Air Gap Membrane Distillation (AGMD), Paper Numerical Validation, ANSYS Fluent Training

Description

The present problem simulates the air gap membrane distillation (AGMD) by ANSYS Fluent software. This simulation is based on the data in the reference article “Numerical investigation of air gap membrane distillation (AGMD): Seeking optimal performance.”

Its results are compared and validated with the results in the article. The chamber of these membrane distillation systems is made of 4 different parts, including the space for the passage of saline water flow as the system feed water, the space for the passage of cold water flow, and the space for the air gap, and the space for membranes or filters.

It is installed between air and hot water. The working mechanism of these membrane distillation systems is such that the flow of hot salt water enters its chamber from one side, and the cold water enters it from the other side.

Eventually, these vapors collide with the cold surface of the space for the passage of cold water flow, and by losing their heat, they distill and turn into pure fresh water.

It is assumed that the water flow inside the model has a Reynolds equal to 100 and, consequently, the inlet velocity of the hot salt water flow, the airflow, and the cold water flow has an inlet velocity of 0.1 m/s; the direction of salt water flow is from left to right, and the direction of air and cold water flow is from right to left.

According to the functional mechanism of this system, heat is transferred from the space related to the flow of hot salt water to the membrane part of the model, and then the same amount of heat is transferred from the membrane part to the space for airflow.

The present model is drawn in 2 dimensions using Design Modeler software. The present model is rectangular and consists of four parts: hot salt water flow space, cold water flow space, air gap, and membrane as a porous medium.

The meshing has been done using ANSYS Meshing software, and the mesh type is structured. The element number is 344400, and boundary layer mesh is also used.

Air Gap Membrane Distillation Methodology

So that the membrane in the form of a filter and an air gap is placed in the space between these two streams, the saline feed water, passing through its common surface with its adjacent membrane, gives off its heat to the membrane environment and is converted to pure vapor inside the membrane.

This steam then passes through the porous medium and enters the air in the air gap with its high temperature. Also, the inlet temperature of salt water is equal to 348.15 K. The inlet temperature of air and cold water is equal to 298.15 K.

In the inner space of the membrane part of the model, water vapor is defined as a flowing fluid. Since the space for the membrane is filtered, a porous medium model is used, which is made of a material called PVDF with a porosity coefficient of 0.85.

Hence, the interface wall between the hot salt water flow and the membrane as a heat sink with a value equivalent to -62122.815 W/m3 (according to relationships 7 to 9 of the article) and the interface wall between the membrane and the airflow as a heat source with a value equal to +62122.815 W/m3.

In the current paper, several different materials are used to define salt water, pure water, water vapor, air, and a type of membrane (PVFD) whose properties include density, specific heat capacity, thermal conductivity, and viscosity.

Air Gap Membrane Distillation Conclusion

At the end of the solution process, the results of the present work were compared and validated with the results in the reference article. Thus, the results are validated by the diagram in Figure 12-c of the paper (in Reynolds mode equal to 100 and surface water temperature equal to 75 ° C).

For this purpose, the fluid bulk temperature in the passage of salt water flow space and the temperature of the upper wall of the membrane space in three different sections, including 0.05 m, 0.10 m, and 0.15 m, are obtained.

To obtain the temperature, the REPORT command is used, and also lines and points are created in the three mentioned sections. The comparison of the results of the present work with the article is shown in the table below.

Also, at the end of the solution, two-dimensional counters of temperature, pressure and velocity were obtained.