Comparative study of Lagrangian breakup models for flash-boiling ammonia spray simulation

Eulerian-Lagrangian CFD (Computational Fluid Dynamics) simulations of liquid ammonia sprays remain challenging, particularly in consistently predicting spray morphology and droplet size across operating conditions. This is because ammonia's physical properties differ significantly from those of common hydrocarbons, and so traditional models do not work effectively. At the same time, ammonia is attracting increasing interest as a fuel for combustion applications such as propulsion systems, gas turbines, and industrial burners. In particular, for thermal-power applications, the direct injection of liquid ammonia represents an attractive solution due to its potential to improve both volumetric and overall thermal efficiency. In this context, we propose a comparative study of two different Eulerian-Lagrangian breakup modeling approaches for simulating evaporative and flash-boiling ammonia sprays. Rather than resolving the micro-scale internal nozzle flashing, we focus on capturing the macroscopic downstream spray behavior, plume expansion, and vaporization in a computationally efficient framework suitable for application-scale simulations. In the first approach, we used correlations for the time and size constants of the KH-RT breakup model, as well as for the spray cone angle. The second approach is based on an effervescent breakup model (fbBreakup), which accounts for bubble growth within droplets and is applied here to ammonia. We evaluated the numerical results against experimental data for the spray issued by a multi-hole injector, including liquid and vapor tip penetrations, overall spray morphology, and local Sauter Mean Diameter (SMD) values. We also quantified the significant cooling effect induced by ammonia during the phase change process. The results show that while the calibrated correlation approach reproduces the spray structure well, the fbBreakup model behaves autonomously without case-specific tuning. We conduct an extensive discussion to highlight the advantages, limitations, and downstream thermal characteristics of both methodologies.