Comparison of saturated and linear laser-induced fluorescence measurementsof nitric oxide in counterflow diffusion flames

Quantitative measurements of NO concentrations ([NO]) have been obtained al ong the centerline of atmospheric ethane-air counterflow diffusion flames b y using saturated and linear laser-induced fluorescence (LIF). In particula r, four flames with strain rates varying from 5 to 48 s(-1) were investigat ed while maintaining a constant fuel dilution in all cases. The utility of a broad-band laser-saturated fluorescence (LSF) technique is assessed by co mparison to similar measurements of NO using linear LIF. The linear LIF mea surements are corrected for variations in the local electronic quenching ra te coefficient by using major species profiles generated by a diffusive fla me code and available correlations for the quenching cross-sections of NO. The corrected LIF profiles compare favorably with the LSF profiles. A four- level model is used to investigate the effects of rotational energy transfe r (RET) on the LSF measurements. The excellent comparison between the quenc hing-corrected linear LIF and the LSF measurements at locally fuel-lean to greater than stoichiometric mixture fractions verifies the validity of the LSF technique for these conditions. The slight but consistent discrepancy b etween the LSF and linear LIF measurements at local equivalence ratios abov e 1.6 may be attributed to a change in the collisional branching ratio from lean to rich stoichiometries and/or the need for further work on the elect ronic quenching cross-sections required for quantitative NO measurements un der fuel-rich conditions. (C) 1999 by The Combustion Institute.