Direct Numerical Simulation of Flow and Mass Transfer in Spacer-Filled Channels
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Spacer-filled channels are employed in membrane modules in many industrial applications where feed-flow spacers (employed to separate membrane sheets and create flow channels) tend to enhance mass transport characteristics, possibly mitigating fouling and concentration polarization phenomena. In this work direct numerical simulation was performed for the flow in the spacer-filled channels to obtain a better understanding of fluid flow and mass transfer phenomena in these channels. A solute with a Schmidt number of 1 at Reynolds numbers of 300, 500 and 800 (based on the bulk velocity and spacer diameter) was considered. The effect of spacer location was also studied for three different configurations, spacer at the centre of the channel, at off-centre location, and attached to the wall. Instantaneous velocity fields and flow structures such as separation of boundary layer on the walls and on the cylinder, eddies on the walls, recirculation regions and vortex shedding were investigated. A Fourier analysis was carried out on the time series velocity data. Using this analysis the Strouhal number was calculated and the development of the flow towards a broader turbulent state at higher Reynolds number was captured. Other statistical characteristics such as time-averaged velocities and wall shear rates are obtained and discussed. The average pressure loss which represents the operation cost of membrane modules was calculated for the channels and found to be highest for spacer at the centre of the channel and lowest for spacer attached to the wall. Scalar transport equation is directly solved along with Navier-Stokes equation to get the concentration field. Local Sherwood number is obtained on the walls and the relationship between shear stress, vortex shedding, and mass transfer enhancement was explored. The overall Sherwood number and Stanton number of the channels, which indicate the mass transfer performance of the channels, are obtained. It was observed that as spacer approaches the wall mass transfer rate is decreasing.