Electronic-Resonance-Enhanced (ERE) Coherent Anti-stokes Raman Scattering (CARS) detection of minor species in reacting flows
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Electronic-resonance-enhanced coherent anti-Stokes Raman scattering (ERE-CARS) is implemented to measure nitric oxide (NO) concentration profiles in laminar, counter-flow, non-premixed hydrogen flames at a global strain rate of 20 s-1. Visible pump (532 nm) and Stokes (591 nm) beams combined with an ultraviolet probe beam (236 nm) near an electronic resonance of NO give significant enhancement of the CARS signal at 226 nm. Spectral scans are recorded between the fuel and oxidizer nozzles by scanning the Stokes beam over spectral width of the Q1(13.5) Raman transition. Flames with three different levels of nitrogen and carbon dioxide dilution in the fuel stream were investigated. Comparisons between measured ERE-CARS signals and computed number densities show good agreement with respect to the shape and the relative magnitude of NO concentration profiles. Spectral scans at the position of peak NO concentrations are obtained for counter-flow flames containing 30% to 90% hydrogen in the fuel stream (nitrogen diluent; pure oxygen in oxidizer stream) as well as 30% to 90% oxygen in the oxidizer stream (nitrogen diluent; pure hydrogen in fuel stream) to simulate both fuel-rich and oxygen-rich flames. A maximum level of NO concentration is found near 40% dilution in both cases. Excellent agreement is observed between measured NO signals and predicted NO number densities. Pathway and sensitivity analyses are conducted to prove that thermal NO dominates over the N2O, NNH and prompt pathways for the conditions of this investigation. These results support the robustness of the ERE-CARS technique for detection of NO in flames and lay the foundation for future applications to coal syngas flames as well as to high-pressure gas turbine combustors. Our measurements are also relevant for future efforts to understand NOx formation and destruction when burning high-hydrogen content fuels under oxygen-rich conditions.
