Measurement of Air-Blast Swirl Cup Atomization Field at Low Temperature With Digital Holographic Technique

A digital off-axis holography system was applied to measure the atomization characteristics near the exit of a nozzle with an air-blast swirl cup under different combinations of oil temperature and air pressure. It is found that both changes of oil temperature and air pressure have strong effect on the near-nozzle droplet field at oil pressure of 0.03 MPa. The larger droplets with sparse distribution and unconspicuous spray cone have been found at lower air pressure, and the droplet diameter in the spray cone center is larger than that at the edge. As air pressure increases, the droplet diameter decreases significantly, the distribution becomes uniform, and the spray cone is obvious. For all conditions, the droplet diameter is mainly distributed within 200 μm. However, at an air pressure of 0.5 kPa, a large number of droplets are above 200 μm, with a broad particle size distribution. As air pressure increases the diameter distribution range tends to narrow and the droplet diameter becomes smaller. A sharp increase of Sauter mean diameter (SMD), even up to 1 944 μm, appears at the edge of spray cone under conditions of oil temperature of -20 ℃, -40 ℃, oil pressure of 0.03 MPa and air pressure of 1.0 kPa. This result indicates that the atomization quality of air-blast swirl nozzle is poor under low pressure and low temperature conditions. This study also reveals that the values of velocity component in the x and y directions have a wider range of distribution with the increase of air pressure, especially for particle diameters from 32 to 66 μm. However, particles whose diameters are above 500 μm are counted less and have a narrow range of velocity distribution. The experimental results indicate that the digital off-axis holography is a powerful tool for three-dimensional visualization of near-nozzle atomization at low oil temperatures. This work can provide reference data for optimization of the nozzle structure design.