Picosecond pulsed digital off-axis holography for near-nozzle droplet size and 3D distribution measurement of a swirl kerosene spray

摘要

Near-nozzle spray formation determines the spray structure of downstream dilute spray, and thus plays an important role in evaluating the performance of air blast atomizer. To capture the exact structure of the atomization field in near-nozzle region, the spray cones from the exit of a two-stage swirler to the 9 mm downstream area with the gas pressure of 0.5 kPa, 1.0 kPa, 3.0 kPa and the fuel pressure of 0.03 MPa and 0.71 MPa, are investigated using a picosecond pulsed laser digital off-axis holography (DOH) system. The spray structures and droplet characteristics in near-nozzle region are visualized with clear image of nonspherical droplets and then quantified. Semi-spray cone angles from the front view (x-y) projections of particle fields are obtained and results indicate that the angle increases with the increase of gas pressure within test range. From the overall view of spray field, the droplet peak diameter decreases with gas pressure, from about 80 um at 0.54 kPa to about 30 mu m at 3.05 kPa. In addition, cumulative volume distribution of droplet obtained in near-nozzle region agrees well with modified Rosin-Rammler distribution. From the local view, spatial distributions of the normalized droplet number density and Sauter mean diameters (SMD) ranging from 30 mu m to 480 mu m of the swirl spray field are statistically analyzed. Results indicate that as the gas pressure increases, the SMD distribution is more uniform. Under all gas pressure conditions, SMD distributions tend to be larger in the near-exit area and the region inside mist cone edge. The work also demonstrates that picosecond pulsed DOH is a powerful tool for 3D diagnostics of spray.