Search

The pyrolytic reactions of n-propyl and i-propyl radicals and their precursors have been investigated at elevated temperatures and pressures (600–900 K and 5–8 bar). The studies were performed in a miniature high repetition rate shock tube with synchrotron vacuum ultraviolet photoionization mass spectrometry yielding comprehensive datasets of temporal and photoionization energy dependencies of reagents and products. The propyl radicals were generated by pyrolysis of alkyl nitrites. In addition to the main decomposition path, NO elimination, the mass spectra allowed an alternative minor path via HNO elimination to be identified and quantified. For the n-propyl precursor the HNO elimination route accounted for 9% of the alkyl nitrite while for the i-propyl precursor it was 2.5%. These values are in good agreement with a recent study on i-butyl radicals in the same apparatus at similar conditions. The ratio of disproportionation/recombination was obtained for n-propyl from the relative concentrations of n-hexane and propene and for i-propyl from 2,3-dimethyl butane and propene. For n-propyl the ratio is 0.3 and 1.2 for i-propyl, both are independent of temperature and pressure over the experimental range. The disproportionation/recombination ratios are about a factor of 2 higher than prior evaluations that are based mainly on room temperature data. The experimental results are some of the first from the apparatus used and comments are made about the technique, challenges encountered, and suggestions for future developments.

Gas densities were measured by x-ray absorption spectroscopy to examine spatial inhomogeneity in density behind incident and reflected shock waves in a miniature (12.7 mm bore) high repetition rate shock tube. Shock waves were generated in argon. The pressure and temperature behind the incident shock were P2 ~ 1.72 bar and T2 ~ 800 K, while those behind the reflected shock were P5 ~ 6.5 bar and T5 ~ 1480 K. Time-resolved line-of-sight transmission measurements using x-rays at 9 keV photon energy were made along various axial and transverse locations covering the entire shock tube cross section. The x-ray transmission at each location was converted to pathlength-integrated densities, which in some conditions could be further converted to pathlength-averaged densities. The measured gas densities were compared with those calculated by the normal shock wave relations, and good agreement was found in the pre-shock and immediately behind the incident shock regions. Similar agreement was found for reflected shock conditions very near the endwall. However, increasingly large differences between the measured and calculated gas densities were found as distance from the endwall increased. Reconstruction by Abel inversion allowed time and radially resolved densities to be obtained from the transverse measurements. While the profiles measured in this experiment are of insufficient signal-to noise level and resolution to definitively assign boundary layer thicknesses, future improvements may be able to do this. The radially resolved densities reveal growing sidewall thermal boundary layers in gases behind the reflected shock. These reconstructions were compared with a model of the thermal boundary layer and are consistent with values of the thermal boundary layer thickness being between 0.25 mm and 1 mm at 0.7 ms after shock reflection.

Cycloalkanes and alkenes are important components of real fuels but there is little kinetic and mechanistic data on the dissociation of most large cyclic and olefinic molecules at elevated temperatures. We present here the first experimental and theoretical investigation of dissociation of cycloheptane and the initial product from ring opening, 1-heptene. Experiments were performed in a diaphragmless shock tube using laser schlieren densitometry. Pyrolysis of cycloheptane (0.5–4% in Kr) was studied over 1450–2000 K and 30–120 Torr. Experiments with 1-heptene (1–4% in Kr) covered 1200–1650 K and 30–120 Torr. A newly developed chemical kinetic mechanism for pyrolysis of cycloheptane and 1-heptene is presented herein. Simulations are in very good agreement with the experimental measurements. Rate coefficients for the initial ring-opening process in cycloheptane, k1, and dissociation of 1-heptene, k2, were determined from the experiments. Both k1 and k2 are in falloff, and the pressure and temperature dependencies were well reproduced by theoretical calculations allowing extrapolation to conditions beyond the scope of this work. These calculations yielded the following expressions for k1 and k2 with the uncertainties estimated as ±40% and ±50% respectively: k 1 , ∞ = 5.94 × 10 17 exp ( − 44 , 521 T ) s − 1 and k 2 , ∞ = 8.86 × 10 16 exp ( − 35 , 887 T ) s − 1 . The results of this study indicate that cycloheptane dissociates similarly to cyclopentane and cyclohexane, i.e. ring-opening via C C scission to a diradical that rapidly isomerizes to a conjugate 1-alkene. The secondary chemistry is dominated by the dissociation products of the 1-alkenes i.e. allyl and n-alkyl radicals. Furthermore, rates of dissociation of the cycloalkanes are size dependent and kcyclopentane

Synchrotron x-ray fluorescence has been used to measure temperatures in optically dense gases where traditional methods would fail. These data provide a benchmark for stringent tests of computational fluid dynamics models for complex systems where physical and chemical processes are intimately linked. The experiments measured krypton number densities in a sooting, atmospheric pressure, nonpremixed coflow flame that is widely used in combustion research. The experiments not only form targets for the models, but the simulations also identify potential sources of uncertainties in the measurements, allowing for future improvements.

An automated, high pressure, high flow rate batch mixing apparatus has been designed and constructed for rapid, stable, and repeatable mixing of multiple gases and vapors. The apparatus operates as an intermittent batch mixer with cycles of topping off fresh mixture to maintain pressure in an accumulator tank until consumed in an experimental apparatus. At high duty cycles, the apparatus can also function at steady state. This style of mixing is suitable for experiments such as high repetition rate shock tubes and other devices with intermittent flow demands. It is compact and portable, facilitating use in locations such as synchrotron light sources. The entire apparatus is heated to permit the mixing of vapors from species with low volatilities. The apparatus is fully automated and runs for extended periods with the only intervention being to refresh reagent supplies. The accuracy and repeatability of the apparatus were verified by periodic gas sampling and analysis with gas chromatography. Multi-component mixtures spanning a wide range of complexity, dilution, and volatility of constituents have been prepared. The compositions of the majority of the mixture were found to be stable over several filling cycles, repeatable, and with the proper calibration of set-point conditions, accurate. Challenges were encountered preparing a mixture from multi-component liquids, and potential solutions are discussed.

Flame-sampling experiments, especially in conjunction with laminar low-pressure premixed flames, are routinely used in combustion chemistry studies to unravel the identities and quantities of key intermediates and their pathways. In many instances, however, an unambiguous interpretation of the experimental and modeling results is hampered by the uncertainties about the probe-induced, perturbed temperature profile. To overcome this limitation, two-dimensional perturbations of the temperature field caused by sampling probes with different geometries have been investigated using synchrotron-based X-ray fluorescence spectroscopy. In these experiments, which were performed at the 7-BM beamline of the Advanced Photon Source (APS) at the Argonne National Laboratory, a continuous beam of hard X-rays at 15 keV was used to excite krypton atoms that were added in a concentration of 5 vol.-% to the unburnt gas mixture and the resulting krypton fluorescence at 12.65 keV was subsequently collected. The highly spatially resolved signal was converted into the local flame temperature to obtain temperature fields at various burner-probe separations as functions of the distance to the burner surface and the radial distance from the centerline. Multiple measurements were performed with different probe geometries and because of the observed impact on the temperature profiles the results clearly revealed the need to specify the sampling probe design to enable quantitative and meaningful comparisons of modeling results with flame-sampled mole fraction data.

International audience; Shock tube techniques for kinetic studies are continuously evolving driven by advances in kinetic modeling and detection techniques. An innovative category of shock tubes has been recently developed for use at Synchrotron facilities. In this work, a new high-repetition-rate shock tube (HRRST) was constructed to employ synchrotron-based double imaging photoelectron/photoion coincidence spectroscopy (i 2 PEPICO) at the beamline DESIRS of the SOLEIL synchrotron. The shock tube design and performance (pressure profiles, repeatability of operations) are presented for the first time together with the detailed description of the coupling with the molecular beam end-station holding the i 2 PEPICO spectrometer. The first experimental results with the HRRST/i 2 PEPICO on ethanol pyrolysis are grouped based on four different experimental conditions, each highlighting functionality of this novel experimental system. Experiments were performed at temperatures between 1232 K and 1525 K, pressures between 6.2 bar and 7.5 bar, with 2.7% or 0.25% ethanol in argon, and photon energy of 10.0 eV or 11.0 eV. The results are supported by kinetic analyses with the CRECK model. This study shows the potential of the HRRST/i 2 PEPICO combination for obtaining detailed mechanistic and kinetic data for complex chemical systems. Mass spectra, photoelectron spectra and time-resolved species profiles were obtained for a wide variety of species from methyl radicals to large polyaromatic hydrocarbons. The different experimental conditions studied indicate how future experiments can be designed to target key regimes facilitating the elucidation of desired kinetic and mechanistic data.

The high temperature gas phase chemistry of the four butyl radical isomers (n-butyl, sec-butyl, iso-butyl, and tert-butyl) was investigated in a combined experimental and theoretical study. Organic nitrites were used as convenient and clean sources of each of the butyl radical isomers. Rate coefficients for dissociation of each nitrite were obtained experimentally and are at, or close to, the high pressure limit. Low pressure experiments were performed in a diaphragmless shock tube with laser schlieren densitometry at post-shock pressures of 65, 130, and 260 Torr and post-shock temperatures of 700–1000 K. Additional experiments were conducted with iso-butyl radicals at 805 K and 8.7 bar to elucidate changes in mechanism at higher pressures. These experiments were performed in a miniature shock tube with synchrotron-based photoionization mass spectrometry. The mass spectra confirmed that scission of the O–NO bond is the primary channel by which the precursors dissociate, but they also provided evidence of a minor channel (

A high speed, high pressure solenoid actuated valve has been developed for use as a driver section for automated shock tubes. The valve is based on a prior design, and significant improvements in the design of the valve are described. The new design retains the performance of prior versions of the valve and creates very reproducible reaction conditions in the shock tube, which are illustrated by several thousand experiments. In addition, the longevity of the valve is improved, failures are reduced, and the maintenance and manufacture of the valve are simplified.

The dissociation of 2-methyl furan at high temperatures has been studied in a combined experimental and theoretical approach to elucidate the details of this multi-channel unimolecular reaction. Laser schlieren densitometry studies were performed in a diaphragmless shock tube over the range 1600 < T < 2300 K and three pressures 60, 120 and 240 Torr. The theoretical study identified many reaction paths, most of which are initiated by the formation of carbenes. Of these paths, five account for 99% consumption of 2MF, and three account for 95% consumption. Simulations of the experimental results with a model that incorporated the theoretical predictions of reaction paths failed to reproduce the experimental data. This was resolved by increasing the rate of loss of an H-atom from the methyl group in 2-methyl furan by a factor of 2-4. The resulting model provides good simulations of the complete set of experimental data. The branching fractions for the three key reactions are both temperature and pressure dependent.

The perturbation of the temperature field caused by a quartz sampling probe has been investigated in a fuel-rich low-pressure premixed ethylene/oxygen/argon/krypton flame using X-ray fluorescence. The experiments were performed at the 7-BM beamline at the Advanced Photon Source (APS) at the Argonne National Laboratory where a continuous beam of X-rays at 15 keV was used to excite krypton atoms that were added to the unburnt flame gases in a concentration of 5% (by volume). The resulting krypton X-ray fluorescence at 12.65 keV was collected and the spatially resolved signal was subsequently converted into the local temperature of the imaged spot. One and two dimensional scans of the temperature field were obtained by translating the entire flame chamber through a pre-programmed sequence of positions on high precision translation stages and measuring the X-ray fluorescence at each location. Multiple measurements were performed at various separations between the burner surface and probe tip, representing sampling positions from the preheat, reaction, and postflame zones of the low-pressure flame. Distortions of up to 1000 K of the burner-probe centerline flame temperature were found with the tip of the probe in the preheat zone and distortions of up to 500 K were observed with it in the reaction and postflame zones. Furthermore, perturbations of the temperature field have been revealed that reach radially as far as 20 mm from the burner-probe centerline and about 3 mm in front of the probe tip. These results clearly reveal the limitations of one-dimensional models for predicting flame-sampling experiments and comments are made with regard to model developments and validations based on quantitative speciation data from low-pressure flames obtained via intrusive sampling techniques.

The chemistry of small unsaturated hydrocarbons, such as 1,3-butadiene (1,3-C4H6), 1,2-butadiene (1,2-C4H6), 2-butyne (2-C4H6), and 1-butyne (1-C4H6), is of central importance to the modeling of combustion systems. These species are important intermediates in combustion processes, and yet their high-temperature chemistry remains poorly understood, with various dissociation and isomerization pathways proposed in the literature. Here we investigate the thermal decompositions of 1,3-C4H6, 1,2-C4H6, 2-C4H6, and 1-C4H6 inside a diaphragmless shock tube, at postshock total pressures of 26–261 Torr and temperatures ranging from 1428 to 2354 K, using laser schlieren densitometry. The experimental work was complemented by high-level ab initio calculations, which collectively provide strong evidence that formally direct dissociation is the major channel for pyrolysis of 1,3-C4H6 and 2-C4H6; these paths have not been previously reported but are critical to reconciling the current work and disparate literature report...

The dissociation of cyclopentane has been investigated in a diaphragmless shock tube with laser schlieren densitometry at nominal post-shock pressures, P 2 , of 35, 70, 150, and 300 Torr and temperatures, T 2 , of 1472–2074 K. These are the first experimental data reported in the high temperature fall-off regime for cyclopentane. The experimental density gradients were simulated using a chemical-kinetic model with good agreement between simulations and experiments. Rate coefficients for dissociation of cyclopentane were obtained and mechanistic details were elucidated. An RRKM model was developed to best fit the experimental results, and the rate coefficients calculated are: k 1( 35 Torr) = (7.93 ± 3.96) × 10 84 T − 19.815 exp(−64366/T) s −1 , k 1( 70 Torr ) = (4.79 ± 2.39) × 10 77 T − 17.705 exp( − 62316/T) s −1 , k 1( 150 Torr ) = (2.42 ± 1.21) × 10 69 T − 15.295 exp( − 59862/T) s −1 , k 1 ( 300 Torr ) = (4.66 ± 2.33) × 10 61 T − 13.065 exp( − 57493/T) s −1 , and k 1∞ = (1.69 ± 0.85) × 10 16 T − 0.005 exp( − 42983/T). Results show that cyclopentane dissociation is similar to that of cyclohexane, and involves formation of a biradical intermediate. The high-pressure limit rates for cyclopentane dissociation are also compared with previous results in the literature and show good agreement with a study by Tsang (1978) [21].

Density profiles of flowing gases within an opaque, heated, silicon carbide (SiC) miniature flow tube were measured, for the first time, by X-ray fluorescence spectroscopy using krypton as the fluorescent species. The experiments were performed with the flow tube at various fixed exterior wall temperatures between 300 K and 1100 K and a highly focused X-ray beam (5 µm × 7 µm; 15 keV). Spatially resolved raster scan maps of gas densities in the miniature reactor and the nearly supersonic jet eluting from the reactor were obtained. The intensity of the fluorescence signal was attenuated by the SiC, and a signal trapping model based on the tube morphology is required to convert the raw fluorescence signal into gas densities. This model was obtained, in part, from simultaneous measurements of the absorption of the incident X-rays by the SiC. Additional radiographic experiments with an unfocused beam were performed to image the SiC tube and complement the raster scans. Excellent correlation was found between the flow pattern revealed by the fluorescence measurements and the wall structure observed from the absorption measurements. Comments are made regarding refinements of the technique that will result in data of sufficient fidelity for validating computational fluid dynamic models of gas flows in heated miniature tubes.

The recombination and dissociation of the resonantly stabilized 2-methylallyl radical has been studied in a diaphragmless shock tube by laser schlieren densitometry (LS) over temperatures of 700–1350 K and pressures of 60–260 Torr. Both 2,5-dimethyl-1,5-hexadiene and the new low temperature precursor 3-methylbut-3-enyl nitrite were used to generate 2-methylallyl radicals under these conditions. Rate coefficients were obtained for dissociation of the precursors, recombination of 2-methylallyl, and dissociation of 2-methylallyl by simulation of the LS profiles. The experiments are complemented by a priori theoretical calculations for both the recombination and dissociation of 2-methylallyl. The experimental results and theoretical predictions are in excellent agreement with one another. The calculated high pressure limit rate coefficient for recombination of 2-methylallyl is log( k 1 ) = 14.737−0.641log T +251.39/(2.303× T ) and that for dissociation of 2-methylallyl is log( k 3 ) = 11.100−1.2295logT−28545/(2.303× T ). The uncertainties in k 1 and k 3 are estimated as factors of 1.5. Rate coefficients are provided over a broad range of pressures for chemical kinetic modeling.

A new shock tube has been constructed for investigations of high-temperature chemical kinetics with an emphasis on combustion chemistry. This instrument includes a diaphragmless driver and electrical control of valving. A diaphragmless design significantly improves repeatability of experimental conditions vs the use of diaphragms and leads to an approximate order of magnitude reduction in turnaround time between experiments. Electrical control of valves, combined with diaphragmless operation, also enables remote and automated operation of the shock tube. The design allows for both incident and reflected shock experiments with multiple diagnostics. The performance of the shock tube is demonstrated by reproducing previous literature measurements on the unimolecular decomposition of isobutyl nitrite and cyclohexene.

The thermal dissociations of three alkyl nitrites (C 3 H 7 ONO, n -C 4 H 9 ONO, and i -C 4 H 9 ONO) have been studied in a diaphragmless shock tube by laser schlieren densitometry over 700 T P k 1 , were obtained with an uncertainty of 30%. Rice–Ramsperger–Kassel–Marcus/Master Equation (RRKM/ME) calculations for dissociation of the nitrites were performed and provide excellent simulations of the experimental results. The high-pressure limit rate coefficients from the RRKM/ME calculations are: k 1 , C 3 H 7 ONO = ( 5.69 ± 1.71 ) × 10 21 T − 1.60 e x p ( − 21458 / T ) s − 1 ; k 1 , n − C 4 H 9 ONO = ( 9.91 ± 2.97 ) × 10 21 T − 1.59 e x p ( − 21588 / T ) s − 1 ; k 1 , i − C 4 H 9 ONO = ( 3.92 ± 1.18 ) × 10 21 T − 1.58 e x p ( − 21162 / T ) s − 1 . At the low temperatures of this work, the alkyl radicals are thermally stable and thus their recombination reactions were also studied. The recombination rate coefficients for ethyl, n- propyl and i- propyl radicals (uncertainties of 50%) are in excellent agreement with literature values, particularly recent theoretical results, and extend the range of experiments to higher temperatures.

In-situ wide-angle X-ray scattering (WAXS) measurements have been conducted on atmospheric-pressure fuel-rich premixed freely propagating ethylene/oxygen flames with argon and nitrogen dilution. In this work, a novel analysis methodology able to provide quantitative information on soot/carbon particle species and concentrations was tested under heavy sooting conditions. The particle composition and concentrations were retrieved by fitting theoretical calculations of structural components from major molecular and nanometric species to the experimental WAXS data. The results show that argon dilution yields predominantly graphene-like components that are less stacked and amorphous carbon that is less structured than under nitrogen dilution. This finding was later confirmed by electron microscopy analysis on samples extracted from similar flames. In addition, the WAXS showed that most of the carbon present in the flames was bound as particles. These results constitute some of the first in-situ observations on structures and concentrations of carbon species in laboratory burner flames operating without stabilization plates.

The decomposition of ortho-benzyne radicals (o-C6H4), generated from the dissociation of a new precursor, fluorobenzene (C6H5F), has been investigated in a diaphragmless shock tube in a combined laser schlieren densitometry, LS, (P2 = 30 ± 2, 59 ± 3, 121 ± 5 Torr, 2050 < T2 < 2980)/time-of-flight mass spectrometry, TOF-MS (P5 = 1150 ± 200 Torr, 2300 < T5 < 2800 K) study. The LS density gradient profiles were simulated, and excellent agreement was found between simulations and experimental profiles. Rate coefficients for C6H5F → o-C6H4 + HF, o-C6H4 → C4H2 + C2H2, and o-C6H4 → C6H3 + H were obtained. Good agreement with Xu et al. [Proc. Combust. Inst., 31(2007) 231-239] with respect to the o-benzyne dissociation branching ratio was found. However a strong pressure dependence was also observed in o-benzyne dissociation which was not seen by Xu et al. For o-C6H4 → C4H2 + C2H2: k2a,120Torr = (1.2 ± 0.4) × 1063 T−14.27 exp(−52,710/T) k2a,60Torr = (1.4 ± 0.5) × 1060 T−13.595 exp(−50,538/T), k2a,30Torr = (4.0 ± 1.3) × 1057 T−13.015 exp(−48,628/T) s−1. The inclusion of high temperature o-benzyne abstraction reactions is necessary to simulate the reacting system, and we report estimates of the rate of o-C6H4 + C6H5F.

The chemistry of small unsaturated hydrocarbons, such as 1,3-butadiene (1,3-C

The recombination and disproportionation of allyl radicals has been studied in a single pulse shock tube with gas chromatographic measurements at 1-10 bar, 650-1300 K, and 1.4-2 ms reaction times. 1,5-Hexadiene and allyl iodide were used as precursors. Simulation of the results using derived rate expressions from a complementary diaphragmless shock tube/laser schlieren densitometry study provided excellent agreement with precursor consumption and formation of all major stable intermediates. No significant pressure dependence was observed at the present conditions. It was found that under the conditions of these experiments, reactions of allyl radicals in the cooling wave had to be accounted for to accurately simulate the experimental results, and this unusual situation is discussed. In the allyl iodide experiments, higher amounts of allene, propene, and benzene were found at lower temperatures than expected. Possible mechanisms are discussed and suggest that iodine containing species are responsible for the low temperature formation of allene, propene, and benzene.

The decomposition of dimethyl ether (CH 3 OCH 3 ) has been investigated behind incident shock waves in a diaphragmless shock tube using laser schlieren densitometry, LS ( T = 1500–2450 K, P = 57 ± 4, 125 ± 5 and 253 ± 12 Torr). The LS density gradient profiles were simulated and excellent agreement was found between the simulations and experimental profiles. Rate coefficients for CH 3 OCH 3 → CH 3 O + CH 3 were obtained. They showed strong fall-off, and at the lower end of the experimental temperature range are close to the low pressure limit. First order rate coefficient expressions were determined over 1500 T k 57Torr = (3.10 ± 1.0) × 10 79 T −19.03 exp(−54417/ T ) s −1 , k 125Torr = (1.12 ± 0.3) × 10 83 T −19.94 exp(−55554/ T ) s −1 and k 253Torr = (1.02 ± 0.3) × 10 73 T −17.09 exp(−51500/ T ) s −1 . The effect of a roaming channel for decomposition of dimethyl ether was assessed and the best agreement was obtained with 1% dissociation of DME via the roaming path.

The decomposition of ethyl iodide and subsequent dissociation of ethyl radicals have been investigated behind incident shock waves in a diaphragmless shock tube by laser-schlieren (LS) densitometry (1150–1870 K, 55 ± 2 Torr and 123 ± 3 Torr). The LS density-gradient profiles were simulated assuming that the initial dissociation of C2H5I proceeded by 87% C–I fission and 13% HI elimination. Excellent agreement was found between the simulations and experimental profiles. Rate coefficients for the C–I scission reaction were obtained and show strong falloff. Gorin model RRKM (Rice, Ramsperger, Kassel, and Marcus) calculations are in excellent agreement with the experimental data with E0 = 55.0 kcal/mol, which is in very good agreement with recent thermochemical measurements and evaluations. However, E0 is approximately 2.7 kcal/mol higher than previous estimates. First-order rate coefficients for dissociation of C2H5I were determined to be k55Torr = 8.65 × 1068T−16.65 exp(−37,890/T) s−1, k123Torr = 3.01 × 1069T−16.68 exp(−38,430/T) s−1, k∞ = 2.52 × 1019T−1.01 exp(−28,775/T) s−1. Rates of dissociation for ethyl radicals were also obtained, and these are in very good agreement with theoretical predictions (Miller J. A. and Klippenstein S. J. Phys Chem Chem Phys 2004, 6, 1192–1202). The simulations show that at low temperatures ethyl radicals are consumed through recombination reactions as well as dissociation, whereas at high temperatures, dissociation dominates. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 44: 433–443, 2012

The decomposition of propargyl iodide, C 3 H 3 I → C 3 H 3 + I, and the following combination of propargyl radical, C 3 H 3 + C 3 H 3 → C 6 H 6, have been observed in the incident shock in mixtures of 1%, 2%, and 5% C 3 H 3 I in Kr, over 800–2000 K, for nominal post-shock pressures of 20 and 130 Torr. The iodide dissociates readily, the endothermic reaction producing an easily resolved positive density gradient whose kinetic modeling allows extraction of precise rates showing strong falloff. A simple Gorin-model RRKM treatment of the measurements results in a k ∞ = 3.35 × 10 18 T −0.9 exp (−22192/ T ) s −1 . The initial positive gradient is immediately followed by a strong negative, showing exothermic reaction from propargyl combination. Modeling of this gradient with an assumed theoretical product distribution, provided by J.A. Miller, results in propargyl combination rates in excellent agreement with current theory and having a strong negative temperature dependence, the rate dropping near a factor of forty over 800–2000 K. Possible errors associated with the need to assume a product distribution are discussed and it is suggested that this should not result in rate uncertainties for the C 3 H 3 combination reaction greater than ±50%.

The dissociation of 1, 2 and 4% 1,4-dioxane dilute in krypton was studied in a shock tube using laser schlieren densitometry, LS, for 1550-2100 K with 56 ± 4 and 123 ± 3 Torr. Products were identified by time-of-flight mass spectrometry, TOF-MS. 1,4-dioxane was found to initially dissociate via C-O bond fission followed by nearly equal contributions from pathways involving 2,6 H-atom transfers to either the O or C atom at the scission site. The 'linear' species thus formed (ethylene glycol vinyl ether and 2-ethoxyacetaldehyde) then dissociate by central fission at rates too fast to resolve. The radicals produced in this fission break down further to generate H, CH(3) and OH, driving a chain decomposition and subsequent exothermic recombination. High-level ab initio calculations were used to develop a potential energy surface for the dissociation. These results were incorporated into an 83 reaction mechanism used to simulate the LS profiles with excellent agreement. Simulations of the TOF-MS experiments were also performed with good agreement for consumption of 1,4-dioxane. Rate coefficients for the overall initial dissociation yielded k(123Torr) = (1.58 ± 0.50) × 10(59) × T(-13.63) × exp(-43970/T) s(-1) and k(58Torr) = (3.16 ± 1.10) × 10(79) × T(-19.13) × exp(-51326/T) s(-1) for 1600T2100 K.

A combination of experiment and theory is applied to the self-reaction kinetics of phenyl radicals. The dissociation of phenyl iodide is observed with both time-of-flight mass spectrometry, TOF-MS, and laser schlieren, LS, diagnostics coupled to a diaphragmless shock tube for temperatures ranging from 1276 to 1853 K. The LS experiments were performed at pressures of 22 +/- 2, 54 +/- 7, and 122 +/- 6 Torr, and the TOF-MS experiments were performed at pressures in the range 500-700 Torr. These observations are sensitive to both the dissociation of phenyl iodide and to the subsequent self-reaction of the phenyl radicals. The experimental observations indicate that both these reactions are more complicated than previously assumed. The phenyl iodide dissociation yields approximately 6% C(6)H(4) + HI in addition to the major and commonly assumed C(6)H(5) + I channel. The self-reaction of phenyl radicals does not proceed solely by recombination, but also through disproportionation to benzene + o-/m-/p-benzynes, with comparable rate coefficients for both. The various channels in the self-reaction of phenyl radicals are studied with ab initio transition state theory based master equation calculations. These calculations elucidate the complex nature of the C(6)H(5) self-reaction and are consistent with the experimental observations. The theoretical predictions are used as a guide in the development of a model for the phenyl iodide pyrolysis that accurately reproduces the observed laser schlieren profiles over the full range of the observations.

The dissociation of diacetyl dilute in krypton has been studied in a shock tube using laser schlieren densitometry at 1200-1800 K and reaction pressures of 55 +/- 2, 120 +/- 3, and 225 +/- 5 Torr. The experimentally determined rate coefficients show falloff and an ab initio/Master Equation/VRC-TST analysis was used to determine pressure-dependent rate coefficient expressions that are in good agreement with the experimental data. From the theoretical calculations k(infinity)(T) = 5.029 x 10(19) (T/298 K)(-3.40) exp(-37665/T) s(-1) for 300T2000 K. The laser schlieren profiles were simulated using a model for methyl recombination with appropriate additions for diacetyl. From the simulations rate coefficients were determined for CH(3) + CH(3) = C(2)H(6) and CH(3) + C(4)H(6)O(2) = CH(3)CO + CH(2)CO + CH(4) (k(T) = 2.818T(4.00) exp(-5737/T) cm(3) mol(-1) s(-1)). Excellent agreement is found between the simulations and experimental profiles, and Troe type parameters have been calculated for the dissociation of diacetyl and the recombination of methyl radicals.

Vibrational relaxation and dissociation of CH{sub 3}I, 2-20% in krypton, have been investigated behind incident shock waves in a diaphragmless shock tube at 20, 66, 148, and 280 Torr and 630-2200 K by laser schlieren densitometry. The effective collision energy obtained from the vibrational relaxation experiments has a small, positive temperature dependence, {Delta}E{sub down} = 63 x (T/298){sup 0.56} cm{sup -1}. First-order rate coefficients for dissociation of CH{sub 3}I show a strong pressure dependence and are close to the low-pressure limit. Restricted-rotor Gorin model RRKM calculations fit the experimental results very well with {Delta}E{sub down} = 378 x (T/298){sup 0.457} cm{sup -1}. The secondary chemistry of this reaction system is dominated by reactions of methyl radicals and the reaction of the H atom with CH{sub 3}I. The results of the decomposition experiments are very well simulated with a model that incorporates methyl recombination and reactions of methylene. Second-order rate coefficients for ethane dissociation to two methyl radicals were derived from the experiments and yield k = (4.50 {+-} 0.50) x 10{sup 17} exp(-32709/T) cm{sup 3} mol{sup -1} s{sup -1}, in good agreement with previous measurements. Rate coefficients for H + CH{sub 3}I were also obtained and give k = (7.50more » {+-} 1.0) x 10{sup 13} exp(-601/T) cm{sup 3} mol{sup -1} s{sup -1}, in reasonable agreement with a previous experimental value.« less

The dissociation and relaxation of CO 2 has been reexamined in the incident shock wave with the laser-schlieren technique. These new experiments covered 1377-6478 K, and 42-750 Torr, and improvements partly described herein have permitted accurate determination of both rate and incubation time. In general the steady rate measurements are in agreement with other recent determinations. The one anomaly is that the new rates are not fully second order; they vary about 50% over 70-600 Torr. This unexpected feature is actually quite consistent with the recent literature, which shows a similar trend. However, attempts to produce this result with RRKM calculations were unsuccessful. Relaxation times are in agreement with available literature, and incubation time to relaxation time ratios lie between 1.5 and 3 over 4000-6600 K, consistent with findings for other molecules. These ratios are much smaller than those recently derived from reflected-shock experiments by Oehlschlaeger et al. (Z. Phys. Chem. 2005, 219, 555). A simple argument suggests such large values are indeed anomalous, although why they are too large is not clear.

The classic pyrolytic decomposition of acetaldehyde has been examined to the higher temperatures used in combustion and also lower pressures with 85 laser-schlieren, shock-tube measurements of density gradient covering 40–500 torr and 1550–2400 K. This work is supplemented and modeled with a CASPT2 based variable reaction coordinate RRKM prediction of the dissociation kinetics. These RRKM predictions are then incorporated in good two-dimensional master equation fits of the strong falloff seen in the laser-schlieren experiments, and also that shown in some previous shock-tube results using UV absorption of the acetaldehyde as diagnostic. The laser-schlieren data provide not only unambiguous dissociation rates but also solid indications of the secondary chemistry. Modeling of the full density gradient profiles offers good estimates of rates for H-atom abstraction from both the acetaldehyde and the HCO radical, again at high temperatures.

The high pressure oxidation of dilute CO mixtures doped with 150–200 ppm of H 2 has been studied behind reflected shock waves in the UIC high pressure single pulse shock tube. The experiments were performed over the temperature range from 1000 to 1500 K and pressures spanning 21–500 bars for stoichiometric ( Φ = 1) and fuel lean ( Φ = 0.5) oxidation. Stable species sampled from the shock tube were analyzed by standard GC, GC/MS techniques. The experimental data obtained in this work were simulated using a detailed model for H 2 /CO combustion that was validated against a variety of experimental observables/targets that span a wide range of conditions. These simulations have shown that within experimental error the model is able to capture the experimental trends for the lower pressure data sets (average nominal pressures of 24 and 43 bars). However the model under predicts the CO and O 2 decay and subsequent CO 2 formation for the higher pressure data sets (average nominal pressures of 256 and 450 bars). The current elevated pressure data sets span a previously unmapped regime and have served to probe HO 2 radical reactions which appear to be among the most sensitive reactions in the model under these conditions. With updated rate parameters for a key HO 2 radical reaction OH + HO 2 = H 2 O + O 2 , the model is able to reconcile the elevated pressure data sets thereby extending its capability to an extreme range of conditions.

The thermal decomposition of nitromethane provides a classic example of the competition between roaming mediated isomerization and simple bond fission. A recent theoretical analysis suggests that as the pressure is increased from 2 to 200 Torr the product distribution undergoes a sharp transition from roaming dominated to bond-fission dominated. Laser schlieren densitometry is used to explore the variation in the effect of roaming on the density gradients for CH3NO2 decomposition in a shock tube for pressures of 30, 60, and 120 Torr at temperatures ranging from 1200 to 1860 K. A complementary theoretical analysis provides a novel exploration of the effects of roaming on the thermal decomposition kinetics. The analysis focuses on the roaming dynamics in a reduced dimensional space consisting of the rigid-body motions of the CH3 and NO2 radicals. A high-level reduced-dimensionality potential energy surface is developed from fits to large-scale multireference ab initio calculations. Rigid body trajectory simulations coupled with master equation kinetics calculations provide high-level a priori predictions for the thermal branching between roaming and dissociation. A statistical model provides a qualitative/semiquantitative interpretation of the results. Modeling efforts explore the relation between the predicted roaming branching and the observed gradients. Overall, the experiments are found to be fairly consistent with the theoretically proposed branching ratio, but they are also consistent with a no-roaming scenario and the underlying reasons are discussed. The theoretical predictions are also compared with prior theoretical predictions, with a related statistical model, and with the extant experimental data for the decomposition of CH3NO2, and for the reaction of CH3 with NO2.

Improvements to equipment lifetime and measurement reproducibility have been made by modifying the actuating mechanism of a diaphragmless shock tube that is used for high temperature gas kinetic studies. The modifications have two major benefits while retaining the simplicity of the original apparatus. First, the reproducibility of shock wave generation has been greatly improved and is demonstrated with 50 nearly identical experiments on the dissociation of cyclohexene at T2 = 1765 ± 13 K and P2 = 120 ± 1 Torr, demonstrating the capability for signal averaging over many experiments. Second, the lifetime of the bellows which forms the heart of the actuator is considerably improved, significantly increasing the time between replacements.

Tunable synchrotron-sourced photoionization time-of-flight mass spectrometry (PI-TOF-MS) is an important technique in combustion chemistry, complementing lab-scale electron impact and laser photoionization studies for a wide variety of reactors, typically at low pressure. For high-temperature and high-pressure chemical kinetics studies, the shock tube is the reactor of choice. Extending the benefits of shock tube/TOF-MS research to include synchrotron sourced PI-TOF-MS required a radical reconception of the shock tube. An automated, miniature, high-repetition-rate shock tube was developed and can be used to study high-pressure reactive systems (T600 K, P100 bar) behind reflected shock waves. In this paper, we present results of a PI-TOF-MS study at the Advanced Light Source at Lawrence Berkeley National Laboratory. Dimethyl ether pyrolysis (2% CH3OCH3/Ar) was observed behind the reflected shock (1400T51700 K, 3P516 bar) with ionization energies between 10 and 13 eV. Individual experiments have extremely low signal levels. However, product species and radical intermediates are well-resolved when averaging over hundreds of shots, which is ordinarily impractical in conventional shock tube studies. The signal levels attained and data throughput rates with this technique are comparable to those with other synchrotron-based PI-TOF-MS reactors, and it is anticipated that this high pressure technique will greatly complement those lower pressure techniques.

The pyrolysis of toluene, the simplest methyl-substituted aromatic molecule, has been studied behind reflected shock waves using a single pulse shock tube. Part 1 in this two-part series focused on the high-pressure experimental results and the high-pressure limiting rate coefficients for the primary steps in toluene decomposition. The present work focuses on the modeling of benzyl decomposition and the growth of key soot precursors (C2H2, C4H2, C8H6, and indene) from toluene pyrolysis with 81 among the 262 reactions in the detailed toluene model representing the chemistry that describes the formation and decomposition of these species. Feasible pathways for benzyl decomposition as well as phenylacetylene and indene formation have been tested. The simulations also show very good agreement with the single pulse shock tube profiles for the growth of key soot precursors such as C2H2, C4H2, C8H6, and indene.

1,2,4,5-Hexatetraene (1245HT) is, according to theory, a key intermediate to benzene from propargyl radicals in a variety of flames; however, it has only been experimentally observed once in previous studies of the C3H3 + C3H3 reaction. To determine if it is indeed an intermediate to benzene formation, 1245HT was synthesized, via a Grignard reaction, and pyrolysized in a single-pulse shock tube at two nominal pressures of 22 and 40 bar over a temperature range from 540 to 1180 K. At temperatures T700 K, 1245HT converts efficiently to 3,4-dimethylenecyclobutene (34DMCB) with a rate constant of k = 10(10.16) x exp(-23.4 kcal/RT), which is in good agreement with the one calculated by Miller and Klippenstein. At higher temperatures, various C6H6 isomers were generated, which is consistent with theory and earlier experimental studies. Thus, the current work strongly supports the theory that 1245HT plays a bridging role in forming benzene from propargyl radicals. RRKM modeling of the current data set has also been carried out with the Miller-Klippenstein potential. It was found that the theory gives reasonably good predictions of the experimental observations of 1245HT, 1,5-hexadiyne (15HD), and 34DMCB in the current study and in our earlier studies of 15HD pyrolysis and propargyl recombination; however, there is considerable discrepancy between experiment and theory for the isomerization route of 1,2-hexadien-5-yne (12HD5Y) --2-ethynyl-1,3-butadiene (2E13BD) --fulvene.

The self-reaction of propargyl (C3H3) radicals has been widely suggested as one of the key routes forming benzene in a variety of aliphatic flames. Currently, in the majority of aromatic models, the C3H3 + C3H3 submechanism often contains one or two C6H6 isomers and a few global reaction steps, which do not adequately represent the actual recombination chemistry. Recent experimental and theoretical studies on the direct propargyl recombination and subsequent C6H6 isomerization have provided sufficient information to revisit and revise the C3H3 + C3H3 reaction submechanism. In the present work, a semidetailed kinetic model consisting of seven isomeric C6H6 species and 14 reaction steps was constructed based on the most recent potential energy surface for this system. The trial model was subjected to systemic optimization by use of a recently developed physically bounded Gauss-Newton (PGN) method against detailed species profiles of direct propargyl recombination and 1,5-hexadiyne (15HD) isomerization obtained from experiments at high temperatures in a shock tube and at low temperatures in a flow reactor, which were all measured at very high pressure (shock tube) or atmospheric (flow reactor) conditions. Predictions of the optimized model were in excellent agreement with all experimental measurements. The optimized C3H3 + C3H3 reaction subset was also tested for flame modeling. Two different aromatic chemistry models that incorporate benzene formation from propargyl radicals as a single step reaction were modified to include the complete submechanism for propargyl recombination. The updated models predict significant percentages of three isomeric species [2-ethynyl-1,3-butadiene (2E13BD), fulvene, and benzene] in premixed fuel-rich acetylene and ethylene flames, reflecting the observed flame structures.

The high-temperature, high-pressure pyrolysis of the prototype aromatic, benzene, has been studied behind reflected shock waves in the UIC High Pressure Single Pulse Shock Tube. Three sets of experiments were performed at nominal pressures of 30 and 50 bars in the high temperature regime from 1200–1800 K. Stable species sampled from the shock tube were analyzed offline using gas chromatographic techniques. The present data set was simulated using the three most recent models, two of the models developed and validated against high-temperature benzene pyrolysis shock-tube data for stable species profiles as well as H atom production rates and the third model, a “work-in-progress” model from our laboratory aimed at resolving the high-pressure combustion of primary aromatics such as benzene and toluene. The simulations reflect the complexities and uncertainties involved not only in describing the primary decay steps but also the subsequent high-temperature secondary chemistry for even the simplest ar...

An iterative physically bounded Gauss−Newton (PGN) method has been formulated to estimate unknown kinetic parameters from experimental measurements. A physically bounded approach is adopted to reduce the size of the search space and ensure search within physically meaningful ranges of kinetic rates. First-order sensitivity information of state variables, with respect to unknown rate parameters, is computed simultaneously with the integration of the governing ordinary differential equations (ODEs). Optimal kinetic parameters are obtained by iteratively solving the Gauss−Newton update equations within the physical parameter range. Four different reaction systems, including both simple and complex networks, have been utilized for validating the performance of the proposed method. Comparisons with other state-of-the-art algorithms showed its efficiency, robustness, and accuracy. The methodology was also applied to analyze ethane oxidation based on the widely cited GRI-Mech 3.0 mechanism to demonstrate the alg...

An extensive experimental study of ethane oxidation and pyrolysis has been conducted in the high pressure shock tube at UIC covering reflected shock pressures from 5–1000 bar, reaction temperatures up to 1550 K and stoichiometric (Φ = 1), fuel rich (Φ = 5), and pyrolytic mixtures. The experimental data has been used to develop a single model that can simulate the whole dataset very well and is the first ethane model capable of simulating experimental results over such an extensive range of pressure, temperature, and stoichiometry. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 306–331, 2005

The thermal dissociation of ethane has been examined in two complementary sets of shock-tube experiments that together cover 1400–2200 K and a pressure range of 70–5700 torr. These experiments used both laser-schlieren and UV absorption of CH3 as diagnostics. In addition, that ethane exhibits a double vibrational relaxation has been confirmed for high temperatures, demonstrating slow IVR in this molecule, and both relaxation times have been determined for 1000–1500 K. Some estimates of dissociation incubation times were possible over about 1900–2100 K. An RRKM model calculation using the earlier Klippenstein–Harding theoretical model provides a remarkably accurate description of the complete range of data with 〈ΔE〉down = 120 (T/300)0.9 cm−1. Over 1400–2200 K, the k∞ from this model is 8.03 × 1028 T−3.52 exp (−95346 cal/mol/RT) s−1. It is suggested that the success of this model shows that there is no non-RRKM behavior in this reaction.

The high-pressure single pulse shock tube (HPST) at the University of Illinois at Chicago has been used to study the oxidation of toluene at reflected shock pressures from 22 to 550 bar and temperatures from 1210 to 1480 K. Experiments were performed for dilute stoichiometric, Φ = 1 , and rich, Φ = 5 , reagent mixtures. Stable species were analyzed using gas chromatography and gas chromatography/mass spectrometry. The resulting data set is the first that provides species concentrations over such extremes of pressure, temperature, and stoichiometry, and it serves as an excellent base for the validation and refinement of future detailed chemical kinetic models. Two literature models for the oxidation of toluene that have been validated against atmospheric pressure turbulent flow reactor and jet stirred reactor data were used to simulate the experimental data. Several modifications were made to one model to more accurately simulate the high-pressure/high-temperature experimental data. The modified model reproduces the Φ = 1 experimental data well and forms the first step in the development of a more comprehensive model for toluene combustion validated over wide ranges of temperature and pressure.

The pyrolysis of 1,5-hexadiyne has been studied in a high-pressure single pulse shock tube to investigate the mechanisms involved in the production of benzene from propargyl radicals. Analysis of the reaction products by gas chromatography and matrix isolation Fourier transform infrared spectroscopy has positively identified six linear C6H6 species and two cyclic C6H6 species. Of these species cis-1,3-hexadien-5-yne and trans-1,3-hexadiene-5-yne have been unambiguously identified for the first time and provide vital information concerning a low-temperature route to benzene that does not involve the formation of fulvene; however, the data also provide support for two high-temperature paths from propargyl radicals to benzene via fulvene. Thus experimental evidence has been gained that supports two different routes to benzene formation. The mechanisms and rate coefficients that have been obtained in this work are discussed.