Shock tube/laser absorption studies of ringed hydrocarbons and their precursors
Keywords: Mechanical Engineering.

Combustion, whether of fossil fuels or renewable fuels, is the dominant way the overwhelming demand for energy is currently met and will be for the foreseeable future. This has led to the production of harmful emissions that have a significant impact on the health of both the planet and humans. In order to improve the efficiency and cleanliness of combustion processes, improved knowledge of fundamental combustion chemistry is required. Recognizing the importance of understanding how intermediate species behave in combustion processes, particularly as related to the species that have been linked to forming harmful emissions, the work covered in this dissertation can be divided into two categories. The first is the development of diagnostics to measure key species that are intermediates in some combustion chemistry, and the second is experimental studies of ringed and small hydrocarbon decomposition chemistry employing those diagnostics, and the kinetic insights arising from that data. Arising from the need to characterize the decomposition chemistry of exo-tetrahydrodicyclopentadiene (JP-10), a diagnostic capable of measuring cyclopentadiene (CPD) in combustion conditions was developed. Using room temperature spectra and high temperature cross-section measurements, a diagnostic wavelength balancing high CPD absorbance with the existing knowledge base of other absorbing species was selected. This diagnostic was developed over a wide temperature range, and the absorbance of CPD characterized at other relevant wavelengths. The new CPD measurement tool was then employed in conjunction with a plethora of other laser diagnostics to characterize JP-10 pyrolysis chemistry between 1166 K to 1522 K at 2.5-3 atm. Using multiple measurement schemes and additional constraints, values for ethylene, propene, cyclopentadiene, methane, benzene, toluene, 1,3 butadiene, allene, and JP-10 fuel were reported, along with the overall decomposition rate constant for JP-10. Select portions of this data were used to constrain a HyChem model of JP-10 chemistry. Allene and propyne, two C3H4 isomers, were found to be intermediate species in the decomposition of many real fuels and contributors to forming polycyclic aromatic hydrocarbons, so it was desired to be able to measure their species time-histories. Employing the new capability of rapid scanning MIRcat-QCL lasers, the high temperature spectra of allene and propyne were measured for the first time. Using the measured spectra, wavelengths for allene and propyne diagnostics were selected carefully to minimize interference. Once selected, the diagnostics were characterized over 1196-1502 K, at 1.3-1.6 atm. These two tools were then used to measure the isomerization rates of allene to propyne and propyne to allene. These rate constants were found to be in good agreement with the average of past experimental determinations and recently computed rate expressions. Finally, small ringed hydrocarbon decomposition was studied, as these species are known to produce resonance-stabilized radicals that lead to PAH formation. Using the CPD diagnostic, improved and coupled with several other measurement tools, acetylene, ethylene, and cyclopentadiene time-histories were measured in cyclopentadiene and cyclopentene pyrolysis. Measurements for CPD pyrolysis were made over 1319 K to 1678 K at 1.2 - 1.5 atm, while cyclopentene measurements were made from 1220 K to 1681 K at pressures between 1.3 and 1.6 atm. Comparisons were made to multiple chemical kinetic models of different approaches, and the differences were examined. The work detailed in these studies illustrates the extensive capabilities that laser diagnostics employed on a shock tube provide for measuring combustion chemistry. The two new diagnostic tools developed for this work will enable future measurements of important combustion intermediates in real fuel chemistry, while the species time-histories and rate constants presented can be used to constrain and validate kinetic models