Morphology analysis on multi-jet flash-boiling sprays under wide ambient pressures

- Multi-jet flash-boiling sprays were studied at ambient pressures of as low as to 2 kPa.
- Collapse of multi-jet flash-boiling sprays did not occur at relatively low Pamb.
- Spray morphology in flash-boiling conditions was sensitive to Pamb.
- Liquid temperature drop and Pamb were believed to be key influencing factors on spray collapse.

The main objective of this study is to investigate the effect of ambient pressure (Pamb) on the morphology of multi-jet flash boiling sprays and to promote the understanding on the collapse mechanism. With the high-speed imaging technique, the n-hexane spray process in a constant volume vessel was captured under the ambient pressures from 2 kPa to 101.3 kPa and the fuel temperatures (Tfuel) from 20 °C to 120 °C. The spray width within the axial distances of 20 nozzle diameters (do) could be increased by increasing Tfuel and/or decreasing Pamb. Beyond 20do, the change in spray width against superheated degree (ΔTSD) was fairly sensitive to Pamb. It decreased with the increase in ΔTSD at Pamb larger than 50 kPa, and increased with ΔTSD at Pamb less than 50 kPa. Based on the new understanding that the collapse in multi-jet flash-boiling sprays was induced by vapor condensation at the nozzle exit and the resultant local pressure drop (i.e. generation of low-pressure zone), the sensitivity of spray width on Pamb could be well explained as follows: relatively larger Pamb (>50 kPa for the used injector) would be conducive to the vapor coagulation and cause stronger vapor condensation (i.e. larger pressure drop) while the vapor dispersion may become dominant at lower Pamb (<50 kPa for the used injector), negatively influencing the intensity of vapor condensation.