Ignition and flame development of high-pressure liquid ammonia spray combustion with simultaneous high-speed OH* and NH2* chemiluminescence imaging

Abstract

Liquid ammonia, benefiting from its convenient storage and high hydrogen content, has gained widespread attention as a carbon-free fuel for combustion devices to achieve carbon emission reduction. In this study, the ignition and flame development characteristics of liquid ammonia spray flame are analyzed with simultaneous high-speed OH*, NH₂* chemiluminescence and flame luminosity imaging. The test is conducted at the ambient pressure of 3 MPa, the ambient temperature of 950 K, and the injection pressure of 40 MPa. The results revealed that liquid ammonia spray flame can be divided into four stages: 1) the ignition process with the appearance of auto-ignition kernels at the jet front; 2) the flame propagation process with auto-ignition kernels expanding to the central spray region; 3) the fully-developed combustion process with the flame filling the core region; 4) the post-combustion process with the flame area decreasing rapidly. OH* signals were first observed at the jet front, and NH₂* signals were observed after OH* signals appeared in aggregated form. Throughout the combustion process, OH* had a wide distribution and a long duration, while the NH₂* not only appeared later but dissipated earlier, and the distribution was smaller than the OH*. Chemical kinetic analysis showed that the primary elementary reactions to produce OH at the ignition moment were O+H₂O=2OH, H+O₂=OH+O, and H₂+O=H+OH, while NH₂ was mainly formed through NH₃+OH=NH₂+H₂O. It was worth noting that after the combustion end, sporadic flames and NH₂* signals can still be observed in the jet region.