In the present work, the mechanism of drop formation from a thin hollow liquid (water) jet with a high-velocity core-gas (air) flow was examined. The breakup modes were identified from the high-speed spark photography. There exist at least three different modes of disintegration, namely, Rayleigh, bubble-breakup, and pure-pulsating modes depending on the air and water flow rates. Tentative maps representing those disintegration modes were proposed with the liquid film Reynolds number and the aerodynamic Weber number taken as the dimensionless parameters.
Spray configuration was visualized and mean (cross-section-averaged) SMD radial distribution of local SMD and volume concentration of drops were measured for various liquid and gas injection velocities and for annulus-gap clearances (0.2 - 0.8 mm). The atomization quality was improved with the higher flow rate of the atomizing gas; however, the dependence of drop size on the liquid flow rate was complicated. With the increase in liquid flow rate, the mean SMD first increased and then decreased, and increased again, up to the maximum, followed by the second decreasing range. Flow regimes (laminar or turbulent) and the momentum strengths of the liquid and the gas streams, and their combinations, were considered as the primary factors determining the overall atomization characteristics. The aerodynamic force was proven to be dominant in the first three ranges of the liquid flow rate, while the hydrodynamic force predominated in the last range. The variation trend of the mean SMD gradually disappeared with the decrease in the liquid sheet thickness. When the sheet thickness decreased to 0.2 mm, the mean SMD simply increased with the increase in liquid flow rate.
