Experimental and numerical (CFD) results, obtained for 12 different flow-aligned spacer structures under different hydrodynamic conditions, were investigated in order to gain insight into the flow and mass transfer profiles inside the channels of membrane modules. The CFD results were analysed in terms of shear stress and mass transfer at the walls. A modified friction factor was introduced to investigate the effect of the hydrodynamic conditions in the channel on the shear stress at the walls. The experimental results showed that the transition between laminar and transitional flow regime could be rigorously determined for the spacers under study. The excellent agreement between the CFD and experimental results suggests that the cyclic inlet and outlet boundary condition can be applied with high confidence. Additionally, it was found that the flow structure is determined for all cases studied mainly by the transverse filaments. The presence of longitudinal filaments in the channel was shown to not significantly affect the flow structure. Finally, mass transfer results demonstrated that the modified friction factor could be used for selecting the best spacer in terms of mass transfer efficiency. (c) 2007 Elsevier B.V. All rights reserved.