MACROSCOPIC CHARACTERIZATION OF FLASH-BOILING MULTI-HOLE SPRAYS USING PLANAR LASER INDUCED EXCIPLEX FLUORESCENCE TECHNIQUE. PART I. ON-AXIS SPRAY STRUCTURE

Fuel temperature and the ambient pressure can dramatically influence spray characteristics such as spray structure, spray penetration, and quantity of vaporized fuel. Higher fuel temperatures that occur in today's direct-injection (DI) engines can make the injected fuel reach superheated conditions, dramatically changing the spray due to flash-boiling. In this paper, the spray structure and vaporization behaviors are examined for a multi-hole gasoline DI injector over a range of superheated conditions. Planer laser induced exciplex fluorescence (PLIEF) is used to characterize the spray, where fluorobenzene (FB) and diethyl-methyl-amine (DEMA) are added into n-hexane as the base fuel simulating gasoline to track the liquid and vapor distribution. The temperature dependence of cross talk between the vapor and liquid signals is examined using a quartz calibration cell, after which a careful correction is applied to the spray image. The results show that multiple plumes from this multi-hole injector collapse with increasing superheat degree until the spray transforms into a single solid plume. The liquid component of the spray collapses less as compared to the vapor component, which becomes a complete gaseous jet concentrated at the centerline axis of the spray. The vapor quantity increases with the superheat degree of fuel linearly in two slopes before and after the collapsing. When the increased superheat degree triggers the collapsing, the vaporization rate escalates dramatically, indicating a rapid phase change due to flash-boiling.