Numerical simulation of the flow in a rotating-disk membrane module

T.G. Kang*, K.S. Moon, J.S. Kim, Korea Aerospace University; G.K. Park, S.U. Kim, BKT Co. Ltd., , Korea

A common problem in crossflow filtration (CFF) is progressive fouling on membrane surfaces by rejected particles, increasing the resistance to permeate flow, which results in decline in the permeate flux. To mitigate the progress of fouling, in dynamic filtration, an active control over the flow in a membrane module is introduced by moving parts such as rotating disks, rotating membranes, impellers, or vibrating components. Though applying higher shear stresses on the membrane surface delays the progress of fouling, it requires more energy compared with a classical CFF module with any moving parts. To realize an energy-efficient dynamic filtration module, it is needed to understand detailed flow characteristics influenced by operating conditions and geometrical parameters of the module. In this regard, we investigate the flow in a dynamic membrane module that consists of a periodic stack of a rotating disk placed between two fixed membranes (See Fig. 1).

Fig 1. A unit structure of the rotating membrane module. The height of the outer disk is 19.4 mm. The radius and thickness of the rotating disk are 72.5 and 4 mm, respectively.

On the membrane surface, there are holes through which the feed flows in and out. A commercial CFD software (ANSYS-CFX 18.0) is employed to solve the flow in the module. All the computations are carried out using a workstation with dual 10-core CPUs and 128GB memories. In the membrane module with specific geometric parameters used in present study, the flow belongs to a laminar flow regime, thus steady incompressible Navier-Stokes equations are solved using a MRF (multiple reference frame) method, which assumes the rotating flow in the computational domain to be quasi-steady. Spiral motion of the flow is observed, but it varies with the vertical coordinate z (See Figs. 2 and 3).

Fig 2. Rotatonal flows at several cross-sections at z=-9.6, -8, -5, and -3 mm.
Fig 3. Countours of the shear stress in the xy planes at z=3 and 9.6 mm.

Detailed fluid flow is affected by the rotating speed of the disk and the geometrical parameters of the disk such as the radius and the thickness. It is worthy to mention that the flow rate of the feed through the inlet and the location (or spatial distribution) of holes on the membrane surface have significant influences on the rotating flows. Improperly chosen locations of the holes create dead zones the fluid motion, resulting in a poor filtration capacity. The present study present an example in which CFD can be an efficient tool in understanding the flow phenomena in a membrane module and choosing a proper set of operating conditions and geometrical parameters to enhance filtration performance.

Session: M4 - Short Oral + Poster Presentations
Day: 14 March 2018
Time: 14:45 - 16:45 h

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