Large-eddy simulation study of direct injection ammonia sprays under engine-relevant conditions in flash boiling scenarios

Ammonia, a promising fuel for internal combustion engines, is known to be highly susceptible to flash boiling under diesel engine-relevant conditions. Here, large-eddy simulation (LES) is used to examine ammonia sprays in ambient nitrogen under high injection pressure. A phase change model, combining boiling, flash boiling, and non-equilibrium evaporation, is utilized. Both single-hole and multi-hole nozzles are investigated. The spray models are calibrated according to various injection modeling parameters. The studied conditions encompass different modes of superheat levels (Rp) in non-reacting operating conditions. First, the single-hole configuration is studied under non-flash boiling conditions, and the spray setup is calibrated and validated. Second, the multi-hole configuration is used to validate the phase change model in flash boiling conditions. Third, numerical simulations are performed on a single-hole setup under high injection pressure to study transition flash boiling for 0 . 4 ≤ Rp ≤ 1 . 0 . Fourth, Voronoi analysis is performed for the first time to compare the droplet clustering behavior under non-flash boiling and flash boiling engine-relevant conditions. The main observations of the study are as follows: (1) The present ammonia spray LES model can be calibrated to match experimental vapor penetration, liquid penetration length, and spray opening angle. (2) Intense flash boiling (low Rp) results in faster vapor penetration and stronger mixing due to faster inter-phase momentum transfer. (3) Lower values of Rp (intense flash boiling) lead to stronger cooling of the spray core, resulting in longer liquid penetration lengths. (4) Voronoi analysis indicates that sprays under non-flash boiling conditions, with slower rate of phase change, have a higher preferential concentration of droplets due to a broader observed range of droplet sizes and Stokes numbers.