A Perspex test cell was made to observe the flow regime directly in plate-and-frame modules under different conditions. The pressure drops in channels with and without spacer were examined. The critical Reynolds number(Rec) were determined with the correlations for Re and friction factors. The results showed that Rec in the spacer filled channel was much smaller than that in the empty one

indicating that the turbulence regime was reached at lower fluid velocity. The results were verified through the dye injecting method. Channels with different surface roughness

spacer arrangement and spacer porosity were also investigated. Finally

a laboratory scale spiral-wound module was made and a similar pressure drop situation was obtained

which confirms that the method is also suitable for the spiral-wound modules.