Insights into Droplet Behavior within High Pressure Diesel Sprays

Hung, C. Insights into Droplet Behavior Within High Pressure Diesel Sprays. University of Wisconsin-Madison, 1998.

This dissertation focuses on information gathered by a Phase Doppler Particle Analyzer (PDPA) about droplet behavior within high pressure Diesel sprays. In particular, high injection pressure and multiple injection effects upon microscopic spray characteristics were studied. Additionally, PDPA beam and signal attenuation in Diesel sprays was examined, and a droplet collision study was conducted.

The initial investigation compared PDPA results for a 105 MPa injection pressure Diesel spray with that previously obtained for a 21 MPa Diesel spray. The high injection pressure produced faster droplets, which were smaller at large axial distances. Comparison of the data with aerodynamic stability criteria demonstrated that increased injection pressure produces a higher percentage of droplets likely to undergo aerodynamic breakup. Transient droplet velocity and size profiles were, however, similar despite the difference in injection pressure. Specific characteristics, such as the appearance of subtle waves in the time-dependent data, were present in both cases.

Within the high injection pressure data, certain periods in time were marked by diminished data acquisition. Of the possible reasons for measurement rejections, the extent of attenuation of the transmitter beams and scattered light by droplets outside the probe volume was tested with beam shields to protect the optical path. The test revealed that any significant beam and signal attenuation was likely to occur in the spray core near the probe volume and not in the outer regions of the spray. The data also indicates interference with air entrainment into the spray by the shields.

Multiple injection from a common rail system was examined in atmospheric and pressurized conditions. The spray proved to be a challenging environment for the PDPA, making it difficult to fairly compare single and multiple injections. On-axis data was hindered in both conditions. Off-axis data demonstrated an abrupt edge for the core of the spray.

The droplet collision study showed three different breakup regimes from the microphotographs. High speed video demonstrated the possibility of secondary collisions by overtaking droplets and suggested some collisions include a gas film between the colliding droplets. PDPA data provided a correlation between mean diameter and collision Weber number.