The effects of burner stabilization on Fenimore NO formation in low-pressure, fuel-rich premixed CH_4/O_2/N_2 flames

摘要

We investigate the effects of varying the degree of burner stabilization on Fenimore NO formation in fuel-rich low-pressure flat CH_4/O_2/N_2 flames. Towards this end, axial profiles of flame temperature and OH, NO and CH mole fractions are measured using laser-induced fluorescence (LIF). The experiments are performed at equivalence ratios between 1.3 and 1.5. The flame temperature is seen to decrease by 200-300 K, with a concomitant decrease in OH mole fraction, upon reducing the total flow rate from 5 to 3 L/min, thus increasing stabilization. At equivalence ratios between 1.3 and 1.5, this decrease in flow rate lowers the maximum CH mole fraction by a factor of 2, and the NO mole fraction by ～40% in all flames studied. Integrating the reaction rate for CH + N_2 to estimate Fenimore NO formation, using the rate coefficient in GRI-Mech 3.0, and the measured temperatures and CH profiles show very good agreement with the measured NO mole fraction for φ = 1.3 and 1.4, supporting the current choice for this rate. This agreement also shows that the increase in residence time caused by increased stabilization is an important factor in the ultimate impact of the changes in CH mole fraction on NO formation. The results at φ = 1.5 suggest that substantial quantities of fixed nitrogen species, e.g., HCN, are only slowly oxidized in the post-flame zone under these conditions, leading to a significant discrepancy between the measured NO mole fraction and that obtained by integrating over the CH profile. Detailed calculations using GRI-Mech 3.0 predict the experimental results at φ = 1.3 nearly quantitatively, but show increasing differences with the measurements for both CH and NO profiles with increasing equivalence ratio.