Simulation of single ammonia droplet combustion at elevated pressures

Ammonia ( NH 3 ) has gained significant attention as a carbon-neutral fuel. While most research on ammonia combustion has focused on its gaseous phase, there is growing interest in the direct combustion of ammonia sprays. This study presents, to the best of our knowledge, the first numerical investigation of single ammonia droplet combustion using the volume-of-fluid approach, with evaporation flux calculated without empirical models. Simulations are conducted under elevated-pressure conditions ( 0 . 75 − 3 MPa ), and results show good agreement with recent experiments at 1 . 5 MPa (Matsuura et al., Proc. Combust. Inst., vol. 40, 2024). To evaluate the impact of water absorption, three liquid-phase models are compared: no-absorption, diffusion-limited, and well-mixed. While water absorption imposes a blocking effect on evaporation, the concurrent rise in droplet temperature largely offsets it, resulting in only a modest change in droplet regression rates. In the diffusion-limited model, the temperature rise is particularly pronounced and can ultimately lead to droplet boiling. A prominent endothermic region, primarily driven by and dissociation, develops in the pre-flame zone and is intensified by high pressure and oxygen levels. Reaction pathway analysis identified six key pathways for kinetics, with the and pathways acting as sinks and the pathway as a major source. The Zeldovich mechanism plays a dual role, consuming in the fuel-rich pre-flame region and forming thermal post-flame. Critically, emissions exhibit a threshold-like dependence on pressure: at moderate oxygen levels, increasing pressure suppresses , whereas at high oxygen concentrations, it significantly enhances emissions by promoting the thermal Zeldovich and pathways. These findings clarify fundamental mechanisms of ammonia droplet combustion and provide guidance for designing low-emission ammonia spray combustion systems. Novelty and significance This study is novel as it represents, to the best of our knowledge, the first numerical investigation of single ammonia droplet combustion using a high-fidelity volume-of-fluid approach with direct, non-empirical calculation of the evaporation flux. A key novelty lies in the systematic evaluation of water absorption effects through three distinct liquid-phase models. The simulations deliver crucial insights into the physicochemical processes, complementing and extending recent microgravity experimental data under elevated pressure conditions. The significance of this work is further underscored by its detailed analysis of emissions and formation pathways across varying ambient pressures and oxygen levels. This research holds potential for advancing the design and optimization of future direct ammonia spray combustion technologies.