Intrinsic combustion instabilities in ammonia-hydrogen/methane non-premixed flames

Intrinsic combustion instabilities that onset in NH${}_3$–H${}_2$ and NH${}_3$–CH${}_4$ non-premixed flames are experimentally characterized. A unique research burner capable of creating a good approximation of the classical one-dimensional chambered non-premixed configuration is used, enabling direct comparison with theoretical stability models based on this simple configuration. Starting from a stable flame near the Burke-Schumann limit, the Damköhler number is gradually reduced by decreasing the fuel concentration, going through the marginal stability state where instabilities onset. Both CO${}_2$ and $N_2$ dilution are considered, and global stability limits are provided for a wide range of fuel blends. The instabilities are diffusive-thermal in nature, where it is shown that unstable NH${}_3$–CH${}_4$ flames exhibit pulsations in their reaction rates due to their large Lewis numbers. The characteristic pulsation frequency is highly dependent on the NH${}_3$ fraction in the fuel. A peculiar phenomenon is reported when H${}_2$ with a small Lewis number is added to the less mobile NH${}_3$ species. Superimposed cellular-pulsating instabilities form in NH${}_3$–H${}_2$ flames, which are thoroughly characterized as a function of the NH${}_3$ fraction.