Direct numerical simulation of thermodiffusively unstable lean NH3/H2-air flames

Carbon-neutral fuels and energy carriers are crucial for decarbonization, and ammonia has recently gained attention as a promising candidate due to its potential as a hydrogen carrier and carbon-free fuel. However, ammonia combustion poses challenges due to its low reactivity and high nitrogen oxide emissions, which require additional research efforts. In this contribution, we use a high-order, low-Mach number reactive flow solver to perform a direct numerical simulation of ammonia premixed combustion and shed light on intrinsic instabilities of the ensuing flame. The thermodiffusive stability limits of mixtures of technical interest are investigated and a numerical reconstruction of the dispersion relation of a target mixture is carried out. Then a direct simulation is performed on a medium-scale domain in order to investigate the impact of intrinsic instability on flame propagation and pollutants formation. The onset of cellular structures typical of intrinsically unstable flames is observed resulting in regions of super-adiabatic temperatures leading to enhanced pollutant formation. The data obtained can be further employed in the context of data-driven models for sub-grid modeling for large eddy simulations