This study numerically investigates the stability limits of a co-axial ammonia/oxygen-enriched air non-premixed flame using direct numerical simulations with detailed chemistry. The analysis shows that flame stability is primarily governed by the momentum flux ratio (J), with distinct extinction mechanisms occurring across different J regimes. A stable, counterflow-like flame is sustained only within a specific range (1 ≤ J ≤ 4). Outside this range, extinction results from insufficient supply of the lower-momentum reactant—oxidizer at high J and fuel at low J. For J < 1, a unique flame lift-off phenomenon is observed, driven by the formation of premixed gases and linked to ammonia’s low burning velocity. Within the stable J range, injection velocity imposes additional stability constraints. At high velocities, extinction is caused by excessive flame stretch, while at low velocities, significant heat loss to the cold burner rim overwhelms heat generation, leading to quenching. These findings are summarized in a stability map that delineates the operable ranges of J and injection velocity, offering valuable guidance for designing stable and efficient ammonia combustors.
JUNG et al. (Wed,) studied this question.