Droplet Sizes and Velocities in a Transient Diesel Fuel Spray 900397

Simultaneous droplet sizes and velocities were obtained for a transient diesel fuel spray in a quiescent chamber at atmospheric temperature and pressure. Instantaneous injection pressure, needle lift, and rate of injection were also measured, allowing calculation of the instantaneous nozzle discharge coefficient. Short-exposure still photographs were obtained at various chamber pressure and densities to further investigate this spray.

Correlations between droplet size and velocity were determined at each crank angle to observe the detailed nature of the transient events occurring in this transient diesel fuel spray. As expected, peak mean and rms velocities are observed in the center of the spray. Measured average velocities are consistent with a calculated value, using the discharge coefficient for the nozzle and the known rate of fuel injection. The spray was nearly symmetric, with higher velocities occurring near the injector tip, and the radial dependence of velocity consistent with that observed from the spray photographs. Factors observed to effect the droplet size and velocity distributions and history include pump speed, fuel quantity delivered, and needle lift. The measured droplet diameters range in size from approximately the same size as the nozzle diameter to micron-sized droplets. The size distributions tend to have expected shapes, with a very strong dependence on time and position in the spray.

The spray photographs taken at low ambient pressure conditions reveal the existence and characteristic frequency of surface waves on the liquid exiting from the nozzle. These waves are less obvious at atmospheric pressures, and completely obscured by aerodynamic effects on the spray structure at chamber densities typical of that encountered in diesel injection.