We report laser absorption measurements of NH, decay within the flame front region of rich, atmospheric pressure ammonia flames. These data are combined with earlier OH, NH, and NH, measurements to obtain new estimates for the oscillator strength of NH,. This value, f = 6.4 x for the 'Q,,7 line in the (0,9,0) + (0,O.O) vibrational band of the A'A, +-X'B, transition, suggests A&(NH) = 87 kcal/mol. The ammonia profiles were also combined with previous data on NO, NH, NH,, and OH to provide an extensive database at fuel equivalence ratios (+) of 1.28, 1.50, and 1.81 for comparison to our kinetic model predictions. This modeling used a one-dimensional flame code which explicitly accounts for the diffusional component in our flame experiments. Modeling results using a conventional mechanism predicted concentration profiles which deviated markedly from our observations. It was possible to obtain much more satisfactory fits by postulating reactions between various NH, ( i = 1, 2 ) species to form N-N bonds. The N,H, (,j = 1-3) species could then lose H atoms via dissociation to ultimately form N,. Inclusion of these reactions in the mechanism allowed us to predict concentration-distance profiles for five different species at three different equivalence ratios that are in good agreement with experiment. The most important component of this mechanism is the recognition that the NH, + NH, reactions dominate the kinetics in rich flames. A most satisfying aspect of these calculations is that the key rate constants in the NH, + NH, sequence were estimated using simple RRK theory.

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We have used laser diagnostics to probe atmospheric pressure ammonia–oxygen flames. Absorption from a tunable dye laser was used to measure concentration profiles of OH, NH, and NH2 radicals at fuel equivalence ratios φ of 1.28, 1.50, and 1.81. The absolute concentrations of OH and NH and the product of the NH2 concentration and the NH2 oscillator strength were derived as a function of height above the flat flame burner. These measurements indicate that the reaction NH2+OH?NH+H2O is equilibrated near the flame front, as well as in the post flame region. It was possible to use these measurements to derive an oscillator strength fi = (2.04±0.44)×10−4 for the PQ1,7 line in the (0,9,0)←(0,0,0) vibrational band of the A 2A1←X 2B1 transition of NH2. Rotational temperatures of OH were obtained from absorption measurements on a variety of rotational lines. These temperatures suggest the possibility of excess rotational energy in OH in the flame front regions of the φ = 1.28 and φ = 1.50 flames.

Laser diagnostics have been used to probe NO in atmospheric pressure flames. Laser induced fluorescence techniques (LIF) were used to measure relative concentration profiles of NO at fuel equivalence ratios φ=1.28, 1.50, and 1.81 in NH3/O2/N2 flames and φ=1.7 and 1.8 in CH4/air/O2 flames. Laser absorption measurements were made to derive an absolute concentration of NO in a lean NH3/O2/N2 flame. This measured NO concentration agreed well with the calculated equilibrium concentration. The fluorescence signals from rich flames were then calibrated by comparing the fluorescence signals to that of the lean flame where absolute concentrations were derived. In rich NH3/O2/N2 flames NO concentrations decay more rapidly throughout the burnt gases than one would expect from the conventional mechanism of ammonia oxidation. This suggests that new reactions such as NH2+NH2 and NH+NH2 to ultimately yield N2 are important in these rich flames. LIF measurements on the CH4/air/O2 flames were able to resolve the growth and decay of ‘‘prompt NO’’ within the flame front. The LIF technique is estimated to have a sensitivity of better than 1 ppm for NO in these atmospheric pressure flames.