A computational fluid dynamics (CFD) model was used to simulate the flow in a narrow spacer-filled channel, which represents the longitudinal section of a spiral-wound module membrane. The flow was computed for different geometries and Reynolds numbers. Calculations show the presence of different flow patterns in the laminar regime. Equations describing the critical Reynolds numbers at which the transition between different patterns occur were derived. It is shown that the critical Reynolds number determines the point at which good mixing conditions are achieved without excessive pressure drop.