Nitric oxide concentration profiles in atmospheric-pressure flames using electronic-resonance-enhanced coherent anti- Stokes Raman scattering (ERE-CARS)
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We report measurements of nitric oxide (NO) in atmospheric-pressure flames using visible pump (532 nm) and Stokes (591 nm) beams with an ultraviolet probe beam (236 nm) near an electronic resonance, giving significantly enhanced CARS signal at 226 nm. For a hydrogen-air flame stabilized over a Hencken burner, good agreement is obtained between ERE-CARS measurements and flame computations using UNICORN, a two-dimensional flame code. Excellent agreement between measured and calculated NO spectra is obtained for heavily sooting acetylene-air flames on the same Hencken burner. The measured shapes of NO concentration profiles determined from ERE-CARS spectra without correcting for collisional effects is in excellent agreement with that predicted using the OPPDIF code in conjunction with GRI 3.0 kinetics for a laminar, counterflow, non-premixed hydrogen-air flame. Effects of fuel-stream dilution (nitrogen and carbon dioxide) on measured NO concentrations are also studied in the counter-flow configuration. For diluted flames, comparisons between measured ERE-CARS signals and computed number densities show good spatial agreement and their relative magnitudes match well. Counter-flow flames with various hydrogen levels in the fuel stream (pure oxygen in the oxidizer stream) and various oxygen levels in the oxidizer stream (pure hydrogen in the fuel stream) are investigated to simulate fuel-rich and oxygen-rich flames and an optimum NO level is found. Pathway and sensitivity analyses are implemented to understand NO formation under these conditions. The current results establish the utility of ERE-CARS for detection of NO in flames with large temperature and concentration gradients as well as in sooting environments.
