Physics of aeration in slug: flow visualization analysis in horizontal pipes

Aeration in slug and associated secondary flow leads to surging of pressure and erosion corrosion in pipe. Surging of pressure develops high mechanical impact on the pipe, and erosion corrosion reduces the thickness of internal wall, thereby resulting in pipe failure. To explore the phenomena of aeration in slug, flow visualization analysis is reported in this paper for intermittent flow sub-regimes and their transition. Analysis is reported for onset of slug, transition of plug to slug flow and development of aeration at the slug front. The visualized images and the motion pictures captured using high-speed photography in the present experiments are used to depict the process of air entrapment during the transition of wavy-stratified flow to slug flow as well as plug flow to slug flow. It is depicted for the first time through our visualization analysis that gas bubble entrapment in slug happens due to plunging kind of wave breaking mechanism. The captured images are also analyzed to describe the phenomena of augmentation of aeration in slug leading to the formation of highly aerated slug flow. Thorough understanding of aeration in slug will help in avoiding the chances of pipe failure.