Joint numerical and experimental study of NO emissions in dual swirl confined hydrogen flames

Large Eddy Simulations (LES) play a significant role in the development of injection systems, and new methods need to be carefully validated for hydrogen combustion, especially for NO emissions. Measuring temporally and spatially resolved local NO concentrations using Laser-Induced Fluorescence (NO-LIF), however, remains challenging in confined configurations. In this work, NO emissions are investigated with LES and NO-LIF in dual-swirl hydrogen-air flames. Two operating conditions are studied, featuring two distinct stabilization modes: an attached flame (A - 2.5 kW) and a lifted flame above the injector lips (L - 5.0 kW). Overall, satisfactory agreement is obtained between experiments and LES. LES identify the main NO production regions and accurately capture the location of peak NO formation for both flame configurations. For the anchored flame (A), however, temperature discrepancies lead to localized quantitative deviations in NO molar fractions. In contrast, for the lifted flame (L), both the spatial distribution and the NO levels are predicted accurately. The paper discusses the effects of models used to describe chemistry, molecular transport, radiation and heat losses on the temperature and NO profiles. Flamelet theory and one-dimensional strained flame computations are also used to describe the effects of strain on the various flame structures observed in the 3D LES. Novelty and significance statement In confined hydrogen-air flames, existing experimental temperature and NO data are almost always restricted to exhaust measurements, which limits LES validation where detailed fields are required. In the present work, local NO molar fractions measured using NO-LIF are compared to LES for the first time in a confined swirled hydrogen burner. Temperatures obtained from Raman scattering are also compared to thermocouple measurements at several locations within the chamber to evaluate their impact on NO-LIF measurements. The comparison with LES demonstrates that the simulations accurately predict the NO distribution in the lifted-flame configuration, for which the temperature field is well captured. The paper also analyzes which factors control the accuracy of NO prediction, showing that prediction of temperature is probably the first factor needing attention while chemical schemes seem to be performing well.