The effect of gas bubbles on Taylor-Couette flow patterns in the transition from laminar to weakly turbulent vortex flow is investigated by quasi-2D and 3D CFD simulation. The CFD model is based on the Eulerian-Eulerian two-fluid approach with constitutive closures for inter-phase forces and liquid turbulence and validated by comparison with previously published experimental data. The bubble spatial distribution patterns and the mechanism by which the dispersed fluid obtains a non-uniform radial distribution is discussed and the relative importance of various inter-phase forces is demonstrated. In addition, the experimental observation of vortex axial wavelength expansion due to the presence of bubbles was reproduced by the CFD simulations, and a two fluid Taylor-Couette flow regime map based on three observed regimes was constructed. Lastly, the influence of the azimuthal Reynolds number and gas superficial velocity on vortex wavelength, liquid velocity, mass transfer and turbulence are discussed in detail. (C) 2015 Elsevier B.V. All rights reserved.