Book of abstract of the 47th Meeting of the Italian Section of The Combustion Institute. Advancing combustion for a sustainable and low-carbon future

This work presents a comprehensive numerical investigation of the evaporation and combustion of isolated ammonia droplets in microgravity, using a one-dimensional spherically symmetric model. The simulation framework includes detailed multicomponent transport in the gas phase, detailed chemical kinetics, radiative heat transfer via the P1 model, and adaptive mesh refinement. In the liquid phase, Maxwell–Stefan theory is used to model species diffusion, including the absorption of water vapor from the surroundings. Thermodynamic equilibrium at the gas–liquid interface is computed using the Peng–Robinson equation of state. Model validation was carried out using the microgravity experimental data of Matsuura et al. (Proc. Comb. Inst. 40, 2024), demonstrating excellent agreement in droplet regression rates and flame standoff distances. Once validated, the model was used to analyze the flame structure and the impact of ambient pressure and O2 concentration. Emphasis was placed on droplet burning rates, flame diameter evolution, and formation of nitrogen oxides (NOx). The analysis quantified the relative importance of N2, N₂O, HNO, NO2, and thermal pathways under different conditions. It was found that higher ambient O2 concentrations significantly enhance NOx formation via the thermal pathway, especially at elevated pressures, whereas the N₂O and N₂ pathways play a key role in NO consumption, acting as DeNOx pathways.