Back to basics – NO concentration measurements in atmospheric lean-to-rich, low-temperature, premixed hydrogen–air flames diluted with argon

Ideally, nitric oxide (NO) production pathways would be measured individually to understand the formation mechanisms at a fundamental level. Unfortunately, the four production routes in hydrocarbon combustion cannot be fully decoupled. Hydrogen combustion at low flame temperatures eliminates prompt-NO and mitigates thermal production, such that only the N₂O and NNH pathways remain as significant production routes. The H₂/O₂ system, whose base chemistry has been studied in great detail, offers an excellent platform to validate nitrogen chemistry by limiting the possibility of error propagation during model calibration. The current work presents measurements of velocity, temperature, and NO concentration in premixed, jet-wall stagnation, hydrogen–air flames at atmospheric pressure, diluted with argon to maintain adiabatic flame temperatures below 1800 K. Measurements of reference flame speeds, Su,ref, obtained with particle tracking velocimetry, highlight the modeling differences in H₂/O₂ chemistry from a selection of thermochemical mechanisms, especially in lean flames affected by preferential diffusion. Laser induced fluorescence measurements in lean-to-rich flames (Ø = 0.7–1.5) yield concentrations of NO from 2 to 0.5 ppm, respectively. Simulated NO profiles cover one order of magnitude in predicted signal intensity. Fortunately, recent mechanisms with accurate descriptions of the N₂O and NNH pathways predict NO concentrations within experimental uncertainties for multiple operating conditions.