Laser-saturated and linear laser-induced fluorescence measurements of nitric oxide in counterflow diffusion flames under non-sooting oxygen-enriched conditions

We report quantitative, spatially resolved, laser-induced fluorescence (LIF) measurements of NO concentration ([NO]) in non-sooting, oxygen-enriched counterflow diffusion flames at atmospheric pressure. Three different flames containing 25%, 50%, and 100% CH4, respectively, in the fuel stream and 100%, 50%, and 35% O-2, respectively, in the oxidizer stream are investigated at a global strain rate of 20 s(-1). Excitation of NO is achieved at 224.45 nm in the gamma(0,0) band and detection is performed in a 2-nm region centered at 235.78 nm in the gamma(0,1) band. Excitation scans ensure no spectral background problems under the high-temperature conditions of oxygen enrichment. Detection scans obtained at the excitation wavelength verify no interferences from any other species, particularly O-2. Quantitative axial profiles of [NO] are presented for all three flames. Linear LIF measurements are compared to laser-saturated fluorescence (LSF) measurements to assess the utility of a broadband LSF technique at temperatures approaching 3000 K. Numerical computations for all counterflow diffusion flames are conducted using OPP-DIF with GRI Mech-3.0. The effect of gas-phase radiation is considered in the modeling, The results indicate excellent agreement between the measured and predicted NO concentrations for all three flames. A comparison of the linear LIF and LSF measurements also yields excellent agreement.