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Measuring Hydroxyl Radicals during the Oxidation of Methane, Ethane, Ethylene, and Acetylene in a Shock Tube Using UV Absorption Spectroscopy
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The hydroxyl (OH) radical is a common intermediate species in any hydrogen- or hydrocarbon-based flame. Investigating OH at elevated temperatures and pressures is not a trivial task, and many considerations must be made to fully study the molecule. Shock tubes can provide the experimenter with a wide range of temperatures and pressures to investigate a variety of combustion characteristics including, but not limited to, OH kinetic profiles. Described in this dissertation is the diagnostic used to measure OH within a shock tube using UV absorption spectroscopy from an enhanced UV Xenon lamp passed through a spectrometer. OH absorption was made over a narrow range of wavelengths around 309.551 nm within the widely studied OH X→A ground vibrational transition region. Experiments have been performed in the shock-tube facility at Texas A&M University using this OH absorption diagnostic. A calibration mixture of stoichiometric H2/O2 diluted in 98% argon by volume was tested initially and compared with a well-known hydrogen-based kinetics mechanism to generate an absorption coefficient correlation. This correlation is valid over the range of conditions observed in the experiments at two pressures near 2 and 13 atm and temperatures from 1182 to 2017 K. Tests were completed using the absorption coefficient correlation on stoichiometric mixtures of methane, methane and water, ethane, ethylene, and acetylene to compare against a comprehensive, detailed chemical kinetics mechanism which considers up through C5 hydrocarbons. Measurements of methane show good agreement in peak OH formation and ignition delay time when compared with the mechanism. Improvements can be made in the shape of the methane-oxygen OH profile, and sensitivity and rate of production analyses were performed with the mechanism to identify key reactions for tuning. Similar results were found for methane-water-oxygen mixtures with no difference in profile shape or ignition delay time noted. There is room for improvement between the mechanism and measured values of OH for ethane-, ethylene-, and acetylene-based mixtures, although interesting pre-ignition features are nonetheless captured relatively well by the mechanism. Uncertainty in the measurement comes from the inherent noise in the photomultiplier tube signal and is ±25-150 ppm for the 2-atm experiments and ±6-25 ppm for the 13-atm experiments.
Aul, Christopher J (2013). Measuring Hydroxyl Radicals during the Oxidation of Methane, Ethane, Ethylene, and Acetylene in a Shock Tube Using UV Absorption Spectroscopy. Doctoral dissertation, Texas A&M University. Available electronically from