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

The amidogen radical (NH 2 ) is a crucial species in ammonia decomposition and de-NOx processes within ammonia flames. Accurate in situ measurements of NH 2 radicals, particularly to determine their spatial distribution in turbulent flames , are scarce. This is mainly due to the lack of methods capable of imaging NH 2 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, NH 2 , NH, and NH 3 in flames with a single dye laser. Effective NH 2 -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 NH 3 (NH 3 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 NH 2 , NH, and NH 3 . 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 NH 2 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 NH 2 -PLIF with an NH 2 concentration of around 800 ppm. The fluorescence signals of NH 2 and NH 3 exhibit partial spectral overlap , but they are spatially separated in premixed flames . Tests whereby NH 2 - and NH 3 -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.