COMPARATIVE STUDY ON THE COLLAPSE MODES UNDER FLASH-BOILING AND NON-FLASH-BOILING CONDITIONS USING A TWO-HOLE GDI INJECTOR

Abstract

Several mechanisms are available for the multi-jet GDI spray collapse under low ambient pressure (≤ 1.0 bar) flash-boiling (LP-FB) and non-flash-boiling (NFB) conditions. The present investigation is to clarify the applicability of the available mechanisms under different thermodynamic states. The tests were carried out in an optically accessible constant volume vessel using a two-hole GDI injector over fuel temperature (T_fuel) ranging from 50° to 130°C and ambient pressure (P_amb) ranging from 0.2 to 9.0 bar. The collapse could be found both under LP-FB and NFB conditions, but their characteristics changing with P_amb and T_fuel showed the opposite trends. The LP-FB collapse enhanced as P_amb decreased or T_fuel increased while the NFB collapse weakened as P_amb decreased and could disappear once Tfuel was beyond 70°C. Further analysis into the temporal spray development showed that the LP-FB collapse occurred at the very beginning of injection accompanied with the occurrence of a new jet between the original jets (termed as "inter-jet jet") while the NFB collapse gradually occurred with the time elapsed. The NFB collapse was attributed to the low-pressure zone caused by the jet entrainment. The disappearance of NFB collapse as T_fuel increased demonstrated that the enhanced evaporation would counteract this low-pressure zone. This also provided the strong support that the LP-FB collapse could not be induced by the jet entrainment because the flashing evaporation was much stronger. After careful examination of the available mechanisms, it was more likely that the occurrence of under-expansion in LP-FB jets and their interactions at the nozzle exit caused the collapse.