Experimental Investigation of Flash-Boiling Atomization of Liquid Ammonia: Influence of Nozzle Aspect Ratio and Injection Conditions on Steady-State Spray Morphology

Optimized mixture formation is critical to overcome the combustion-averse properties of ammonia (NH3) in carbon-free energy systems. This study investigates the steady-state flash-boiling atomization of liquid ammonia in stainless-steel micro-nozzles (D = 100 µm), linking internal flow mechanics to macroscopic spray morphology. By integrating high-speed shadowgraphy and in-line mass flow measurements with high-resolution µ-CT characterization, we demonstrate that internal manufacturing defects such as nearly planar inlets and localized roughness act as dominant heterogeneous nucleation sites that override idealized CAD predictions. A key finding is the identification of a stochastic regime bifurcation unique to the ultra-short nozzle (L/D = 1.53), which alternates between non-flashing liquid jets and fully-flashing plumes under identical thermo-hydraulic conditions. In contrast, longer nozzles (L/D ≥ 4.21) remain hydraulically locked in stable flashing regimes. Furthermore, localized nozzle-tip condensation is shown to produce peripheral droplets that may compromise flame stability. By providing a comprehensive data set that links the hydraulic characterization and µ-CT scans of the nozzle internal geometry with spray dynamics, this work establishes a high-fidelity empirical foundation for the development of predictive numerical models and CFD simulations in ammonia-fueled injection systems.