Simultaneous planar laser-induced fluorescence measurement of reactant NH3, radical NH, and pollutant NO in ammonia-hydrogen flames using a single dye laser

Abstract

The stability, local extinction, and NOx production of ammonia-hydrogen (NH3-H2) flames are significantly impacted by turbulence-chemistry interactions. A powerful technique to study these interactions is planar laser-induced fluorescence (PLIF). However, multi-scalar PLIF typically requires complex and expensive systems, with multiple laser sources. This study proposes a novel PLIF technique using a single dye laser to perform simultaneous, single-shot imaging of the reactant NH3, radical NH, and pollutant NO in NH3-H2 flames. According to the excitation scans and the emission spectra of NH3, NH, and NO, three wavelength couples can be used to image the three species simultaneously via two-photon excitation of NH3C'−X (2,0), single-photon excitation of NH A3Π−X3Σ− (0,0) near 304 nm, and single-photon excitation of NO A2Σ+−X2Π (0,1) near 237 nm. Wavelengths near 237 nm and 304 nm are obtained simultaneously with only one dye laser by combining outputs of the frequency-doubling and frequency mixing units. NH3-, NH-, and NO-PLIF imaging performance is analyzed by quantifying the signal-to-noise ratios and detection limits. This technique is then used to visualize the structure of premixed and non-premixed flames over wide ranges of NH3 and H2 concentrations in the fuel blend. Results show that the premixed NH3-H2 flames have a compact structure, while the non-premixed flames exhibit a large gap between NH3- and NH-PLIF layers, where ammonia undergoes significant decomposition before reaching the reaction layer. The inner edge of the NO-PLIF layer overlaps well with the reaction layer represented by the NH-PLIF layer in all flames. The fully developed turbulent structure downstream of the NH3-H2 flame causes pinching-off of the premixed flame front and local extinction of the diffusion flame front.