Fluid Structure Interaction analysis of lateral fibre movement in submerged membrane reactors

Abstract

A Fluid Structural Interaction (FSI) approach was used to simulate fluid flow, surface shear and filtration flux as a function of aeration induced lateral fibre movement in a submerged membrane system. The two-way FSI approach integrated Computational Fluid Dynamics (CFD), to model the three-dimensional pressure/loads of the air–liquid two-phase flow, and Transient Structural Analysis, to calculate pressure induced displacement of fibres with different material, diameter and looseness. These novel, a priori simulations provide insights into the effects of both membrane intrinsic properties, fibre packing density, and two-phase flow on the critical factors for fouling control. Fibre displacement and membrane surface shear profiles on a 300mm long fibre were spatially and temporarily variable, with irregular periodical patterns established approximately 10s after initiation of aeration at 4.7Nm3/h. Average surface shear was 67% higher for 1.3mm diameter fibres compared to a 1.0mm fibre of identical Young's modulus and looseness. Increasing the fibre looseness from 0.5% to 1% increased the average surface shear by 50.4% (0.56–1.13Pa) for fibres of identical diameter and Young's modulus, whereas reducing the Young's modulus from 76 to 20MPa for fibres with identical diameter and looseness only increased average surface shear by 9.7%. The FSI modelled fibre displacement showed good agreement with experimentally measured fibre displacement data, with 8.3% difference in amplitude and 9.1% in period. The impact of fibre movement on pure water flux was assessed by quantifying the reduction in shell side pressure due to changes in flow of the surrounding liquid. This reduction in shell side pressure, together with the increase of the pressure drop in the lumen side caused by the distortion of fibre geometry, resulted in a decrease in transmembrane pressure and filtration flux for the moving fibre compared to a fixed fibre. FSI simulations of the behaviour of an idealised multi-fibre bundle indicated that reducing the separation between adjacent fibres from 1.0mm to 0.2mm enhanced interactions between adjacent fibres that increased surface shear from less than 0.05 to 0.23Pa. This novel FSI approach provides insight into aeration induced fibre movement enabling critical assessment of the effects of material, fibre diameter and looseness on surface shear and flux in submerged hollow fibre membrane systems.