Air Gap Membrane Distillation (AGMD)

Description

This project simulates air gap membrane distillation (AGMD) using ANSYS Fluent. Producing potable water from saline or impure sources is a central goal of clean water engineering, and AGMD is one of the membrane distillation technologies developed specifically for this purpose.

Water desalination systems fall into two broad categories: thermal desalination and membrane desalination. In the thermal method, a phase change is used to produce fresh water, whereas in the membrane method, specialized membranes separate the water from its impurities. Membrane distillation (MD) systems combine the two approaches — they rely on both a phase change and a dedicated filter membrane. Several MD configurations exist; the one studied here is Air Gap Membrane Distillation (AGMD).

The AGMD system consists of four zones: the feed channel, the membrane layer, the air gap, and the cooling channel. Hot water flows through the feed layer while cold water flows in the opposite direction through the cooling layer. The membrane layer sits next to the feed water, and the air gap is placed between the membrane and the cooling channel. In operation, the water first undergoes surface evaporation, and the resulting pure vapor then condenses on the cold surface. For simplicity, the hot feed water is assumed to have already been converted to steam, so saturated steam flows through the feed channel, ready to condense in the air gap.

The geometry was modeled in 2D using Design Modeler, and the model was meshed in ANSYS Meshing using a structured grid of 150,000 cells.

Methodology

Because the system involves the steam turning into water through condensation, together with the presence of an air gap, three phases must be represented, so a multiphase model is required rather than a single fluid. The VOF (Volume of Fluid) model is used, since it cleanly separates the different phases and resolves a distinct interface between them — the best choice for capturing a sharp boundary between the water and vapor phases. Air is defined as the primary phase, with liquid water and water vapor as the secondary phases; the volume fraction of each secondary phase is solved through its transport equation.

A phase change occurs between the water and vapor phases, so a mass transfer is defined between them based on the evaporation-condensation mechanism. This mechanism governs the phase change between liquid and vapor, with Lee's equations used to calculate the mass transfer rate; these equations depend on the saturation temperature and the frequency coefficients of evaporation and condensation. In addition, the membrane is represented as a porous medium, with a porosity parameter — the ratio of void volume to total volume — defining its permeability.

Conclusion

On completion of the solution, contours of temperature, the phase change rate between water and vapor, and the volume fraction of each of the water and vapor phases were obtained.

The results show the temperature dropping on the cold side of the air gap, with the temperature contours clearly revealing the thermal boundary layer. The highest condensation (phase change) rate occurs in the regions where the temperature falls, and the negative sign of the phase change rate indicates the transformation from vapor to liquid. Examining the volume fraction contour of the distilled water reveals a film of liquid forming on the cold plate of the air gap; this freshly produced fresh water then runs to the bottom of the air gap under gravity.

Overall, the results confirm that the desalination system operates correctly and that the membrane distillation mechanism performs as intended — demonstrating how AGMD converts hot saline feed into clean, condensed fresh water, and how CFD can be used to evaluate and optimize such clean-water technologies.