Abstract
To characterize the performance of Membrane Distillation (MD) modules, channels filled with woven spacers were investigated by Computational Fluid Dynamics (including Direct Numerical Simulations and the use of the SST k-ω turbulence model) and by parallel experiments with Thermochromic Liquid Crystals. The cases considered here regard mutually orthogonal filaments with a spacer pitch to channel height ratio P/H=2, two spacer orientations θ with respect to the main flow (0° and 45°), and bulk Reynolds numbers Re from ∼200 to ∼2000, an interval of great interest in practical MD applications. For both values of θ, CFD predicted steady-state flow for Re up to ∼300, and chaotic flow for Re larger than ∼400. In the intermediate range Re≈300-400, periodic flow regimes were predicted for both orientations. These regimes were of particular interest and complexity, as they exhibited a slow global oscillation of the flow superimposed on high order harmonics corresponding to fast local oscillations. Experiments confirmed the appearance of unsteadiness for Re>∼300. Heat transfer and friction were little affected by unsteadiness and exhibited a smooth behaviour with Re. The agreement with the experimental results was good using DNS, and acceptable using RANS.
Highlights
Introduction and literature reviewSpacers are widely used in a variety of membrane processes, among which Membrane Distillation (MD), both as structural elements and as mixing promoters, aimed at reducing polarization and enhancing heat or mass transport [1, 2, 3]
The main differences regard the values of Nu maxima, which are somewhat overestimated by the numerical simulations, and the narrow regions of high Nu predicted by Computational Fluid Dynamics (CFD) along the diagonal parallel to the main flow, not present in the experiments
The aim of the present work was to answer the following questions, which are all of some considerable importance for the optimum design of Membrane Distillation modules and their spacers: I) In the complex geometry of a woven spacer filled channel, what are the possible flow regimes, the associated Reynolds number ranges and the modes of transition from a regime to another? II) How do these transitions affect friction and heat transfer? III) Is there a computational approach capable of reproducing experimental data through the whole
Summary
Spacers are widely used in a variety of membrane processes, among which Membrane Distillation (MD), both as structural elements and as mixing promoters, aimed at reducing polarization and enhancing heat or mass transport [1, 2, 3] They cause pressure drop to increase [4]. An increasing number of CFD studies have been devoted to the simulation of unsteady flows Limiting this brief survey to fully three-dimensional simulations (the only that can capture the complexity of the flow in a real spacer-filled channel), Koutsou et al [10] performed Direct Numerical Simulations (DNS) with FluentTM, highlighting that unsteadiness may occur even at low Reynolds numbers, and Tamburini et al [11] compared DNS results for early turbulent flow in channels filled with overlapped-type spacers, obtained by ANSYS-CFXTM, with experimental measurements using Thermochromic Liquid Crystals. Mahdavifar et al [12] performed DNS with OpenFOAM to investigate the effects of the spacer-to-wall clearance on the flow field; OpenFOAM
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