Single-phase flow patterns in stirred reactors were investigated both computational, with FLUENT, and experimental, by Laser Doppler Velocimetry (LDV). This was done for three different impeller types, viz. two axial flow impellers and a disc-turbine. The effect of geometrical modifications to the vessel geometry was studied as well. In particular, the exact baffle arrangement has a strong impact on the flow pattern. It will be shown that the single-phase flow patterns exhibit unexpected features not recognized before. An accurate numerical computation requires full three dimensional grids. Further, anisotropic turbulence models, in this case the Algebraic Stress Model, give better predictions than the k-epsilon turbulence model. When these requirements are met, the predicted flow patterns compare very well with the experimental data. Thus, a quick assessment of the influence of geometrical variables can be made just using numerical simulations.