Spray Structure and Atomisation Mechanisms of Flash-Boiling Methanol under Reduced Ambient Pressure and Split Injection

Methanol is a promising low-carbon fuel for future propulsion systems. However, its high latent heat of vaporisation and poor auto-ignition characteristics increase the dependence on efficient spray atomisation and mixture preparation. In this study, the spray structure and atomisation mechanisms of flash-boiling methanol were experimentally investigated using an eight-hole ECN Spray M injector under reduced ambient pressure conditions. Experiments were conducted at injection pressures of 50–200 bar, fuel temperatures between 70 and 90 °C, and atmospheric absolute gas pressures of 1.0, 0.5 and 0.3 bar, corresponding to a saturation pressure ratio (𝑅𝑅𝑝𝑝 = 𝑃𝑃𝑠𝑠𝑠𝑠𝑠𝑠/𝑃𝑃𝑠𝑠𝑎𝑎𝑎𝑎) in the range of 0.8 to 3.2. Highspeed shadowgraph imaging was employed to investigate spray regime transition, plume interaction, spray collapse and injector-tip wetting across the reported range and for the cases of pulsed, split injection scenarios commonly encountered in direct injection combustion strategies. Results showed that an increase in pressure ratio promoted the transition from non-flashing to strong flash-boiling regimes. intensifying vapour generation and inward plume entrainment. Under strong flash-boiling conditions, neighbouring plumes merged, forming a central spray structure preceded by an axially protruding tip, extending by up to 6.5 mm beneath the main spray core of approximately 20 mm in length for a fuel pressure of 100 bar and 𝑅𝑅𝑝𝑝 = 3.2. Significant plume widening was observed from 0.04 ms after the start of injection due to rapid radial vapour expansion and enhanced plume interaction. Micro-scale imaging further revealed vapour bubble formation, growth and bursting that resulted in poorly atomised large droplets and ligament structures near to the injector tip, following the needle closing phase. Sequential pulsed injection experiments revealed that temperature induced effects within the liquid fuel captive within the nozzle cavity, leads to internal vapour formation. accelerated external flashing and jet expansion that influenced the spray penetration during repeated injections. Consistently, the third injection exhibited approximately 4% lower penetration than the first injection under continuous flashing conditions. The results provide insight into flash-boiling-assisted methanol injection strategies for improved mixture preparation in future low-carbon and dual-fuel engine applications.