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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 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.