Modeling droplet size distributions in flashing sprays with 3D LES using an enhanced ELSA approach

Abstract

Flashing sprays play a critical role in high-altitude propulsion systems, where rapid phase transitions and complex transonic effects govern spray breakup and droplet formation. This study applies a novel Flashing Liquid Atomization Model (FLAM), within a hybrid Eulerian–Lagrangian framework to predict droplet size distributions at spray breakup. The governing equations of a one-fluid formulation for two-phase flows are solved on the Eulerian grid, while post-breakup droplets are tracked as Lagrangian particles. By incorporating surface density transport, the FLAM model eliminates the need to predefine droplet properties, instead providing locally determined droplet diameters. The droplet evaporation process is modeled as a combination of flash evaporation and diffusion, accounting for injection into conditions below the triple point. The approach is validated against experimental data from liquid nitrogen injection in near-vacuum conditions. Results reveal circumferential inhomogeneities in droplet size distributions, challenging the assumption of uniform atomization in flashing sprays. The three-dimensional Large Eddy Simulations (LES) capture turbulence-driven droplet collisions and shock-induced surface density destruction, leading to a more uniform droplet distribution while preserving the qualitative trends observed in previous two-dimensional Reynolds-Averaged Navier–Stokes (RANS) investigations. This study highlights the importance of capturing localized breakup physics for accurately modeling flashing sprays. The findings provide new insights into the interplay of phase change, turbulence, and transonic effects in cryogenic jet injection, contributing to the advancement of hybrid Eulerian–Lagrangian spray modeling for high-altitude propulsion systems.