Refining Flash Boiling Spray Stage Classification Through Improved Understanding of in-Nozzle Bubble Nucleation

Flash boiling sprays are governed by different atomization mechanisms at different stages, leading to distinct spray characteristics. Therefore, a clear stage classification is essential for understanding their atomization process and for controlling spray behavior. Based on direct observations of in-nozzle bubble dynamics and external spray features, this study proposes a refined three-stage classification of superheated sprays: in addition to the well-recognized transitional flash boiling and flare flash boiling stages, an unboiled superheated stage is identified. The study suggests that the transitions between these stages are intrinsically linked to bubble nucleation and growth mechanisms inside the nozzle. The presence of the unboiled superheated stage is explained through classical nucleation theory. Furthermore, by incorporating a heterogeneous nucleation correction, a combined experimental-theoretical method is proposed to predict the onset of in-nozzle nucleation, which marks the transition from the unboiled superheated stage to transitional flash boiling. Sensitivity analysis identifies the main factors influencing nucleation onset and highlights potential strategies for actively controlling nucleation onset and spray stage transitions.