Three-dimensional simulation of gas-liquid flows in square bubble columns using the Euler-Euler approach and k-E mixture turbulence model are presented. The population balance equation is solved by the homogeneous MUSIG model and breakup and coalescence are specified by a model according to Luo. Especially the effect of different boundary conditions at the inlet and the resulting impact on bubble size distribution was studied. Radial and vertical profiles of Sauter mean diameter are compared with those experimental values reported in literature.