A novel ELSA model for flash evaporation

The development of novel upper-stage rocket engines with storable cryogenic liquids and laser ignition requires a detailed understanding of the processes prior to ignition. Due to the low atmospheric pressure at high altitudes or in space, the liquid is injected below its saturation conditions, causing bubble nucleation, growth, and spray break-up, referred to as flash evaporation. Given the multiscale nature of the phenomenon, modeling and simulating this intricate process pose challenges. Common strategies, like the Eulerian-Lagrangian method, encounter limitations in representing spray break-up dynamics and providing initial data, such as the droplet size and number distribution. On the other hand, Euler-Euler methods cannot resolve the interface for all length scales involved, and the information about the surface interface is lost. Therefore, this work introduces an extension of the Eulerian-Lagrangian spray atomization model (ELSA) for flashing sprays, used to regain the information about mean droplet size. The model incorporates terms for flashing break-up and droplet evaporation and is validated against a cryogenic liquid nitrogen test case, reflecting conditions pertinent to cryogenic fuel injection at high altitudes. Comparative analyses with standard ELSA and a second model from the literature highlight the significance of accounting for the flashing process in spray break-up and droplet formation dynamics. This work contributes to a refined understanding of the break-up process in flashing sprays, offering potential advancements in spray modeling for aerospace applications.