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Understanding N2O formation and consumption in ammonia combustion is crucial in realizing the impact of ammonia as an alternative fuel to mitigate the impact of climate change. This study demonstrates the feasibility of using Raman spectroscopy for in-situ N2O measurements in ammonia flames. Raman spectra were acquired along a NH3/H2/N2-air flame in a laminar opposed jet burner, using a pulsed laser combined with a three-disk rotating shutter system to suppress the luminous flame background. This setup enabled the clear detection of the N2O Raman spectrum. Raman libraries of N2, O2, H2, NO, and N2O were fitted to the spectra using a newly developed fitting routine. This yielded qualitative N2O mole fractions along the flame that align closely with numerical simulations based on recently published chemical reaction models for ammonia oxidation, paving the way for future quantitative N2O measurements in ammonia flames. Since no prior libraries for temperature-dependent N2O Raman spectra were available, a methodology for its simulation is introduced. High-resolution N2O spectra were acquired between 295 and 1091 K as validation data for the simulation. Despite minor deviations, the simulation effectively captures the spectral shape and temperature dependence of the Raman cross sections, enabling its use in the spectral fitting routine towards quantitative in-situ concentration measurements.
Polyhedral Bunsen flames, induced by hydrodynamic and thermo-diffusive instabilities, are characterized by periodic trough and cusp cellular structures along the conical flame front. In this study, the effects of flow velocity, hydrogen content, and equivalence ratio on the internal cellular structure of premixed fuel-lean hydrogen/methane/air polyhedral flames are experimentally investigated. A high-spatial-resolution one-dimensional Raman/Rayleigh scattering system is employed to measure the internal scalar structures of polyhedral flames in troughs and cusps. Planar laser-induced fluorescence of hydroxyl radicals and chemiluminescence imaging measurements are used to quantify the flame front morphology. In the experiments, stationary polyhedral flames with varying flow velocities from 1.65 to 2.50 m/s, hydrogen contents from 50 to 83%, and equivalence ratios from 0.53 to 0.64 are selected and measured. The results indicate that the positively curved troughs exhibit significantly higher hydrogen mole fractions and local equivalence ratios compared to the negatively curved cusps, due to the respective focusing/defocusing effect of trough/cusp structure on highly diffusive hydrogen. The hydrogen mole fraction and local equivalence ratio differences between troughs and cusps are first increased and then decreased with increasing measurement height from 5 to 13 mm, due to the three-dimensional effect of the flame front. With increasing flow velocity from 1.65 to 2.50 m/s, the hydrogen mole fraction and local equivalence ratio differences between troughs and cusps decrease, which is attributed to the overall decreasing curvatures in troughs and cusps due to the decreased residence time and increased velocity-induced strain. With increasing hydrogen content from 50 to 83%, the hydrogen mole fraction and local equivalence ratio differences between troughs and cusps are amplified, due to the enhanced effects of the flame front curvature and the differential diffusion of hydrogen. With increasing equivalence ratio from 0.53 to 0.64, a clear increasing trend in hydrogen mole fraction and equivalence ratio differences between troughs and cusps is observed at constant flow velocity condition, which is a trade-off result between increasing effective Lewis number and increasing curvatures in troughs and cusps.