Air Gap Membrane Distillation (AGMD), ANSYS Fluent CFD Simulation Tutorial

Description

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

Two general methods, thermal and membrane, can be used for water desalination. In the thermal method, heat and phase change is needed to produce fresh water. While in the membrane method, special membranes must be used to separate water from impurities. Sometimes, these two methods of desalination can be combined. Membrane distillation (MD) desalination systems are included in this category.

This method combines the thermal and membrane methods. Because there is a phase change in the system, filters are responsible for trapping impure particles.

In this system, there are two separate paths. On one side of the system, the cold feed water moves and is heated by the heater. Then this hot brine water moves from the other side of the system. Between these two sides, the condensation zone is placed.

Between the condensing zone and the cold side of the system, a special condensing plate (where the steam is converted to water) is placed, and between the condensing zone and the hot side of the system, a special membrane is placed.

The main desalination mechanism in this method is based on the condensation process, but before condensation occurs, the water flow is purified by the membrane. Now, if hot water vapor passes through the membrane and is ready to condense, four situations may occur:

• Directly enters a cold stream and condenses with it: DCMD (Direct Contact Membrane Distillation)
• It is transferred to an air gap cooled by a cold stream and condensed in this gap: AGMD (Air Gap Membrane Distillation)
• It goes to the part that is sucked by the vacuum pump, and then it is condensed in an independent condenser outside the system, and the net flow is separated: VMD (Vacuum Membrane Distillation)
• It goes to the part where the gas flow is swept and then condensed in an independent condenser outside the system, and the net flow is separated: SGMD (Sweeping Gas Membrane Distillation)

This problem has investigated the performance of the AGMD desalination system. This system is designed simply and consists of four parts. From the system’s right side, the cold water flow enters. From the system’s left side, the flow of hot feed water enters in the opposite direction. Next to the feed water side, the membrane is placed, and then an air gap is placed next to the cold side.

For simplicity, it is assumed that the hot feed water has already been converted to steam. So, in this section, saturated steam enters the system 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 the 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.