Ammonia Pyrolysis behind Reflected Shock Waves: Multispecies Measurements and Modeling

During the oxidation of ammonia (NH 3 ), pyrolysis reactions strongly influence ignition delay times, intermediate radical populations, and production of nitrogen oxides (NO x ), none of which are adequately predicted by NH 3 -containing chemical kinetic models. Moreover, simultaneous, multispecies data sets relevant to NH 3 pyrolysis, useful for elucidating areas for model refinement, are generally lacking. Thus, the pyrolysis of 0.1–1.0% NH 3 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 (NH 2, 597.4 nm), and NH 3 (225.3 nm) concentrations, as well as preshock NH 3 concentrations (10.4 μm). In this way, the first simultaneous measurements of NH, NH 2, and NH 3 during NH 3 pyrolysis were obtained, and refined measurements of the oscillator strength of the targeted NH 2 absorption feature were enabled. Key discrepancies between these multispecies data and existing model predictions motivated the development of a refined NH 3 pyrolysis model, resulting in improved predictions of measured time-histories. Predictions of late-time NH 2 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 NH 3 -relevant species.