This work evaluates the ability of multiphase computational fluid dynamics (CFD) models to predict known flow regimes in air-water bubble columns. An initial grid-resolution study shows that grid spacing of 0.25 cm or smaller must be used for adequate resolution. The ability of the two-fluid model to predict homogeneous- and transitional-flow behavior is analyzed next, and the flow predictions are found to be highly dependent on the model formulation (that is, bubble-induced turbulence, drag, virtual mass, lift, rotation, and strain). At low gas velocities, homogeneous flow is observed for only a particular set of force models. At higher gas velocities, the same set of models yields reasonable predictions of transitional flow for small columns. Bubble size and liquid coflow also affect flow structures and flow stability at high gas flow rates. Scale-up to larger column diameters is studied for both the homogeneous- and transitional-flow regimes. In the homogeneous regime, the flow behavior is found to be independent of column diameter. However, because of neglect of coalescence, transition to churn-turbulent flow is not observed at high gas velocities for large column diameters. Q 2005 American Institute of Chemical Engineers.