X-ray lithography and micro-machining have been used to study gas-assisted
liquid
atomization in which a liquid film was impinged by a large number of sonic
micro-gas
jets. Three distinct breakup regimes were demonstrated. Two of these regimes
share
characteristics with previously observed atomization processes: a bubble
bursting at a
free surface (Newitt et al. 1954; Boulton-Stone & Blake 1993)
and liquid sheet
disintegration in a high gas/liquid relative velocity environment (Dombrowski
& Johns
1963). The present work shows that suitable control of the gas/liquid
interface creates
a third regime, a new primary atomization mechanism, in which single liquid
droplets
are ejected directly from the liquid film without experiencing an intermediate
ligament
formation stage. The interaction produces a stretched liquid sheet directly
above each
gas orifice. This effectively pre-films the liquid prior to its breakup.
Following this,
surface tension contracts the stretched film of liquid into a sphere which
subsequently
detaches from the liquid sheet and is entrained by the gas jet that momentarily
pierces
the film. After droplet ejection, the stretched liquid film collapses,
covering the gas
orifice, and the process repeats. This new mechanism is capable of the
efficient creation
of finely atomized sprays at low droplet ejection velocities (e.g. 20 μm
Sauter mean
diameter methanol sprays using air at 239 kPa, with air-to-liquid mass
ratios below 1.0,
and droplet velocities lower than 2.0 m s−1).
Independent control of the gas and the
liquid flows allows the droplet creation process to be effectively de-coupled
from the
initial droplet momentum, a characteristic not observed with standard gas-assisted
atomization mechanisms.