3.2. Flow regime map
Experimental data on flow patterns and the transition boundaries
are usually mapped on a two-dimensional plot. Two basic
types of coordinates may be used for the flow regime maps –
one that uses dimensional coordinates such as superficial velocities,
and another that uses some kind of dimensionless group.
Dimensionless analysis is the straightforward approach to select
the proper dimensionless coordinates. The flow pattern transition
boundaries should be a function of all these variables. Maps based
on dimensionless coordinates are more general, for example, they
should be independent of fluid properties and channel size. Unfortunately,
determining the correct dimensionless coordinates for
flow pattern is not at all trivial. Moreover, there is no guarantee
that two dimensionless coordinates are sufficient in the majority
of cases. The maps obtained for a given geometry and channel size
are usually based on superficial liquid and gas velocities. The usual
experimental approach is to apply these variables to all transitions
in all channels, assuming that entrance effects, channel geometry
and size, roughness, solid–liquid–gas contact wetting angle, etc.,
have no influence on flow pattern. The chance of this approach
being correct, outside the specific range of operating conditions
that has been used to correlate the data, is very slim.
The range of superficial liquid and gas velocities covered in the
present study is shown in Figs. 6 and 7 as shaded rectangular area.