter evaluations of the experimental rate constants for the title reactions over an even wider T-range. These experimental three parameter fits to the rate constants in units, cm 3 molecule −1 s −1, are k OH + Cyclopentane = 1.390 × 10 − 16 T 1.779 exp ( 97 K / T ) cm 3 molecule − 1 s − 1 ( 209–1341 K ) , k OH + Cyclohexane = 3.169 × 10 − 16 T 1.679 exp ( 119 K / T ) cm 3 molecule − 1 s − 1 ( 225–1347 K ) , k OH + Methylcyclopentane = 6.903 × 10 − 18 T 2.148 exp ( 536 K / T ) cm 3 molecule − 1 s − 1 ( 296–1344 K ) , k OH + Methylcyclohexane = 2.341 × 10 − 18 T 2.325 exp ( 602 K / T ) cm 3 molecule − 1 s − 1 ( 296–1273 K ) . High level electronic structure methods were used to characterize the first three reactions in order to provide reliable extrapolations of the rate constants from 250–2000 K. The results of the theoretical predictions for OH + cyclohexane and OH + methylcyclopentane were sufficient to make a theoretical prediction for OH + methylcyclohexane. The present recommended rate expressions for OH with cyclohexane, and methylcyclohexane, give rate constants that are 15–25% higher (over the T-range 800–1300 K) than the rate constants utilized in recent modeling efforts aimed at addressing the oxidation of cyclohexane and methylcyclohexane. The current measurements reduce the uncertainties in rate constants for the primary cycloalkane consumption channel in a high temperature oxidation environment.

Ignition and Emission Characteristics of Surrogate and Practical Jet Fuels

Aircrafts have been identified as a significant source of particulate matter (PM) emissions, which have been associated with adverse health effects and decreased ambient air quality. Recent published studies have shown that synthetic fuels, such as iso-paraffinic Fischer−Tropsch (F−T) fuels, can significantly reduce PM emissions compared to operation with petroleum-derived fuels. It is believed that the absence of aromatics in synthetic fuels slows the molecular growth to higher ringed polycyclic aromatic hydrocarbons (PAHs), which ultimately leads to lower soot emissions. However, it is not known if differences in chemical composition between synthetic and conventional (JP-8) fuel also change the ignition and combustion characteristics, which likely impact high altitude relight and combustor lean blowout (LBO) limits. This study was conducted to investigate how differences in fuel composition can impact the chemical ignition and emissions characteristics of a fuel and to help deconvolute the effect of physical and chemical processes on combustion efficiency. Only the chemical ignition delay was considered as the fuels were prevaporized and premixed prior to ignition. The fuels investigated in this study were a F−T processed synthetic jet fuel (synjet), a conventional JP-8, 2-methylheptane (surrogate synjet fuel), and a mixture of 80% heptane and 20% toluene (surrogate JP-8). The experiments were conducted using a modified single-pulse reflected shock tube instrumented to measure fuel ignition delay times and PAH and soot yields. The shock tube results show that both the actual and surrogate synjet fuels produced lower soot and PAH emissions than the actual and surrogate JP-8 fuels at temperatures of interest. These results are consistent with previous turbine engine test results with the actual fuels. It was observed, however, that, for the conditions tested, both surrogate and actual synjet and JP-8 fuels evaluated had similar ignition delays, which may imply that there is no correlation between soot emissions and chemical ignition delay. However, these results may be controlled by the test conditions, and accordingly, future tests at different conditions (e.g., higher equivalence ratios, pressures, lower pre-ignition temperatures) are warranted.

High temperature rate constants for OH+ alkanes

Rate constants for H-atom abstractions by OH radicals from a series of alkanes (propane, n-butane, i-butane and neo-pentane) have been measured at high temperatures with the reflected shock tube technique using multi-pass absorption spectrometric detection of OH radicals at 308nm. The experiments represent the first direct measurements of these rate constants at T>1000K and span a wide T-range, 797–1259K.The present work utilized 80 optical passes corresponding to a total path length of ∼7m. As a result of this increased path length, the high [OH] detection sensitivity permitted pseudo-first-order analysis for unambiguously measuring the total rate constants. The experimental rate constants can be represented in Arrhenius form as,kC3H8+OH=6.671×10-11exp(-1543K/T)cm3molecule-1s-1(797–1248K)kn-C4H10+OH=9.674×10-11exp(-1569K/T)cm3molecule-1s-1(800–1236K)ki-C4H10+OH=9.114×10-11exp(-1654K/T)cm3molecule-1s-1(846–1221K)kneo-C5H12+OH=1.060×10-10exp(-1947K/T)cm3molecule-1s-1(841–1259K)The present results have been combined with prior lower-T measurements to generate three-parameter rate expressions that adequately represent the available direct measurements (within 25%) over a wide temperature regime (250–1250K). High-level ab-initio electronic structure theory computations of the molecular properties of reactants, products and transition states have been used to estimate theoretical rate constants with conventional transition state theory (CTST). The theoretical rate constants are excellent representations of the available experimental data (deviations less than 25%) and thereby offer a reliable method for extrapolation to higher-T as well as for extracting branching ratios for primary, secondary and tertiary abstractions.

Thermal Decomposition of CF3 and the Reaction of CF2 + OH → CF2O + H

The reflected shock tube technique with multipass absorption spectrometric detection (at a total path length of approximately 1.75 m) of OH-radicals at 308 nm has been used to study the dissociation of CF3-radicals [CF3 + Kr --> CF2 + F + Kr (a)] between 1,803 and 2,204 K at three pressures between approximately 230 and 680 Torr. The OH-radical concentration buildup resulted from the fast reaction F + H2O --> OH + HF (b). Hence, OH is a marker for F-atoms. To extract rate constants for reaction (a), the [OH] profiles were modeled with a chemical mechanism. The initial rise in [OH] was mostly sensitive to reactions (a) and (b), but the long time values were additionally affected by CF2 + OH --> CF2O + H (c). Over the experimental temperature range, rate constants for (a) and (c) were determined from the mechanistic fits to be kCF3+Kr = 4.61 x 10-9 exp(-30,020 K/T) and kCF2+OH = (1.6 +/- 0.6) x 10-10, both in units of cm3 molecule-1 s-1. Reaction (a), its reverse recombination reaction reaction (-a), and reaction (c) are also studied theoretically. Reactions (c) and (-a) are studied with direct CASPT2 variable reaction coordinate transition state theory. A master equation analysis for reaction (a) incorporating the ab initio determined reactive flux for reaction (-a) suggests that this reaction is close to but not quite in the low-pressure limit for the pressures studied experimentally. In contrast, reaction (c) is predicted to be in the high-pressure limit due to the high exothermicity of the products. A comparison with past and present experimental results demonstrates good agreement between the theoretical predictions and the present data for both (a) and (c).

CH3 + O2 → Η2CO + OH Revisited

New reflected shock tube kinetics experiments have been performed on the reaction CH(3) + O(2) --> H(2)CO + OH over the temperature range 1244-1502 K. This study was carried out using a White cell multipass (path length = 7 m) optical system observing OH-radical absorption at 308 nm. Within experimental error, the new results are in excellent agreement with an earlier study from this laboratory and have therefore been combined with the earlier data, yielding an updated Arrhenius description for the rate constant, k = (1.06 +/- 0.32) x 10(-12) exp(-6801 +/- 439 K/T) cm(3) molecule(-1) s(-1). This result is compared to earlier determinations, evaluations, and theory.

Reflected Shock Tube and Theoretical Studies of High-Temperature Rate Constants for OH + CF3H ⇆ CF3 + H2O and CF3 + OH → Products

The reflected shock tube technique with multipass absorption spectrometric detection of OH radicals at 308 nm, using either 36 or 60 optical passes corresponding to total path lengths of 3.25 or 5.25 m, respectively, has been used to study the bimolecular reactions, OH+CF3H-->CF3+H2O (1) and CF3+H2O-->OH+CF3H (-1), between 995 and 1663 K. During the course of the study, estimates of rate constants for CF3+OH-->products (2) could also be determined. Experiments on reaction -1 were transformed through equilibrium constants to k1, giving the Arrhenius expression k1=(9.7+/-2.1)x10(-12) exp(-4398+/-275K/T) cm3 molecule(-1) s(-1). Over the temperature range, 1318-1663 K, the results for reaction 2 were constant at k2=(1.5+/-0.4)x10(-11) cm3 molecule(-1) s(-1). Reactions 1 and -1 were also studied with variational transition state theory (VTST) employing QCISD(T) properties for the transition state. These a priori VTST predictions were in good agreement with the present experimental results but were too low at the lower temperatures of earlier experiments, suggesting that either the barrier height was overestimated by about 1.3 kcal/mol or that the effect of tunneling was greatly underestimated. The present experimental results have been combined with the most accurate earlier studies to derive an evaluation over the extended temperature range of 252-1663 K. The three parameter expression k1=2.08x10(-17) T1.5513 exp(-1848 K/T) cm3 molecule(-1) s(-1) describes the rate behavior over this temperature range. Alternatively, the expression k1,th=1.78x10(-23) T3.406 exp(-837 K/T) cm3 molecule(-1) s(-1) obtained from empirically adjusted VTST calculations over the 250-2250 K range agrees with the experimental evaluation to within a factor of 1.6. Reaction 2 was also studied with direct CASPT2 variable reaction coordinate transition state theory. The resulting predictions for the capture rate are found to be in good agreement with the mean of the experimental results and can be represented by the expression k2,th=2.42x10(-11) T-0.0650 exp(134 K/T) cm3 molecule(-1) s(-1) over the 200-2500 K temperature range. The products of this reaction are predicted to be CF2O+HF.