High-temperature laminar flame speed measurements of ammonia/methane blends behind reflected shock waves

Laminar flame speeds of ammonia and methane blends diluted in airgon (79 % Ar/21 % O2) were measured for the first time at temperatures above 500 K. Experimental temperatures ranged from 295 K in static, room temperature experiments to over 1200 K, enabled by the use of reflected shock waves. Experiments for estimating the minimum laser-spark-ignition energies necessary to ignite ammonia/methane blends were performed at room temperature. A monotonic decrease on the minimum ignition energy was observed with increasing methane concentration at all studied equivalence ratios. Over the full 295 – 1204 K temperature range, laminar flame speed measurements were compared against various models available in the literature. The models vary significantly for blends with large proportions of ammonia, whereas all models predict similar flame speeds for pure methane cases. Mixture-scaling was performed to convert fuel/airgon flame speed measurements to equivalent fuel/air data, and good agreement is found between literature results and the mixture-scaled data from the present work. Empirical correlations were constructed using the power-law and non-Arrhenius models, with the latter showing the best agreement with the measurements and literature values. Sensitivity analyses were performed to identify the most important reactions for various models at different levels of methane concentration. As the proportion of methane increases, important carbon-related reactions overtake key nitrogen reactions in controlling laminar flame speed.