In situ temperature measurements in sooting methane/air flames using synchrotron x-ray fluorescence of seeded krypton atoms.

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.

Initiation reactions in the high temperature decomposition of styrene.

The thermal decomposition of styrene was investigated in a combined experimental, theory and modeling study with particular emphasis placed on the initial dissociation reactions. Two sets of shock tube/time-of-flight mass spectrometry (TOF-MS) experiments were performed to identify reaction products and their order of appearance. One set of experiments was conducted with a miniature high repetition rate shock tube at the Advanced Light Source at Lawrence Berkeley National Laboratory using synchrotron vacuum ultraviolet photoionization. The other set of experiments was performed in a diaphragmless shock tube (DFST) using electron impact ionization. The datasets span 1660-2260 K and 0.5-12 atm. The results show a marked transition from aromatic products at low temperatures to polyacetylenes, up to C8H2, at high temperatures. The TOF-MS experiments were complemented by DFST/LS (laser schlieren densitometry) experiments covering 1800-2250 K and 60-240 Torr. These were particularly sensitive to the initial dissociation reactions. These reactions were investigated theoretically and revealed the dissociation of styrene to be a complex multichannel process with strong pressure and temperature dependencies that were evaluated with multi-well master equation simulations. Simulations of the LS data with a mechanism developed in this work are in excellent agreement with the experimental data. From these simulations, rate coefficients for the dissociation of styrene were obtained that are in good agreement with the theoretical predictions. The simulation results also provide fair predictions of the temperature and pressure dependencies of the products observed in the TOF-MS studies. Prior experimental studies of styrene pyrolysis concluded that the main products were benzene and acetylene. In contrast, this study finds that the majority of styrene dissociates to create five styryl radical isomers. Of these, α-styryl accounts for about 50% with the other isomers consuming approximately 20%. It was also found that C-C bond scission to phenyl and vinyl radicals consumes up to 25% of styrene. Finally the dissociation of styrene to benzene and vinylidene accounts for roughly 5% of styrene consumption. Comments are made on the apparent differences between the results of this work and prior literature.

An experimental and theoretical study of the high temperature reactions of the four butyl radical isomers.

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 (<7.7%) through HNO loss and formation of an aldehyde. These high pressure experiments were also used to determine the disproportionation/recombination ratio for iso-butyl radicals as 0.3. Reanalysis of the lower-temperature literature and the present data yielded rate constants for the disproportionation reaction, iso-butyl + iso-butyl = iso-butene + iso-butane. A chemical kinetics model was developed for the reactions of the butyl isomers that included new paths for highly energized adducts. These adducts are formed by the addition of H, CH3 or C2H5 to the butyl radicals. Accompanying theoretical investigations show that chemically activated pathways are competitive with stabilization of the adduct by collision under the conditions of the laser schlieren experiments. These calculations also show that at 10 bar and T < 1000 K stabilization is the only important reaction, but at higher temperatures, even at 10 bar, chemically activated product channels should also be considered. Branching fractions and rate coefficients are presented for these reactions. This study also highlights the importance of the radical structure for determining branching ratios for disproportionation and recombination of alkyl radicals, and these were facilitated by theoretical calculations of recombination rate coefficients for the four butyl radical isomers. The results reveal previously unknown features of butyl radical chemistry under conditions that are relevant to a wide range of applications and reaction mechanisms are presented that incorporate pressure dependent rate coefficients for the key steps.

Solenoid actuated driver valve for high repetition rate shock tubes.

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.