OH and NO profiles in premixed NH3/O2/N2 low-pressure flames measured by calibrated-LIF: Comparison with modeling
Nour El Baba, Pascale Desgroux, Nathalie Lamoureux
Abstract
To adopt the best NOx reduction strategies for ammonia flames, the use of reliable kinetic models is essential. Many mechanisms were developed and validated on experimental data obtained in different reactors. However, to date, very few experiments were carried out in flames with reactive mixtures containing only NH3 as fuel, which limits the evaluation of the performance of kinetic models under realistic combustion conditions. The purpose of this work is to partially fill this gap by providing a new set of reliable species profiles obtained in low-pressure premixed NH3/O2/N2 flames stabilized on a porous burner, with an equivalence ratio ranging from 0.87 to 1.33. NO and OH species were measured in situ by laser-induced fluorescence (LIF). The temperature was measured by multiline NO LIF thermometry. NO-LIF profiles, performed on the Q2(26.5) transition of the A-X(0–0) band near 225.6 nm, were corrected for laser energy attenuation caused by absorption of NH3 in this spectral range. NO-LIF signals were converted in absolute mole fraction at a selected height above the burner by comparing their ratio with the LIF signal measured in a reference methane flame, in which the mole fraction was determined using the standard addition method. OH LIF signals were calibrated using absorption technique in the NH3 flames. Six recent kinetic mechanisms were evaluated against this new dataset. Significant discrepancies, reaching 50% were observed both between models and in relation to experimental profiles. While the shape of the experimental OH profiles is well predicted, the 6 models predict a constant NO profile in the burnt gases of the nearly stoichiometric and lean flames, in contrast to the experimental profiles, which show a 37% decrease. The analysis of production rate and sensitivity did not elucidate the kinetic origin of this deviation, but it did suggest several avenues for future investigation.