Effects of pre-evaporation on spray characteristics and flame structures in liquid ammonia jet flames

Compared with gaseous ammonia, direct use of liquid ammonia (LNH₃) in gas turbines reduces system complexity, cost, and start-up time. However, LNH₃ is prone to flash boiling, and its spray is strongly influenced by droplet pre-evaporation, which modifies flame behavior. To quantify this effect, LNH₃ jet spray flames are studied by varying the ammonia–air mixing-section length to control the pre-evaporation level. The spray characteristics are measured using Particle/Droplet Image Analysis (PDIA), while the spray and flame structures are characterized by simultaneous Mie scattering and OH-PLIF diagnostics. Large-Eddy Simulations (LESs) are employed to investigate spray evaporation, mixing, and combustion in detail. Results show that increasing the mixing distance between LNH₃ and air disrupts the dense liquid core, reduces the number of irregular objects and ligaments near the nozzle, and accelerates droplet evaporation and heat absorption. The resulting evaporative cooling lowers the local temperature, suppressing downstream breakup and further evaporation of LNH₃. Pre-evaporation does not alter the dominant premixed combustion mode or the single-reaction-layer flame structure. Pre-evaporation introduces two competing effects: enhanced evaporation and mixing promote combustion, whereas evaporative cooling delays ignition and suppresses chemical reactions. This trade-off means flame structure varies nonlinearly with pre-evaporation, and a moderate pre-evaporation level does not reduce flame height. This study provides theoretical guidance for optimizing the structure of LNH₃ spray flames in gas turbines.