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

The main objective of this study is to investigate the effect of ambient pressure (P_amb) 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 (T_fuel) from 20 °C to 120 °C. The spray width within the axial distances of 20 nozzle diameters (do) could be increased by increasing T_fuel and/or decreasing P_amb. Beyond 20do, the change in spray width against superheated degree (ΔT_SD) was fairly sensitive to P_amb. It decreased with the increase in ΔT_SD at P_amb larger than 50 kPa, and increased with ΔT_SD at P_amb 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 P_amb could be well explained as follows: relatively larger P_amb (>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 P_amb (<50 kPa for the used injector), negatively influencing the intensity of vapor condensation.