Simultaneous PLIF imaging of NH₂, NH, and NH₃ in ammonia-hydrogen-nitrogen flames using a single dye laser

The amidogen radical (NH₂) is a crucial species in ammonia decomposition and de-NOx processes within ammonia flames. Accurate in situ measurements of NH₂ radicals, particularly to determine their spatial distribution in turbulent flames, are scarce. This is mainly due to the lack of methods capable of imaging NH₂ with a sufficiently high signal-to-noise ratio and in a time-resolved fashion. This study proposes a novel multi-species imaging method based on planar laser-induced fluorescence (PLIF) capable of simultaneously imaging, in single-shot, NH₂, NH, and NH₃ in flames with a single dye laser. Effective NH₂-PLIF is achieved by probing the A²A₁(0,12,0) ← X²B₁(0,2,0) transitions with a laser beam at ∼609 nm. This beam is available as the residual of a frequency-doubling unit required to yield another beam at ∼304.5 nm and that is used to excite both NH (A³Π-X³Σ⁻ (1,0) band) and NH₃ (NH₃ C'-X (2,0) band). According to the excitation scans, two wavelength couples, 609.474/304.737 nm and 609.726/304.863 nm, are suggested to simultaneously excite NH₂, NH, and NH₃. This method is used to visualize the 2D structure of premixed laminar and turbulent ammonia-hydrogen-nitrogen flames with different equivalence ratios and fuel compositions. Consistent with expectations built from 1D simulations, results show that the NH₂ layer partially overlaps with that of NH and primarily sits on the reactants’ side of the NH layer. An SNR near 4 is achieved for NH₂-PLIF with an NH₂ concentration of around 800 ppm. The fluorescence signals of NH₂ and NH₃ exhibit partial spectral overlap, but they are spatially separated in premixed flames. Tests whereby NH₂- and NH₃-PLIF signals are captured simultaneously on the same camera show that this method may be used to mark the location of the preheat zone in premixed ammonia flames with only one camera and one dye laser.