CHARACTERISTICS OF THE SPRAY FROM A DIESEL INJECTOR

Spatial and temporal profiles of the velocity of the entrained air and 60 and 30-mu-m droplets, together with the associated fluxes, from a 5-hole diesel spray exhausting into atmosphere at a repetition rate of 10 Hz have been measured with a phase-Doppler anemometer. The nozzle diameters, fuel charge per hole, injection duration and the area-averaged spray velocity during this duration were 0.18 mm, 2.35 mm3, 0.7 ms and U0 = 132 m/s, respectively. The Sauter mean diameter of the fuel droplets decreased from a maximum centreline value of around 80-mu-m at 100 diameters from the nozzle to 38-mu-m at 780 diameters, and a similar decrease was observed between 1 and 2 ms after the start of injection at the upstream location. The flux carried by the 30-mu-m droplets was up to twice that associated with the 60-mu-m droplets, 2 ms after injection, although the velocities of the larger droplets were consistently higher than those of the smaller droplets. The maximum measured ensemble-averaged relative velocity was 0.45 U0 for 60-mu-m droplets just after the arrival of the spray at 550 diameters from the nozzle. The magnitudes of the Weber number imply that droplet breakup was always confined to the leading edge of the spray and was limited to, at most, the initial 1/2 ms of the passage of the spray past a given point. Breakup was mostly complete by 550 diameters from the nozzle. Thus, the measured decrease in the mean diameter was due to small droplets, generated by breakup at the leading edge of the spray, losing velocity due to aerodynamic drag and falling behind the leading edge. Droplets generated late in the injection schedule were likely to overtake those generated earlier and together with the fan-spreading effect, which arises from the combination of the root mean square (RMS) droplet radial velocity and the radial profile of the ensemble-averaged droplet axial velocity, led to RMS velocities in the axial component of the droplets that were not associated with the transfer of turbulent motion from the air.