Chemistry Reference
In-Depth Information
Figure 8.6.
Mechanical processes for liquid aerosol formation: (a) surface impact of a high-
pressure liquid stream; (b) airjet impact—collision of high-velocity liquid and gas streams;
(c) high-pressure spray nozzles; (d) spinning disk centrifugal atomizers.
force (rotary atomization) are the simplest and most important situations because
they require knowledge of only one material velocity—that of the liquid. Spray pro-
duction by the action of an incident air stream on a jet of liquid involves, of course,
the velocity of both the liquid and the air.
8.7.1.2. Nozzle Atomization
If a liquid is forced through an orifice (nozzle) under a pressure, the velocity of the
liquid in the channel of the orifice becomes so high that turbulent flow is encoun-
tered; that is, the liquid will not flow smoothly in lines parallel to the walls of the
orifice but will flow in complex patterns with eddies, swirls, and vortices. When
the liquid leaves the orifice in this turbulent—or, to use a more fashionable term,
chaotic—state, the angular forces in the vortices will act against the surface tension
of the liquid to strip off units of liquid to form droplets.
For a simple, classical analysis of the situation, assume that as the liquid leaves
the orifice it has not only a linear velocity due to the pressure forcing it through the
system but also some angular velocity o resulting from its chaotic flow pattern.
Liquid will therefore rotate within the jet with a period of 2p/o. The rotation creates
a local centrifugal force F
o
. For a column of exiting liquid of radius r and height dz
that force is given by
2
3
prr
3
o
2
dz
F
o
¼
ð
8
:
8
Þ
