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Turbulent modification of upward bubbly channel flow with surfactant

Yoichiro Matsumoto
Department of Mechanical Engineering, University of Tokyo, Japan

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. In order to remove the complicating factors of bubbly flow such as large deformation and coalescence of bubbles, we conduct our experiment at low void fraction less than 1% using 1mm mono-dispersed bubbles by the 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 produce 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. In both cases, bubbles have mono-dispersed size of about 1mm diameter. 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 center of the channel. Then, the turbulent fluctuation and the Reynolds stress in the liquid phase are very much reduced in this region. However the construction of bubble cluster and modification of turbulence structure by the clustering are not clearly understood. In the present study, we conduct experiment to analyze these mechanisms. We start from the experimental observation of lateral migration of single bubble in the channel under the presence of surfactant. Then, bubble-bubble interactions with the small number of bubbles are observed to investigate the initial stage of constructing bubble clusters. Finally, large-scale turbulent modifications are discussed with the formation of bubble clusters.