The available energy for atomization in two fluid atomizers is independent of liquid flow and pressure. The necessary (kinetic) energy for atomization is supplied by compressed air. The atomization is created due to high frictional shearing forces between the liquid surface and the air having a high velocity even at sonic velocities and sometimes rotated to obtain maximum atomization. See Fig. 50.
Fig. 50. Two-fluid nozzle
Two fluid atomization is the only successful nozzle method of producing very small particles, especially from highly viscous liquids. The relation between drop size and operation conditions has been sought and many scientists have tried to calculate the mean diameter. The best known is:
(13)
Where:
ds = volume particle mean diameter (microns)
V = velocity of the air relative to the liquid at the nozzle orifice (ft/sec.)
Ơ = surface tension (dynes/cm)
PL = liquid density (lbs/ft3)
μ = viscosity (centipoise)
J = air/liquid volume ratio at air and liquid orifices resp.