Near-nozzle flash-boiling flow of iso-octane, methanol, and ammonia in the engine combustion network spray G injector

Non-carbon and e-fuels such as ammonia and methanol are attractive options for the transportation community's decarbonization efforts. To utilize these highly volatile fuels, their in-nozzle flow and near-nozzle spray behavior must be better understood. The current work uses computational fluid dynamics (CFD) simulations to make a comparison of the internal flow and near-nozzle behavior of methanol, ammonia, and iso-octane injection. The behavior of the near-nozzle spray of these fuels was studied by subjecting them to flash-boiling and subcooled conditions through the Engine Combustion Network's (ECN) eight-hole Spray G injector. Three-dimensional transient CFD simulations were performed using an in-house CFD solver that accounts for thermodynamic non-equilibrium aspects of phase change and needle motion. The conditions for comparing the behavior of methanol and ammonia were inspired by the standard subcooled (Spray G) and flash-boiling (Spray G2) conditions stipulated by ECN using iso-octane as the working fluid. The run conditions for ammonia and methanol injection were chosen to reveal the influence of fuel properties and the challenges posed for high pressure direct injection. Iso-octane was also simulated at an elevated injection temperature, in addition to the ECN's G and G2 conditions, to assess the influence of increased saturation pressure on the ensuing spray. The findings indicate that saturation pressure and the enthalpy of vaporization (hfg) are critical to our understanding of the in-nozzle flow and near-nozzle spray behavior of volatile fuels. The non-dimensional quantity representing the ratio of the saturation pressure to the chamber pressure, Rp, is necessary for describing the spectrum of flash-boiling but not sufficient. A second-order effect is induced due to the influence of the enthalpy of vaporization when sprays are subjected to the same Rp values. This effect of the hfg is required for understanding the extent of vaporization. Additionally, it was also observed that sprays with extremely high values of Rp exhibit a strong underexpanded near-nozzle jet behavior in addition to a bell-shaped morphology coupled with a complete spray collapse.