Abstract
A direct numerical simulation (DNS) study is performed to investigate the flow field and interfacial species transfer around freely rising Taylor bubbles in a cylindrical pipe. The Eötvös number Eo is varied from 4.9 to 8.7, the bubble Reynolds number Re from 36 to 303, and the Schmidt number Sc from 1 to 256, covering both the laminar–turbulent transition and fully turbulent regimes. The predicted bubble shape, liquid-film thickness, terminal Froude number, and terminal Sherwood number are compared with experimental data and established correlations, showing good agreement and thereby validating the simulations. On the basis of the validated DNS results, detailed characteristics of the flow and concentration fields are analyzed. At Eo = 4.9, the wake is non-axisymmetric, consistent with the transition regime. At higher Eo, corresponding to the fully turbulent regime, a large open recirculation zone develops downstream of the bubble. A peak in the concentration fluctuation c′ is observed within the liquid film. Its magnitude increases with increasing Schmidt number and follows a scaling law c′ ∼ Sc1/4. In the wake, a pronounced peak in c′ occurs near the wall jet and decays gradually downstream. The wake statistics further suggest scaling behaviors of c̄∼Sc1/4 and c′ ∼ Sc1/4, although additional cases are required for confirmation. Overall, this study demonstrates that DNS is a powerful tool for revealing detailed mechanisms of turbulent flow and interfacial species transfer associated with Taylor bubbles. A key remaining challenge is robust control of parasitic currents, which may become significant owing to interactions between the sharp gas–liquid interface and turbulent fluctuations.