CFD simulations of droplet evaporation in ammonia-air gas mixtures

In this study, a transient, two-dimensional axisymmetric volume-of-fluid (VOF) multiphase model is developed to investigate the absorption of ammonia (NH 3 ) gas by an evaporating, stationary water droplet. The primary objective is to establish a computational fluid dynamics (CFD) framework capable of resolving the coupled heat and mass transfer process occurring within a liquid droplet exposed to a gas mixture consisting of air and a soluble component. Since fully transient CFD simulations of the complete evaporation process are computationally prohibitive, a hybrid flux-based approach is employed. Transient, axisymmetric VOF simulations are performed over the initial 0-7 s, corresponding to the period of strongest NH 3 absorption and the most pronounced surface-temperature increase. Time-resolved interfacial heat and mass fluxes obtained from these simulations are subsequently integrated and coupled with global energy and mass balance equations to predict the droplet temperature evolution and total evaporation time with good accuracy and substantially reduced computational cost. The results show that NH 3 absorption induces a significant increase in droplet surface temperature, which in turn enhances the evaporation rate. For NH 3 -air mixtures containing 5 and 10 vol.% NH 3 , the evaporation of a water droplet with an initial diameter of 410 μm at 297 K leads to maximum surface temperature increases of approximately 7°C and 10°C, respectively. As a consequence, the total evaporation time is reduced by about 28% and 47% compared to evaporation in pure air. These reductions are referenced to the CFD-predicted baseline evaporation time in pure air (815 s), ensuring a consistent numerical comparison. Overall, the findings highlight the strong coupling between NH 3 absorption and droplet evaporation in mixed-gas environments and provide insights relevant to spray drying, humidification, and liquid-gas interaction processes. • Transient 2D VOF CFD model developed for NH 3 absorption by evaporating droplets • Hybrid CFD-flux approach reduces computational cost while capturing transient fluxes • NH 3 uptake raises droplet surface temperature, enhancing evaporation rates • Droplet surface temperature initially drops, then rises due to exothermic NH3 uptake • NH3 uptake temporarily increases droplet volume as absorption exceeds evaporation