Experimental study of nitric oxide distributions in non-premixed and premixed ammonia/hydrogen-air counterflow flames

This study reports an experimental investigation of quantitative Nitric Oxide (NO) distribution in both premixed and non-premixed NH3/H2-air flames using a counterflow burner at atmospheric pressure. One-dimensional (1D) NO laser-induced fluorescence (LIF) spectroscopy and Raman/Rayleigh spectroscopy were conducted to accurately resolve the quantitative 1D NO profile in terms of mixture fraction, temperature, and physical space. We calibrated a saturated NO-LIF model in 5 premixed lean H2/N2/NO-air flames with different seeded NO levels in a McKenna burner and validated its accuracy in three H2-N2-NO-air counterflow diffusion flames. The overall uncertainty of NO quantification was less than 90 ppm. Our measurements were compared with simulations using different ammonia chemical kinetic models, revealing that current models have over 30% uncertainty in predicting peak NO concentrations (mole fraction) in 1D non-premixed and premixed flames and over 100% uncertainty in lower temperature regions. In premixed flames, measured NO concentrations fell within the intermediate range of current chemical kinetic models at lean and stoichiometric conditions, but were lower than the models at rich conditions. In non-premixed flames, all models overestimated the peak NO concentrations by more than 1000 ppm. It is noted that the measured peak NO concentrations increased with higher NH3/H2 ratios (from 4/6 to 8/2), strain rates (from 80 to 140 1/s), and N2 dilution ratios in a 1:1 NH3/H2 mixture (from 0 to 30%). Although most models could qualitatively predict the trends, they were inaccurate in quantifying NO. Additionally, the measured width of the NO profile in mixture fraction space expanded with increasing NH3/H2 ratio, N2 dilution ratio, and strain rate. While models could qualitatively predict this behavior, they consistently underestimated NO in the fuel-rich, lower-temperature region, resulting in a narrower NO profile width. The Manna model showed a better prediction of NO distribution in the fuel rich portion of non-premixed flames, accounting for NH3-NO interactions at lower temperatures. These findings highlight the critical need to improve models to accurately predict NO concentrations in ammonia-containing flames and their behavior in fuel rich regions.