In this study, fuel sprays of a 1-hole injector and a 6-hole injector were investigated under a wide range of subcooled and flash-boiling conditions using high-speed imaging in a constant-volume spray chamber. The fuel pressure was held constant at 15 MPa, and the fuel temperature was varied from 25 degrees C to 85 degrees C. Ambient pressure in the spray chamber was varied from 20 kPa to 200 kPa and the chamber temperature was held constant at 25 degrees C. The results show decreasing ambient pressure led to flash boiling for sprays from the 1-hole injector and consequently enhanced fuel atomization; however, the spray processes of the multi-hole injector were not well represented by the single-hole injector when considering spray collapse. For the 6-hole injector, the results showed plume interaction was more important than chamber density in controlling the spray characteristics. Specifically, expanded fuel plumes induced by flash boiling increased interaction between adjacent fuel plumes and consequently affected the spray structure. Slight plume interaction resulted in moderate spray collapse, shorter spray penetration and wider spray angles. Strong plume interaction triggered severe spray collapse, leading to longer spray penetration and smaller spray angles. Plume interaction started in the near nozzle region, and sustained interaction depended on the injector configuration and the superheat degree. At strong flash boiling conditions (where the ratio of the ambient pressure to the fuel saturation pressure was less than 0.3), plume interaction increased with time and as the spray moved downward away from the nozzle exit due to fast fuel atomization and evaporation. The potential for spray collapse of a mull-hole injector was primarily attributed to the effects of the fuel properties, injector configuration and operating conditions (e.g. ambient pressure, fuel temperature and pressure, etc.) on the development of the plume width relative to changes in the distance between adjacent plumes. Adjusting the parameters to increase the distance between adjacent fuel plumes or decrease plume width, will decrease plume interaction and spray collapse can be suppressed, and vice versa.