High Temperature Single Pulse Shock Tube Studies of Combustion Relevant Chemistry

An increasing demand for energy, as well as a need to decrease harmful emissions, necessitates better utilization of hydrocarbon fuels derived from biological and fossil sources. The single pulse shock tube experiment has been used extensively to gather validation data for the combustion of real and surrogate fuels, probe individual reactions, and aid the development of chemical kinetic models. A new lower pressure, single pulse shock tube has been constructed at the University of Illinois at Chicago. Its operation has been demonstrated in the 1-10 bar range, 650-1500 K, and ~1.7 ms reaction time. This thesis covers the work on three distinct, but broadly related studies. New pyrolysis experiments were conducted with n-heptane and oxidation experiments of mixtures of n-heptane-ethylene-methane and n-heptane-isooctane, examining how the reactivity may be influenced by the interactive chemistry of fuels and their pyrolytic and oxidative decomposition products. Several chemical kinetic models from literature were used in simulating the experimental results and were found to be inadequate for use in interpreting the results. In the second study, the recombination and disproportionation of allyl radicals has been studied in the lower pressure, single pulse shock tube. 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. The final study focused on the chemical kinetic effects of the double bond position in unsaturated biodiesel molecules and long alkenes. High pressure, single pulse shock tube experiments were conducted with four decene isomers, two methyl nonenoate isomers, and two octene isomers. Increased reactivity was observed with decenes and octenes when the double bond was moved toward the center. Significantly different yields in most of the intermediate species measured were observed. The results for the methyl nonenoate isomers showed a difference in the relative yields of some stable intermediates but no increase in reactivity. Chemical kinetic models, developed with the aid of the Reaction Mechanism Generator and an updated database of relevant reaction rate rules, were used to interpret the results. Uncertainty analysis in the predicted results, using a simple Monte Carlo method, showed significant variation in the final results

Chemical Kinetic Influences of Alkyl Chain Structure on the High Pressure and Temperature Oxidation of a Representative Unsaturated Biodiesel: Methyl Nonenoate

The high pressure and temperature oxidation of methyl <i>trans</i>-2-nonenoate, methyl <i>trans</i>-3-nonenoate, 1-octene, and <i>trans</i>-2-octene are investigated experimentally to probe the influence of the double bond position on the chemical kinetics of long esters and alkenes. Single pulse shock tube experiments are performed in the ranges <i>p</i> = 3.8–6.2 MPa and <i>T</i> = 850–1500 K, with an average reaction time of 2 ms. Gas chromatographic measurements indicate increased reactivity for <i>trans</i>-2-octene compared to 1-octene, whereas both methyl nonenoate isomers have reactivities similar to that of 1-octene. A difference in the yield of stable intermediates is observed for the octenes when compared to the methyl nonenoates. Chemical kinetic models are developed with the aid of the Reaction Mechanism Generator to interpret the experimental results. The models are created using two different base chemistry submodels to investigate the influence of the foundational chemistry (i.e., C0–C4), whereas Monte Carlo simulations are performed to examine the quality of agreement with the experimental results. Significant uncertainties are found in the chemistry of unsaturated esters with the double bonds located close to the ester groups. This work highlights the importance of the foundational chemistry in predictive chemical kinetics of biodiesel combustion at engine relevant conditions

Single Pulse Shock Tube Study of Allyl Radical Recombination

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