Spray development of flash-boiling methanol single-hole sprays under varying ambient conditions

Green methanol is a promising low-carbon fuel for future maritime engines, but its low boiling temperature promotes flash boiling under engine-relevant injection conditions and thereby strongly affects spray formation and mixture preparation. In this study, methanol sprays were investigated experimentally on an optically accessible cross-flow bench using diffuse backlight illumination imaging. A single-hole GDI injector (SHA) from the Engine Combustion Network was utilized to precisely isolate the impact of flash boiling on spray characteristics, eliminating the combined effects of spray-to-spray interactions and enabling a clearer understanding of the phenomena. The influence of fuel temperature, crossflow, ambient pressure and ambient air temperature on spray morphology, projected spray area and penetration was analyzed. Increasing fuel temperature strongly enhanced flash boiling, resulting in larger spray area, stronger radial expansion and reduced penetration. Compared with isooctane, methanol showed a significantly stronger flash-boiling response due to its lower boiling temperature and higher pressure ratio. Crossflow affected superheated sprays much more strongly than subcooled sprays, leading to pronounced deflection, aerodynamic modification and, at high crossflow rates, partial confinement of the flash-boiling-induced spray expansion. Ambient pressure influenced the spray through both thermodynamic and aerodynamic mechanisms: increasing pressure reduced the pressure ratio and weakened flash boiling, while also enhancing aerodynamic interaction. Consequently, the dependence of spray area and penetration on fuel temperature became weaker and, at elevated pressures, non-monotonic. By contrast, ambient air temperature had only a minor influence on penetration and affected spray area mainly under subcooled conditions. Overall, the results show that methanol spray development under engine-relevant conditions is governed by the coupled interaction of flash-boiling intensity and aerodynamic processes. These findings contribute to the understanding and optimization of methanol injection systems for future low-carbon marine engines.