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 qua si-equimolar ammonia-nitric oxide flat flame burning at 54 torr has be en 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 consumptio n 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 th at temperature range the Tate coefficient k1 remains practically const ant 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 a nd 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 prov iding the essential hydrogen atoms and hydroxyl radicals for the prima ry attack of NH3. Reaction r.1c replaces the classical H + O2 branchin g reaction occurring in all O2 containing systems. The flame structure has been simulated using the PREMIX code with the experimental temper ature profile as an input parameter. Calculated data compared with exp erimental ones agree very well when Bian et al.'s modified model is us ed while substantial disagreements are noticed with Miller and Bowman' s mechanism.