(37h) Multiscale Analysis and Simulation of Bubbly Flow

Bubble columns are considered as favorable reactors in process engineering. Scale-up and optimization of these reactors require an extensive understanding of gas-liquid two-phase flow in order to gain further insight into mixing and transfer properties. However, the gas-liquid flow and/or gas-liquid-solid flow in bubble columns is quite complicated not only because of its versatile macro-scale flow regimes, but due to those intricate structures occurring at meso and micro scales, posing great challenges for CFD simulation of multi-phase flow in these reactors. Simulation based on Eulerian-Eulerian method, though usually employed in commercial CFD packages, is still not powerful enough to capture real physics due to our limited understanding on the multi-scale flow structure and hence inadequate development of closure laws including drag, lift and turbulence models. The aim of this study is to use the dual-bubble-size (DBS) model to develop a closure law for the ratio of effective drag coefficient to bubble diameter, and then apply this model in simulation of gas-liquid two-phase flow, mass transfer and chemical reaction for FT synthesis. In addition to mass and momentum conservation equations, the stability condition reflecting the compromise between two different dominant mechanisms serves as another constraint for the model, and therefore it is expected to offer a more accurate drag closure for CFD simulation. Prior to this integration, we investigate the respective effects of bubble diameter and correction factor and find that the effect of bubble diameter is limited, whereas the correction factor due to the bubble swarm effect is eminent and appropriate correction factor has to be selected for different correlations of standard drag efficient to be in accord with experiments. By contrast, the new model can well predict the radial gas holdup distribution, the total gas holdup as well as the two-phase flow field without the need of adjusting model parameters, and then it is used for a scale-up process of F-T systhesis, showing its great potential and advantage in understanding the complex nature of multiscale structure of gas-liquid flow in bubble columns.