Ammonia Pyrolysis behind Reflected Shock Waves: Multispecies Measurements and Modeling

During the oxidation of ammonia (NH3), pyrolysis reactions strongly influence ignition delay times, intermediate radical populations, and production of nitrogen oxides (NOx), none of which are adequately predicted by NH3-containing chemical kinetic models. Moreover, simultaneous, multispecies data sets relevant to NH3 pyrolysis, useful for elucidating areas for model refinement, are generally lacking. Thus, the pyrolysis of 0.1-1.0% NH3 dilute in argon was studied behind reflected shock waves (2100-3500 K, pressures near 1 atm) using a combination of laser absorption diagnostics to measure postshock imidogen radical (NH, 336.1 nm), amino radical (NH2, 597.4 nm), and NH3 (225.3 nm) concentrations, as well as preshock NH3 concentrations (10.4 μm). In this way, the first simultaneous measurements of NH, NH2, and NH3 during NH3 pyrolysis were obtained, and refined measurements of the oscillator strength of the targeted NH2 absorption feature were enabled. Key discrepancies between these multispecies data and existing model predictions motivated the development of a refined NH3 pyrolysis model, resulting in improved predictions of measured time-histories. Predictions of late-time NH2 consumption and early-time NH formation were significantly improved. Remaining model discrepancies motivate the generation of similar multispecies data sets using hydrazines and hydrazoic acid as precursors for important NH3-relevant species.