A finite element model was developed to study momentum and mass transport in cross flow membrane filtration systems with open and spacer-filled channels. Simulated conditions represented a range of membrane desalination applications and considered both open and spacer-filled straight through channels, as well as a geometry representing lab-scale cross flow membrane filtration cells. The finite element model agreed with classical analytical models for pressure drop, fluid shear, and concentration polarization in straight-through open channels at low flux and high cross flow. In simulations of representative lab-scale cross flow membrane filters, stagnant regions at the entrance and exit regions, as well as developing flow lead to significant deviations between analytic model and finite element model results. In spacer-filled channels, axial pressure drops were substantially higher than in open channels, varying by a factor of 2 for different spacer configurations (cavity < zigzag < submerged), while shear rates were on average enhanced, but appeared to create stagnant zones where the filaments contacted the membrane. On average, feed channel spacers reduced the extent of concentration polarization because of the enhanced wall shear rates. However, the stagnant regions (in front and behind some spacer filaments) led to enhanced concentration polarization, which suggests certain spacer designs could promote local scale and cake formation in spiral wound elements. (c) 2005 Elsevier B.V. All rights reserved.