A unified spray model for large eddy simulations under non-flashing and flash boiling conditions: Effects of in-nozzle flow and external thermal breakup in liquid ammonia injection

Abstract

As a carbon-free fuel, liquid ammonia is promising to be applied in gas turbines and marine engines to facilitate the decarbonization of energy and transportation sectors. However, ammonia has a high saturation pressure which leads to the transition from non-flashing to flash boiling atomization mechanisms and introduces challenges in modeling the liquid ammonia spray. It is essential to study the spray behavior and propose a unified spray model with high accuracy and implementation efficiency under wide operation conditions. In this study, a numerical model is developed under the Lagrangian-Eulerian framework to consider various breakup mechanisms. This model is then adopted for the spray simulations of liquid ammonia and validated against measurements. Firstly, the spray patterns are classified as normal evaporation (Rp ≥ 1.0), external flash boiling (1.0 < Rp ≤ 0.3), transitional and fully flash boiling (Rp < 0.3) through atomization mechanism analysis. Aiming at various spray patterns, a numerical model is developed involving the in-nozzle flow effect, external thermal breakup, and secondary aerodynamic breakup. The model comparison and validation results under different conditions show the proper prediction ability of the present model with accurate spray penetration and morphology. Compared with the typical aerodynamic breakup model, the spray expansion through the radial velocity increment of child droplets is well reproduced in the present model by considering the external thermal breakup. In addition, the improved boundary conditions that account for the in-nozzle flow effects enable a better prediction under the transitional and fully flash boiling region. Then the spray characteristics analysis of liquid ammonia under various conditions is conducted. It is found that the flash boiling plays an important role in primary atomization and generates smaller droplets. The initial spray expansion due to in-nozzle flow, later low air resistance, and continuous acceleration of small droplets leads to relatively slow and then fast penetration of flash boiling spray. Furthermore, a more complete spray mixing and evaporation process in both axial and radial directions is observed under the Rp0.1 condition. Nevertheless, the cooling effects resulted from the high latent heat of ammonia and subsequent wetting problem for combustion chamber wall should also be considered in practical applications. This study fills the gaps between measurements and predictions of the liquid ammonia spray under the transition from non-flashing to flash boiling conditions.