Autoignition of Oxygen-Enriched Ammonia Combustion in a Turbulent Mixing Layer under Gas-Turbine-like Conditions

In this study, the autoignition of oxygen-enriched ammonia diffusion flames under gas turbine-like conditions is investigated using two-dimensional (2D) direct numerical simulation (DNS) and carefully designed zero-dimensional (0D) simulations with a detailed reaction mechanism. Three oxygen concentrations (25, 30, and 35%) are considered in the oxidizer stream, and the air (21% of oxygen) condition is also calculated as a reference. The zero-dimensional calculations indicate that there is a “most-reactive mixture fraction” (Z_MR) independent of the oxygen concentration for the autoignition of oxygen-enriched ammonia flame, which is still valid in the 2D-DNSs. The autoignition process in the turbulent mixing layer could be divided into inert mixing, preignition, and postignition stages. As the oxygen concentration increases, the periods of inert mixing and preignition sages are shortened, resulting in earlier autoignition. The ignition kernels are located at regions of the mixture fraction value of the Z_MR and low scalar dissipation rates (SDR). As the oxygen concentration increases, autoignition kernels could form at a higher SDR, indicating enhanced combustion stability. NH and NH₂ can be regarded as suitable candidates for marking the heat release rate (HRR) of oxygen-enriched ammonia flames. NO formation is enhanced as the oxygen concentration increases, which is because both the NO production (thermal, HNO, and NH_i) and consumption (N₂O) pathways are enhanced with increasing oxygen concentration; the increment in production is more significant than that in consumption.