Internal Nozzle Simulations of Flash Boiling Liquid Ammonia Sprays for Engine Applications

Ammonia (NH3) is seen as a promising fuel, especially for marine applications, because it contains zero carbon, has robust global infrastructure, and only requires modest pressures (< 1 MPa) to store as a liquid at room temperature. When stored as a liquid, ammonia has a higher volumetric energy density than compressed gaseous hydrogen, the other main zero-carbon fuel candidate. Due to the very high saturation vapor pressure of ammonia, it is expected that liquid ammonia sprays will undergo flash boiling at most engine operating conditions, potentially requiring changes to current injection strategies. Liquid ammonia injectors are under active research and development due to compatibility issues with materials used in standard injectors. In addition, safety considerations in setting up experiments have contributed to only limited experimental data being currently available. This study examines ammonia sprays using volume of fluid (VOF) computational fluid dynamics (CFD) simulations with increasing pressure ratios to better understand liquid ammonia spray behaviors, especially under extreme flash boiling conditions. The geometry for a single-hole research injector (Spray M) from the Engine Combustion Network (ECN) was used based on preliminary experimental data available for this injector. Results show experimental trends are captured. Liquid ammonia sprays under engine-like conditions also undergo extreme flash boiling, generating large amounts of vapor within the injector counterbore and creating underexpanded jet-like conditions in the near-nozzle region. Details of the conditions at the injector tip are explored to understand implications for the setup of future parcel-based simulations for full engine simulations.