Comprehensive linear stability analysis for intrinsic instabilities in premixed ammonia/hydrogen/air flames

Two-dimensional direct numerical simulations of planar laminar premixed ammonia/hydrogen/air flames are conducted for a wide range of equivalence ratios, hydrogen (\(\rm H_2\)) fractions in the fuel blend, pressures, and unburned temperatures to study intrinsic flame instabilities (IFIs) in the linear regime. For stoichiometric and lean mixtures at ambient conditions, a non-monotonic behavior of thermo-diffusive instabilities with increasing (\(\rm H_2\)) fraction is observed. Strongest instabilities occur for molar (\(\rm H_2\)) fractions of 40%. The analysis shows that this behavior is linked to the joint effect of variations of the effective Lewis number and Zeldovich number. IFIs in ammonia/hydrogen blends further show a non-monotonic trend with respect to pressure, which is found to be linked to the chemistry of the hydroperoxyl radical \(\rm HO_2\). The addition of \(\rm NH_3\) opens new reaction pathways for the consumption of \(\rm HO_2\) resulting in a chain carrying behavior in contrast to its chain terminating nature in pure \(\rm H_2\)/air flames. Theoretically derived dispersion relations can predict the non-monotonic behavior for lean conditions. However, these are found to be sensitive to the different methods for evaluating the Zeldovich number available in the literature.