Experimental and Numerical Study of Flow Within Spacer-Filled Channels
Mojab, Seyed Mohammad
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A combined experimental and numerical study is presented on the hydrodynamic characteristics of flow within spacer-filled channels representative of those used in spiral wound membrane (SWM) modules. Spacers are used in the membrane modules to maintain a uniform gap between the membrane layers, as well as to control the flow which may reduce the effect of fouling and concentration polarization phenomena. A 10× scaled-up model of a spacer-filled channel was constructed that enabled detailed non-intrusive particle image velocimetry (PIV) measurements of the average velocity and fluctuation flow fields. The flow characteristics were investigated for Reynolds numbers (based on the hydraulic diameter and the interstitial velocity) ranging from 100 to 1000. Computational studies were also conducted using computational fluid dynamics (CFD) and validated with the experimental results. It was found that the main flow splits into two main streams which move parallel to the spacer filaments with 90° direction difference to each other. The interaction of these two streams at the channel centre-plane where they exchange momentum creates secondary swirling motions in the main flow streams. Counterclockwise swirling flows (primary vortices) that rotate with the main flow streams and small clockwise swirling motions (secondary vortices) observed at the corners of the channel and spacer filaments regions for flow at Re ≥ 350 were identified. Four different flow regimes were observed. Laminar-steady (Re ≤ 200), laminar-unsteady-periodic (Re around 300), unsteady (Re ≥ 350) and onset of turbulent flow (Re = 1000). A better design of feed spacers is proposed for the purpose of improving the membrane module performance. This new spacer forces the flow streams toward the membrane walls with a zigzag motion of flow between the spacer unit cells as well as promotes flow instabilities at a lower Reynolds number compared to the CONWED spacer.