COMPARISON OF EXPERIMENTAL AND CALCULATED STRUCTURES OF AN AMMONIA NITRIC-OXIDE FLAME - IMPORTANCE OF THE NH2+NO REACTION

Using molecular beam sampling mass spectrometry the structure of a quasi-equimolar ammonia-nitric oxide flat flame burning at 54 torr has been determined. From the mole fraction profiles of major species (H, H-2, NH2, NH3, H2O, N2, NO, Ar, N2O) and from the rates of NO consumption or N2 formation, the overall rate constant k1 of the reaction NH2 + NO --> products (r.1) has been measured between 1500 and 2100 K. In that temperature range the Tate coefficient k1 remains practically constant 3 x 10(12) cm3 mol-1 s-1. The ''pool flux'' concept, based on the variation of the total bonds flux throughout the flame, has allowed us to determine the impact of the decomposition reaction in this flame. This event can be ascribed to the process NH2 + NO --> N2 + H + OH (r.1c) or through the decomposition of N2H originating from NH2 + NO --> N2H + OH (r.1b) The branching ratio beta = (k1c/k1) has been deduced and varies from 0.5 at 1500 K to 0.8 at 2150 K. The reaction r.1c plays an essential role on the burning abilities of NH3/NO mixtures by providing the essential hydrogen atoms and hydroxyl radicals for the primary attack of NH3. Reaction r.1c replaces the classical H + O2 branching reaction occurring in all O2 containing systems. The flame structure has been simulated using the PREMIX code with the experimental temperature profile as an input parameter. Calculated data compared with experimental ones agree very well when Bian et al.'s modified model is used while substantial disagreements are noticed with Miller and Bowman's mechanism.