Fluid dynamics and mass transport in rotating channels with application to Centrifugal Membrane Separation




Pharoah, Jon George

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Centrifugal membrane and density separation (CMS) is a novel technology proposed for treatment of waste water and industrial process streams. This cross flow filtration process combines the energy recovery inherent to centrifugal reverse osmosis (CRO) with the potential alleviation of membrane fouling and concentration polarization due to the favourable effects of centrifugal and Coriolis accelerations. This dissertation presents a computational study of CMS undertaken to understand the basic hydrodynamics and mass transfer of the processes and to provide insight for the design of CMS devices. Two distinct membrane models were developed, the porous wall model (PWM) and the source term model (STM), and incorporated into Computational Fluid Dynamics (CFD) codes which solve the full Navier-Stokes equations coupled to a scalar transport equation which accounts for dissolved species. These models are used to simulate two and three dimensional laminar flows in both non-rotating and rotating reverse osmosis membrane cartridges and to predict permeate fluxes. Plate and frame geometries are first examined and it is determined that CMS benefits most from channels with streamwise directions directed radially. It is also shown that the benefits of CMS can be attributed largely to the secondary flows and mixing associated with Coriolis acceleration, and the PWM and the STM are found to perform similarly in the case of reverse osmosis. Next, the STM is used to perform a parametric study of the flow and mass transfer in rectangular and square rotating channels. It is shown that while normal rotation is preferable to spanwise rotation, relatively small deviations from the spanwise orientation are adequate to achieve most of the normal rotation performance, and that differences between the two orientations are minimal in the case of square channels. Also, the flow characteristics are again shown to correlate well with the magnitude of the Coriolis acceleration. Flows in triangular and circular channels are also considered, and are shown to perform similarly to rectangular channels. These channel orientations have application in hollow fiber membrane modules and potentially in spiral wound membrane modules. Finally, the flow and mass transfer in channels with periodic streamwise obstacles are considered. Such obstacles are related to feed spacers used in spiral wound membrane elements and impact considerably on the flow characteristics and mass transfer performance. Flow obstacles are shown to increase mass transfer performance in all cases, with alternating surface mounted performing best. A preliminary investigation is undertaken into rotating flows with periodic obstacles, and the flow fields are shown to depend strongly on the blockage ratio and on the Rossby number. In most cases, it is found that mass transfer performance does not necessarily correlate with either wall shear stress or the local flow field. Several general conclusions regarding CMS can be drawn from this work. It is preferable to operate a CMS devices at low flow rates, which is contrary to conventional wisdom in membrane separation. Secondly, the mixing induced by channel rotation is both more effective and more efficient than the mixing induced by the feed spacers considered here. Finally, the magnitude of the Coriolis acceleration is the dominant parameter in determining CMS performance. This means that a CMS device can either operate at relatively low rotational speeds with flow in the radial direction, or at higher speeds but lower angles of inclination with respect to the rotational axis.



Membrane separation, Membranes (Technology), Sewage, Purification