Laser induced fluorescence and absorption measurements of NO in NH3/O2 and CH4/air flames

Abstract

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.