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

Abstract

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.