This study optimizes the design of a spiral-wound membrane module by considering the curvature of the spacer-filled channel. A three-dimensional computational fluid dynamic (3D CFD) technique and an experimental set up with a curved channel filled with a two-layer-filament spacer were used to analyze fluid flow in the channel and the shear stress on the permeable membrane surfaces. Emphasis was placed on the relative size of the filaments between the inner and outer layers, as compared to previous studies that emphasized the characteristic angle toward the feed direction of the spacer, the diameter, the shape and spacing of filaments in the spacer, and the multi-layer arrangement of the spacer. Numerical results and experimental data show that there are inherent changes in the hydrodynamic behavior of the curved spacer-filled channel between the inner and outer permeable walls due to curvature variations. A spacer with unequal filament diameters between the inner and outer layers could be adopted to reduce the imbalance of shear stress between the inner and outer walls so as to extend the service time of the membrane module. (c) 2008 Elsevier B.V. All rights reserved