Characterization of fluid dynamics in spacer-filled channels for membrane filtration using Doppler optical coherence tomography

The efficiency of membrane filtration is limited by the presence of concentration polarization (CP) and membrane fouling. The development of novel techniques to mitigate these negative effects requires an in-depth understanding of the fluid dynamics inside the fluid channel filled with a spacer that serves as a turbulence promoter. The current study was to apply Doppler optical coherence tomography (OCT) to characterize the velocity profile normal to the membrane surface, which is difficult to obtain by conventional techniques used for membrane characterization. The Doppler OCT system was used to visualize the flow patterns inside a unit cell of the spacer in a modified membrane filtration module. The orientation of the spacer was varied with respect to the bulk flow direction. A series of Doppler images was obtained to demonstrate the subtle interactions between the fluid and the spacer filaments; these characterization results then were used to interpret the performance variation of a reverse osmosis (RO) process with the same spacer configurations. The experimental results presented in this paper validate the ability of the Doppler OCT technique to characterize the fluid dynamics in the spacer-filled channel during membrane filtration. The proposed approach will be a useful tool to optimize the spacer design for better filtration performance.