Mass transfer from a Taylor bubble to the surrounding flowing liquid at the micro-scale: a numerical approach

Gas–liquid slug flow is characterized by complex and intermittent hydrodynamic features that offer an efficient alternative to promote biofilm control. In the present work, the mechanism of transferring a gaseous solute into a co-current liquid in a micro-scale slug flow system was inspected in detail. Specifically, the gas–liquid mass transfer from an individual Taylor bubble filled with oxygen was numerically studied using CFD techniques. To accurately describe the referred phenomenon, the hydrodynamic and concentration fields were simultaneously solved. Furthermore, the interface capturing based on the VOF methodology was also coupled to this solution approach. Three sub-categories within slug flow pattern were identified based on the flow behavior in the liquid phase: no liquid in recirculation (Case A); closed wake below the bubble tail (Case B); and recirculation ahead and below bubble (Case C). Regarding the solute distribution, in Case A the solute is dispersed only backwards, it accumulates in the closed wake structure in Case B, and it reaches the wall within the film region in Case C. Local and average mass transfer coefficients were also estimated for the different cases. The influence of the two most relevant dimensionless groups (Reynolds and Capillary numbers) was also briefly analyzed. Global mass transfer coefficients results confirmed that the penetration theory can provide reasonable estimations for systems like Case C.