The laminar flow structure and concentration distribution in a narrow rectangular charnel simulating the feed channel of nanofiltration (NF) spiral-wound modules are investigated. The continuity, Navier-Stokes equations and the solute continuity equation are solved by the control volume formulation. To validate the numerical predictions, NF permeation experiments with an aqueous solution of sodium chloride (2 g/l) at 25degreesC were performed in a laboratory cell with a rectangular feed channel (2 mm height x 30 mm width x 20 cm length) filled with a ladder-type spacer with a transverse inter-filament distance of 3.8 mm. The numerical results show that the average concentration polarization for the membrane wall with adjacent transverse filaments is independent of the distance to the channel inlet, while for the membrane wall without adjacent filaments the average concentration polarization increases with the channel length. This is due to the fact that in the first case the concentration boundary layer is periodically disrupted by the transverse filaments while in the second case the concentration boundary layer grows continuously along the channel length. The experimental results of the NaCl apparent rejection coefficients are compared to the model predictions, the agreement being good. These results clearly establish how crucial the spacers configuration is in the optimization of the spiral wound module efficiency.