The flow pattern is the representation of experimental data on two-dimensional graphs that shows the transition zones of fluid regime inside the tubes. 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 for the heat exchange is the annular type because it has a liquid film, in phase changing, that flows along the walls of the tube. During the phase changing, heat coefficient is bigger than that in no phase changing one. In the other types outflow, instead, 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.