Abstract
The evolution of the hydroxyl radical in its ground state (OH) and excited state (OH∗) during ammonia (NH3) oxidation was studied behind reflected shocks in a shock tube. Three equivalence ratios were studied (0.5, 1, and 2), with the reactant mixtures of NH3 and oxygen (O2) dilute in 99% argon, over the temperature range 1910 to 2510K, at pressures near 1.4atm. A mid-infrared laser-absorption diagnostic was used to validate pre-shock NH3 concentrations before each experiment. An ultraviolet laser-absorption diagnostic targeting a strong rovibronic transition of OH enabled high-sensitivity, quantitative measurements of OH time-histories, and an emission diagnostic enabled simultaneous, qualitative time–history measurements of OH∗. Post-reflected-shock time-histories were analyzed to extract characteristic ignition timescales, OH rise slopes, and OH post-ignition concentrations, all of which are suitable parameters for kinetic model validation. Measured parameters, including OH and OH∗ ignition delay times (IDTs), revealed key distinctions between ground-state and excited-state OH formation timescales that change across the parameter space studied in this work. Measurements were compared against composite chemical kinetic models assembled from two base oxidation mechanisms, three pyrolysis submodels, and two OH∗ submodels. One combination was found to best predict the experimental data, though no model successfully predicted all measured parameters at all conditions. Detailed rate-of-production analysis revealed that reaction of OH with NH3 serves to depress early-time populations of OH at stoichiometric and lean conditions, leading to earlier OH∗ IDTs relative to OH IDTs. One of the two studied OH∗ submodels was found to better predict measured OH∗ time-histories and IDTs, which was attributed to an improved rate and third-body efficiency for the reaction ▪ . Altogether, the measurements presented in this work represent a valuable source of validation data for NH3 model development and emphasize the utility of laser-absorption diagnostics for such validation.
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