The flow pattern are the representation of experimental data on two-dimensional graphs that shows the transition zones of the regime.
With a map of this type you can determine the flow regime.
With the increasing of gas speed, the outflow passes from stratified -> stratified -wavy-> bubbles -> annular.
The optimum condition of exchange is the annular type because it has a liquid film, in phase changing,
that flows along the walls of the tube. And the phase changing heat coefficient is bigger than that in no phase changing one.
While in the other outflows types there are gas zones, more or less large, in direct contact with the tube,
which don't change phase and therefore the gas only overheats