Turbulent modification of upward bubbly channel flow with surfactant

Abstract

Bubbly flow in a vertical channel is investigated. Our main objective of this research is to clarify the mechanism of the turbulence modification due to the presence of bubbles. We conduct our experiment at low void fraction less than 1% using 1mm mono-dispersed bubbles by addition of small amount of surfactant. The surfactant addition cause so-called Marangoni effect and change the boundary condition of the bubble surface, though do not change the turbulent structure directly in this case. Our previous study reveals that the slightly contaminated bubbles migrate toward the wall in upward turbulent flow, which produces the bubble clustering structure near the wall. On the other hand, highly contaminated bubbles do not have this tendency and uniformly distributed across the channel. The difference of migration behavior is well-explained with the discussion of lift force acting on bubbles and particles. This strong accumulation of bubbles near the wall changes the turbulent structure drastically. Once the bubble clusters are formed, they lift up the near-wall region and give a plug-like flow structure at the centre of the channel. Then, the turbulent fluctuation and the Reynolds stress in the liquid phase are very much reduced in this region.