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this case, the entrainment mechanisms can be grouped into two main categories ,
i.e., film entrainment and pool entrainment. A wavy liquid film can be en-
trained into gas flow in several different ways such as roll-wave shearing
off, wave under cutting, droplet impingement and liquid bridge disintegration.
For annular flow shearing off of the roll-wave crests is the most important
entrainment mechanism [2,14], see Fig. 1. The liquid bulge or bridge disin-
tegration is similar to the large droplet disintegration discussed above in
terms of the critical Weber number. This mechanism may be important in -
counter-current situations or near the churn to annular flow tfansition. The
wave under cutting occurs at very high gas flow and at low Reynolds numbers.
Therefore, for highly viscous fluids, this is an important mechanism to con-
sider [14].
In the pool entrainment, droplets are generated by bubbles streaming
through the horizontal pool level. Because of the bubble burst mechanism,
droplets of two different size groups are produced and carried up by the gas
flow. The entrainment often depends on the height from the pool level due
to the existence of falling drops which cannot be suspended in the gas stream
[26].
As mentioned above, most of the droplets in annular two-phase flow are
produced by the entrainment. There is very strong experimental evidence in-
dicating that the drops are too small to be generated by the standard droplet
disintegration mechanism. In other words, the critical Weber number based on
the relative velocity between gas and drops gives much larger drop sizes than
experimentally observed. Therefore, the majority of droplets should have been
generated at the time of entrainment and not during the flight as droplets in
gas flow. This implies that the relative velocity between the gas core flow
and liquid film flow is the governing factor in determining the droplet size.
Furthermore, the dominant mechanism of entrainment in annular two-phase flow
is shearing off of roll-wave crests by streaming gas flow. In view of these,
a criterion for drop size in annular flow is derived by considering the roll-
wave entrainment mechanism in the next section.
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