Experimental study on generation and growth of bubbles in external flash-boiling spray regime

•Diffusive backlight imaging of flash boiling spray by injecting superheated water jets into a vacuum chamber, a scenario relevant to regenerative cooling in advanced liquid propulsion systems.•Parameters like bubble nucleation flux, bubble growth rate, and spray angle within the external flash boiling spray regime was estimated based on images and compared to simple theoretical models.•The observed nucleation flux aligned well with the nucleation theory-based model, primarily for high superheat conditions.•By mapping the evolution of flash boiling spray based on key non-dimensional numbers, the significance of considering injector pressure for bubble generation within the injector was emphasized. High-performance liquid propulsion systems, such as hypersonic scramjets and liquid rockets, utilize regenerative cooling by employing fuel as a coolant to shield components from thermal damage. The flash-boiling spray, resulting from superheated liquid injection and prevalent in regenerative cooling, has received considerable research interest for enhancing atomization and combustion efficiency. In this study, superheated water jets at various temperatures between 30 and 90 ℃ were injected into a vacuum chamber at different pressures above 5 kPa, and their spray patterns were acquired through diffusive backlight imaging. Images were analyzed to determine key parameters, such as bubble nucleation flux, bubble growth rate, and spray angle, in an external flash-boiling spray regime. The estimated nucleation flux agreed well with the predictions the model based on nucleation theory in the region of sufficiently high superheat. The dynamics of bubble growth was investigated and explained using a bubble growth model. The jet-expansion energy was computed by measuring the spray angle and compared with the available energy. By mapping the evolution of flash-boiling spray in relation to relevant nondimensional numbers, the study underscores the necessity of accounting for the pressure inside the injector to adequately model bubble generation inside the injector. Furthermore, this study substantiates the feasibility of visually recognizing and analyzing individual bubbles in the external flash-boiling regime, thereby enabling the generation of experimental data crucial for the validation of bubble nucleation and growth models.