Measurement of 3D airblast swirl atomization field at low temperature with off-axis holography

A 25 kHz picosecond pulsed digital off-axis holography system is applied to measure the near-nozzle atomization characteristics of an airblast swirl atomizer under low oil temperature conditions. Experiments are carried out for the near-nozzle atomization process 30 mm down-stream from the nozzle exit under air pressure of 1 kPa and oil pressure of 0.03 MPa. The oil temperature varies from –40 ℃ to 28 ℃. It is found that the atomization field in the near-nozzle region contains non-spherical droplets. Typical dynamic processes of atomization such as the breakup of films and filaments are visualized with clear images. The sizes and three-dimensional（3D） positions of droplets ranging from 30 to 1500 μm are obtained through particle identifica-tion and locating, thereafter statistics on 3D distribution of Sauter Mean Diameters（SMD） is obtained. It is found that under conditions of oil pressure of 0.03 MPa and air pressure of 1 kPa, the droplet diameter is mainly distributed within 200 μm, and droplets with size range 30–40 μm account for the largest proportion, above 15%. The 3D droplet size distribution is expressed as a cone where the central droplet size is larger than that on the edge; the decrease of oil temperature significantly deteriorates the atomization effect, which reduces the volume of the spray cone, and the density and the uniformity of atomized droplets; With oil temperature decreasing from 28 ℃ to –20 ℃, the central droplet diameter of the downstream cross-section increases from about 300 μm to over 450 μm and can be locally larger than 650 μm; with the oil temperature of –40 ℃, a large liquid core with multi-branched liquid films and filaments appears downstream of the nozzle, resulting in a longer breakup distance. The experimental results also demonstrate that the high-speed digital off-axis holography is a powerful tool for three-dimensional visualization and diagnostics of near-nozzle atomization under low oil temperatures conditions. This work can provide reference data for optimization of the nozzle structure design and the demonstration of the atomization model.