Improved dynamics, atomization, and vaporization models for liquid ammonia spray simulations under diverse ambient conditions

The use of liquid ammonia as fuel in energy conversion devices presents attractive prospects due to its advantage of zero carbon dioxide emissions. However, the dynamics, atomization, and phase change behaviors of liquid ammonia spray are significantly different from those of conventional gasoline and diesel fuels because of its unique physical properties (e.g., low boiling point, low viscosity, and high latent heat of vaporization), making an accurate simulation of liquid ammonia spray under a wide range of ambient conditions highly challenging. To address this, improved drag, atomization, and vaporization sub-models are proposed in this study to more precisely simulate the ammonia spray process. The effects of finite viscosity and droplet distortion (ranging from prolate spheroid to oblate spheroid) are considered to replicate the drag and vaporization characteristics of ammonia spray. Additionally, an updated flash-boiling atomization model is introduced to reproduce the thermal breakup process by introducing the influence of liquid viscosity in the breakup dispersion equation. The bubble growth rate within the liquid droplet is also enhanced using the latest theories. The validity of the improved models is evaluated through comparisons of experiments and large-eddy simulation (LES) of ammonia spray under typical evaporating and flash-boiling conditions. It is found that the new drag model predicts a lower drag coefficient and a higher evaporation rate than the traditional model under the evaporating conditions and can more accurately reproduce the experimental data. Moreover, the improved flash-boiling atomization model, coupled with the new drag model, demonstrates the highest accuracy in predicting spray dynamics and morphology of flash-boiling sprays compared to other models. After extensive validations, the improved spray models can effectively reproduce the atomization and vaporization characteristics of liquid ammonia sprays across a wide range of operating conditions.