Predictive modeling of flash-boiling ammonia sprays using a volume expansion ratio

Ammonia is attracting interest as a carbon-free fuel for internal combustion engines. Because it can be stored as a liquid under moderate pressure, liquid injection is often considered; under low ambient pressure, liquid injection can induce flash-boiling, which significantly alters spray morphology and affects mixture formation and combustion processes. Therefore, accurate modeling of flash-boiling sprays is necessary in computational fluid dynamics (CFD) simulations. This study proposes a practical modeling framework for flash-boiling liquid ammonia sprays. The model assumes that near-nozzle spray morphology is affected by vapor generation within the liquid phase and subsequent volumetric expansion during depressurization. Accordingly, a volume expansion ratio, V exp , defined as the ratio of volumetric flow rates before and after flash-boiling, is introduced. Correlations for spray width and spray angle are formulated as functions of V exp and used as initial conditions in discrete droplet model calculations. Because V exp depends on fuel thermophysical properties and thermodynamic conditions, spray morphology variations with fuel species, fuel temperature, and ambient pressure can be represented through changes in V exp . The framework was validated using spray visualization experiments for ammonia and propane under various ambient pressures and fuel temperatures. Data for the near-nozzle spray width and spray angle collapsed reasonably well when plotted against V exp , and the proposed correlations predicted these quantities with R 2 values of 0.89 and 0.75, respectively. CFD simulations reproduced the measured spray morphology reasonably without condition-specific tuning for V exp ≲ 200 . The framework offers a practical approach for designing liquid ammonia injection systems.