Flame chemistry and stability of ammonia counterflow diffusion flames: Effects of hydrogen addition and oxygen enrichment

This study investigates ammonia (NH$_3$) counterflow diffusion flames (CDFs) under various conditions and evaluates contemporary kinetic mechanisms against experiments. First, ammonia CDFs with hydrogen additions and air are examined; second, pure ammonia flames with oxygen-enriched oxidizers are investigated; third, the influence of H$_2$ addition on ammonia-hydrogen-argon CDFs is studied. Argon dilution maintains identical maximum flame temperatures, thereby achieving decoupling of thermal and kinetic effects. Extinction strain rates (ESR) are used to assess flame stability and kinetic mechanism performance. Flame structure is characterized by temperature and optical diagnostics of key excited species, including OH*, NH*, NH$_2$*, and NO*. For selected CDFs, profiles of NH$_3$, H$_2$, and NO are quantified using a microprobe coupled with gas-phase diagnostics. Experimental results show good agreement with numerical predictions of ESR and temperatures in ammonia-hydrogen flames. However, under oxygen-enriched conditions, all mechanisms underpredict the peak temperatures. The mechanism proposed by Konnov [Combust. Flame 253 (2023) 112789] accurately reproduces NH* and NH$_2$* profiles, whereas deviations remain for NO*. ... mehrAt higher hydrogen admixtures, the local depletion of NH$_3$ and H$_2$ becomes spatially separated due to preferential hydrogen diffusion. The measured chemiluminescence peaks coincide with these depletion zones, forming a double-peak structure that indicates a second reaction zone dominated by H$_2$ oxidation. Numerical predictions reproduce NH$_3$ and H$_2$ depletion, but fail to capture the corresponding chemiluminescence double peaks. For OH*, the reaction OH* + H$_2$O ⇌ H$_2$O$_2$ + H suppresses double-peak formation, whereas the governing pathways for NH*, NH$_2$*, and NO* remain to be identified.