Unsteadiness and transition to turbulence in woven spacer filled channels for Membrane Distillation

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-w 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 PIH=2, two spacer orientations theta with respect to the main flow (0 degrees and 45 degrees), and bulk Reynolds numbers Re from similar to 200 to similar to 2000, an interval of great interest in practical MD applications. For both values of theta, CFD predicted steady-state flow for Re up to similar to 300, and chaotic flow for Re larger than similar to 400. In the intermediate range Re approximate to 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>similar to 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.