Flash boiling fuel initial disturbance in a transparent step-hole nozzle and its effect on external flows

Fuel breakup process plays a vital role in spray atomization. Compared with the sub-cooled sprays, the breakup process of flash boiling sprays is more complex because of the fierce phase changing process inside of the nozzle. Existing experiments on flash boiling fuel breakup process were mainly based on straight-through flow channels, but most injectors in practical applications are step-hole nozzle, especially for GDI (Gasoline direct injection) injectors. The relationship between fuel initial disturbance inside of the nozzle and external flow breakup process is unclear. To address the said issues, high-speed optical imaging methods were adopted to investigate the initial disturbance and primary breakup process of a fuel jet from a two-dimensional transparent step-hole nozzle. Results show that a stronger initial disturbance could enhance the external fuel breakup. The external fuel breakup process disappears when the disturbance frequency of the nozzle internal flow goes up to 20 kHz under flash boiling conditions. In addition, experiments based on a straight-hole nozzle was carried out in this study for comparison purposes. Compared with the straight-hole nozzle, the use of step-hole nozzles could enhance the external flow breakup efficiency under various test conditions.