Investigation into drag coefficient for sub-millimeter bubbles in gas–liquid bubbly flow: Experiments and CFD simulations

The sub-millimeter bubble technique can enhance the gas–liquid inter-phase mass transfer and show immense potential to increase production efficiency. The two-fluid model (TFM), suitable for the CFD simulations on the scale of actual reactors, is a powerful tool for exploring the application of the sub-millimeter bubble technique. Although the accuracy of TFM is largely affected by the drag models, the applicability of the drag models for gas–liquid two-phase flow of sub-millimeter bubbles is scarcely investigated. In the present work, the applicability of the single-bubble drag models was apprised with experimental data of single sub-millimeter bubbles. Then, the bubble swarm effect and its influence on the calculation of drag coefficients were investigated for the gas–liquid two-phase flow of sub-millimeter bubbles under a wide range of gas holdups (0.02–0.21). The accelerating effect was observed in the experiments of gas–liquid two-phase flows. A reasonable explanation for the accelerating effect was discussed based on the bubble size distribution obtained by applying our original image processing method. Based on the experiments, an equation for the drag modification factors was proposed to include the bubble swarm effect. The accuracy and applicability of the proposed equation were validated by applying it together with the accurate single-bubble drag model in the CFD simulation by TFM. The present work investigated the hydrodynamic characteristics of sub-millimeter bubbly flows experimentally and provided an accurate CFD method for further investigation.