Description

This simulation is about air gap membrane distillation (AGMD) via ANSYS Fluent software.

Water desalination systems include two general methodologies: "thermal desalination" and "membrane desalination." In the "thermal method," phase exchange occurs to produce fresh water; while in "the membrane method," specified membranes are used to separate water from impurities.

Membrane distillation (MD) systems combine the thermal and membrane methods. In these systems, both phase exchange occurs, and specified filters are used.

There are different MD systems. In this project, Air-Gap Membrane Distillation (AGMD) is studied.
This AGMD system consists of four zones; including the feed channel, membrane layer, air gap, and cooling channel. Hot water flows in the feed layer, and cold water flows in the cooling layer in the opposite direction. Next to the feed water layer, the membrane layer is placed, and then an air gap is placed next to the cooling water layer.

AGMD performance system is considered in the processing systems category. It means, that in the first step, the water experiences surface evaporation, and in the second step, the pure water vapor condenses on the condensate surface.
Therefore, for simplicity, it is assumed that the hot feed water has already been converted to steam. So, the saturated steam flows in the feed channel to be ready to condense in the air gap section.

The geometry of the present project is modeled in two dimensions with Design Modeler software.

Then the model meshed with ANSYS Meshing software. The model's grid is structured, and 150000 cells have been created.

Methodology: Air Gap Membrane Distillation (AGMD)

In this simulation, three different phases are used. The steam inside the system is supposed to turn into water, and condensation occurs. On the other hand, there is also an air gap within the system. So a multiphase flow must be used instead of a single fluid.

The VOF (volume of fluid) model has been used to define the multiphase model. This multiphase model can completely separate different phases from each other and display a distinct boundary between phases.

For the boundary between the water phase and the vapor phase in the system to be clear, the VOF model is the best option.

Air is defined as the primary phase of the model, and water liquid, and water vapor are defined as the secondary phases of the model. When the multiphase model is used, a parameter called volume fraction is provided for the secondary phases of the model to solve their transport equations.

A phase change occurs between the water and vapor phases. So a mass transfer between these two phases is defined. This mass transfer is defined based on the evaporation-condensation mechanism.

This mechanism deals with the phase change process between liquid and vapor. In the evaporation-condensation mechanism, Lee's equations calculate the mass transfer rate. These equations depend on the saturation temperature and the frequency coefficient of evaporation and condensation.

Also, a porous medium has been used for the membrane part of the system. A parameter called porosity is defined to express the permeability of the porous medium. This coefficient represents the ratio of empty volume to total volume.

Conclusion

After the end of the simulation solution, the contours related to temperature, phase change rate between water and vapor, and the volume fraction of each of the water and vapor have been obtained.

The results show that the temperature decreases on the cold side of the air gap. The temperature contours show the thermal boundary layer well. The highest condensation or phase change rate occurs in the regions with the temperature drop. The negative sign for the phase change rate indicates the transformation from vapor to liquid phase.

Also, when the volume fraction contour of distilled water is investigated, a film of liquid appears on the cold plate of the air gap. This produced fresh water goes to the bottom of the air gap due to gravity.

The results prove that the current water desalination system works correctly and the membrane distillation mechanism works correctly.