Nitric oxide concentration measurements in low-temperature, premixed hydrogen-air stagnation flames at elevated pressures

Abstract

Understanding the formation of nitric oxide (NO) at elevated pressures closer to practical applications remains one of the challenges to improve nitrogen chemistry modelling. In the lean, premixed conditions favoured to mitigate the formation of thermal NO, secondary production pathways become significant. In an effort to better understand the importance of the NNH and N2O pathways, the present work reports NO concentrations obtained with laser-induced fluorescence (LIF) in near-stoichoimetric (ϕ=0.975), premixed, hydrogen-air stagnation flames at pressures of 2, 4, 6, and 8 atm. To minimize the formation of thermal NO and achieve stable flat flames, the premixed mixture is diluted with helium to maintain flame temperatures below 1800 K. Quantitative concentration measurements of ∼1.6–0.15 ppm are obtained as pressure, and dilution, increase. Particle tracking velocimetry (PTV) and multi-line NO-LIF thermometry provide velocity and temperature profiles, respectively, to complete the dataset. Experimentally-measured boundary conditions allow for direct comparison between simulations and experiments. Although discrepancy is observed between model predictions, recent thermochemical mechanisms exhibit a good agreement with the measured concentrations. Investigation of the formation mechanisms, performed with reaction pathway analysis, identifies the NNH pathway as the dominant production route with significant recycling of reactive nitrogen from NNH→N2O→N2. This recycling reduces with pressure as the pressure-dependent reaction of N2 with atomic oxygen is favoured. Limitations due to flame stability prevents investigations where a net production of N2O from N2 would be expected. This dataset, available for the development and validation of thermochemical mechanisms, is the first effort to measure NO concentration in supra-atmospheric hydrogen-air flames.