A numerical and experimental study is presented, aimed at obtaining a better understanding of transport phenomena in spirally wound membrane elements. where feed flow spacers are used to enhance mass transport characteristics and mitigate fouling and concentration polarization phenomena. Direct numerical simulations of the Navier-Stokes equations are performed in three-dimensional geometries which closely represent real spacer-filled membrane channels. A range of Reynolds numbers characteristic of such membrane modules is covered. The results obtained clearly suggest that a transition to unsteady flow occurs at relatively low Reynolds numbers. Qualitative flow features such as the development and separation of boundary layers, vortex formation, the presence of high shear regions and recirculation zones, and the underlying mechanisms are examined. In addition, quantitative statistical characteristics are obtained and compared for a range of spacer geometrical characteristics, including frequency spectra of flow fluctuations, as well as wall shear stresses and pressure drop. The tatter are directly related to mass transport enhancement and can provide input to quantitative criteria for the optimization of spacer geometrical characteristics. Finally, pressure drop measurements are performed in a specially fabricated spacer-filled channel and the results show good agreement with the numerical predictions. (c) 2007 Elsevier B.V. All rights reserved.