The Effect of Mixture Variation and Initial Temperature on the NH2* Thickness of Spherically Propagating Laminar Ammonia Flames

Interest in ammonia (NH3) in combustion has increased in recent years as a carbon-free fuel alternative. Therefore, understanding its combustion characteristics is crucial. One way to increase the knowledge of ammonia combustion is by investigating the flame zone of a laminar flame. Using a high-spatial-resolution flame zone measurement technique developed by the current research group, the flame zone of different NH3-containing mixtures was measured experimentally. Those measurements were achieved by investigating spherically propagating flames using a chemiluminescence imaging diagnostic with the focus on NH2* profiles. The effect of the fuel mixture on the profile shape was investigated by examining two different mixtures. The first was an oxy-ammonia mixture consisting of NH3 + oxygen-enriched oxidizer where the oxygen (O2) concentration was varied from 25% to 40%. The second was a blend of NH3-H2 where the NH3 concentration (XNH3) was varied from 0.5 to 0.8. Additionally, the effect of the initial temperature was investigated by varying it from 293 to 373 K for three different mixtures, namely NH3 + (35% O2 + 65% N2), (0.7 NH3 + 0.3 H2) / air, and (0.45 H2 + 0.4 NH3 + 0.15 N2) / air. In all investigated mixtures, the initial pressure was fixed at 1 atm and the equivalence ratio was fixed at Φ = 1.0. The study revealed that increasing the O2 concentration in the oxy-ammonia mixture produced thinner flames. On the contrary, increasing the XNH3 in the NH3-H2 blend produced slightly thicker flames. Varying the initial temperature has two different responses for the three designated mixtures. In the oxy-ammonia mixture and the NH3-H2 blend, increasing the initial temperature resulted in the flame being thinner. On the other hand, increasing the initial temperature produced a slightly thicker flame for the H2-NH3-N2 blend. The predicted NH2* profile thicknesses from chemical kinetics agree with the measurements except for the H2-NH3-N2 blend, where the kinetics model underpredicted the thickness by a significant difference.