Flame/droplet interaction and combustion mode in a liquid ammonia jet flame

Ammonia combustion is considered an effective approach to reducing carbon emissions, with liquid ammonia combustion being a cutting-edge topic in ammonia research. Compared to gaseous ammonia, liquid ammonia simplifies fuel supply systems, reduces manufacturing and maintenance costs, and improves responsiveness. To enable investigation of the fundamental combustion processes relating to applications of liquid ammonia, this study conducts carrier phase direct numerical simulation (cp-DNS) of an experimental liquid ammonia jet flame. The research focuses on analyzing the flame structure, flame/droplet interactions, and combustion modes of liquid ammonia jet flame. The cp-DNS coupled with detailed chemistry is performed under the Eulerian-Lagrangian framework, employing an evaporation and flash combined phase change model. The numerical simulation results are validated against experimental data using particle image velocimetry (PIV), and a laser diagnostic system for simultaneous Mie scattering and OH planar laser-induced fluorescence (PLIF) measurements. Three modes of flame/droplet interactions in the liquid ammonia jet flame are identified and quantified: no interaction (Mode A), weak interaction (Mode B), and strong interaction (Mode C). Due to the rapid flash evaporation of liquid ammonia droplets, the dominant interaction mode is Mode A, with Modes B and C contributing to only 4.2% of the total flame surface. Mode B accounts for a higher proportion than Mode C in the liquid ammonia jet flame. Flame/droplet interactions are stronger in the downstream region compared to the upstream region. Furthermore, the result points out that the premixed combustion mode is dominant in the liquid ammonia jet flame. However, in the downstream region, flame/droplet interactions lead to a slight increase in the proportion of diffusion combustion. These findings provide a valuable insight for advancing liquid ammonia combustion modeling and practical system development. Novelty and significance statement This study presents the first carrier-phase direct numerical simulation (cp-DNS) of a liquid ammonia jet flame in a laboratory-scale burner, with validation against experimental measurements. The findings provide valuable insights into the mechanisms of flame/droplet interaction and the combustion modes of liquid ammonia flames. Three distinct modes of flame/droplet interaction are identified. The study further reveals that the combustion mode of liquid ammonia flames is predominantly premixed. This work enhances the fundamental understanding of liquid ammonia flame behavior and provides a solid foundation for future research on liquid ammonia combustion.