Visualization experimental study on transient cavitating flow and spray characteristics of methanol high-pressure direct injector nozzles

In the context of the green and low-carbon transformation of internal combustion engines, methanol is regarded as a promising alternative fuel owing to its high oxygen content, low carbon-to-hydrogen ratio, and excellent combustion characteristics featuring minimal particulate formation, coupled with its compatibility with existing fuel infrastructure due to liquid-phase storage at ambient conditions and potential for production through carbon–neutral pathways utilizing biomass or CO2 hydrogenation. However, its low cetane number and high latent heat of vaporization present challenges to ignition stability and spray consistency during high-pressure injection. To address these issues, a high-pressure common-rail fuel injection experimental platform was developed using a prototype-based transparent nozzle model integrated with the original full-size injector. Using the backlight uniform illumination method, a systematic experimental study was conducted to investigate the effects of injection pressure and nozzle geometry (including straight orifices, convergent orifices, different SAC chamber depths, and various numbers of orifices) on the transient nozzle internal cavitation flow and near-field spray characteristics of methanol, while simultaneously characterizing the associated dynamic needle-motion response. Comparative experiments between methanol and diesel revealed that methanol injections tend to generate more pronounced aggregation of vapor structures and larger spray cone angles, while also causing more pronounced material degradation on the nozzle. Increasing injection pressure mainly affects the transient evolution of cavitation and spray during the needle-opening and needle-closing stages, while no clear monotonic effect is observed on the quasi-steady cavitation intensity or spray cone angle. The depth of the SAC chamber and the number of orifices significantly affect the needle lift and the stability of cavitation structures. Smaller-volume SAC chambers promote earlier cavitation formation, whereas larger-volume chambers contribute to sustained cavitation and prolonged tail-spray. Compared with diesel, methanol exhibits higher cavitation intensity and broader spray diffusion characteristics during stable injection. This study provides fundamental experimental evidence for understanding the transient cavitation and near-field spray behavior of high-pressure methanol injection and offers useful references for subsequent methanol nozzle optimization.