Modelling of flow distribution within spacer-filled channels fed by dividing manifolds as found in stacks for membrane-based technologies

Abstract

Manifolds are critical to the performance of different membrane-based technologies. These stack components distribute the inlet and outlet flow with the aim of providing a homogenous flow pattern inside the spacer-filled channels. This work analyzes the role of manifolds on the flow pattern within the channels of membrane stacks used primarily for electrodialysis and acid-base flow batteries. First, residence time distribution (RTD) curves are predicted for a spacer-filled channel by means of three strategies: (a) computational fluid dynamics (CFD) simulation using a periodic unit cell model, (b) parametric models: axial dispersion model (ADM) and the plug dispersion exchange (PED) model using Danckwerts-type boundary conditions, and (c) the ADM and PED model using the segregate flow laminar model (SFLM) as flow pattern at the left-hand side (z=0-) in the Danckwerts-type inlet boundary condition. In contrast to previous studies, it is shown that the dividing manifold generates experimental RTD curves with several peaks characteristic of uneven distribution and long-tailing phenomena with small fluctuations related to the woven spacer and the combining manifold, respectively. The periodic unit cell CFD model, the ADM and PED model with convectional Danckwerts-type boundary conditions do not predict the RTD experimental curves because they do not consider the effect of RTD curves for the manifolds. On the other hand, the SFLM described these RTD curves adequately, as obtained by CFD simulations; this model used the left-hand side of Danckwerts inlet boundary condition for the PDE model, which had a good agreement to the global RTD experimental curves and their peaks. This study demonstrates that the global RTD curve of electrolytes in rectangular, spacer-filled channels must consider the effect of dividing and combining manifolds as inlet/outlet distributors. The strategy developed here can be used to design and scale-up membrane stacks for industrial and energy conversion applications.