Mitigating polarization in flat-sheet membrane distillation through CFD-driven spacer design.

Membrane distillation (MD) is a thermally driven separation process that has attracted attention for its ability to couple with low-grade or renewable heat sources, making it well suited for sustainable water desalination amid rising freshwater demand and climate change pressures. A key open issue remains the optimization of fluid-dynamics within flat-sheet MD modules to minimize flow maldistribution and maximize both heat and mass transfer, and thus module performance. In this work, we numerically investigate and optimize spacer geometries - components that play a critical role in governing these transport processes. Here, using a validated computational fluid dynamics (CFD) approach, we simulate the hydrodynamics and thermal behavior of the direct contact membrane distillation (DCMD) process. Our model is first validated against published experimental data, ensuring that predicted temperature and velocity fields match observed performance metrics. We then conduct an extensive parametric study across a range of novel spacer designs - varying filament shape, orientation, and spacing - to assess their influence on flow uniformity, temperature and concentration polarization, and overall thermal efficiency. Compared to the best performing spacer configuration reported in the literature, our twisted and elliptical spacer geometries achieve up to a 5.4% reduction in temperature polarization coefficient, indicating a measurable enhancement in heat and mass transfer efficiency. These findings provide a clear roadmap for future experimental implementation of innovative spacers in MD modules, with the goal of significantly improving desalination performance and reducing energy consumption.