Phenomenology of Bubble Breakup and Coalescence in Sudden Expansions and Contractions in Vertical Pipes

The work discusses the phenomenology of bubble breakup and coalescence in a vertical pipe mounted with an expansion followed by a contraction. The flow conditions are such as to permit bubble-to-bubble-to-bubble, slug-to-bubble-to-slug and slug-to-slug-to-slug flow pattern transitions. Optical techniques are used to characterize the interactions between the phases (liquid–gas, gas–gas) and the resulting bubble breakup and coalescence processes. Thirteen sequences of events are used to illustrate the relevant processes. Breakup events are majorly ruled by the dynamic pressure forces of the turbulent motion. However, shear induced breakup is considerably observed for small and large bubbles. The formation of a gas pocket in the contraction is particularly identified as an important promoter of bubble breakup. Maps with the spatial location of bubble breakup for small and large bubbles and the changes in the velocities and lengths of Taylor bubbles as a result of their fragmentation and merging are also discussed for both singularities.