Emission Characteristics of NO*, OH*, and NH* in Ammonia/Hydrogen Flames

Ammonia/hydrogen/nitrogen (NH3/H2/N2) blends are used to increase reactivity when using NH3 as a fuel in combustion systems. Two recent studies have investigated the UV emission spectra of premixed swirling NH3/H2-air flames and O2-enriched laminar NH3-air flames. Nevertheless, the high-resolution UV emission spectra of premixed laminar NH3/H2/N2-air flames at various equivalence ratios (Φ) and flame temperatures at atmospheric pressure are not reported in detail. Hence, the objective of this study is to document high-resolution UV emission spectra of NH3/H2/N2-air blended flames. A fuel blend comprised of 40% NH3, 45% H2, and 15% N2 by volume was used, which has similar combustion characteristics as methane (CH4). Two high-resolution gratings inside a UV-enhanced spectrometer were used to record spectra from NH3/H2/N2-air flames stabilized on a McKenna flat-flame burner at various Φ and varying flame temperatures using three different O2/N2 oxidizer ratios at atmospheric pressure. The UV emission spectra mainly consisted of NO*, OH*, and NH* emissions. The NO* and NH* signals increase and subsequently decrease, whereas the OH* signals decrease as Φ increases from 0.8 to 1.2. The NO*, OH*, and NH* signals decrease as the percentage of O2 to N2 increases from 15% to 30%. This reduction in signal for all three species may be attributed to the decrease in the reaction zone size. NO* and OH* emissions are then compared against simulations to identify the effects of flame temperature on the spectral features and intensity regions. The simulated spectra at a higher temperature than the experimental flame resulted in better agreement with NO* emissions. However, the simulated spectra at two different temperatures had good agreement with the OH* signals in certain wavelength regions. These measurements aim to provide comprehensive experimental data regarding the influence of flame temperature on the emissions of NO*, OH*, and NH*.