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Air Gap Membrane Distillation (AGMD) CFD Simulation

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The present problem validated the article “Numerical investigation of air gap membrane distillation (AGMD): Seeking optimal performance” by ANSYS Fluent.

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Description

Paper Description

The present problem simulates the process of air gap membrane distillation (AGMD). This simulation is based on the data in the reference article “Numerical investigation of air gap membrane distillation (AGMD): Seeking optimal performance” and its results are compared and validated with the results in the article.

The chamber of these membrane distillation desalination systems is made by four 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, the space for 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 flow of cold water enters it from the other side.

So that the membrane in the form of a filter as well as 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.

Paper Description

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 equals 100 and, consequently, the inlet velocity of the hot salt water flow, the air flow and the cold water flow has an inlet velocity of 0.1 m.s-1; But 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.

Also, the inlet temperature of salt water is equal to 348.15 K and 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, and 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.

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 air flow.

Paper Description

Hence, the interface wall between the hot salt water flow and the membrane as a heat sink with a value equivalent to -62122.815 Wm-3 (according to relationships 7 to 9 of the article) and the interface surface wall between the membrane and the air flow as a heat source with a value equal to +62122.815 Wm-3.

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.

materialsaline waterpure waterwater vaporairPVFD
density (kg.m-3)1013.2995.20.55421.2251175
specific heat (j.kg-1.K-1)4064.84182.120141006.431325
thermal conductivity (W.m-1.K-1)0.6420.6130.02610.02420.2622
viscosity (kg.m-1.s-1)0.0005860.0008380.00001340.000017894

Geometry & Mesh

The present model is drawn in two dimensions using Design Modeler software. The present model is rectangular in shape and consists of four parts: hot salt water flow space, cold water flow space, air gap, and membrane as a porous medium. The following figure shows a view of the geometry.

AGMD

The meshing of the model 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 . The following figure shows the mesh.

AGMD

CFD Simulation

To simulate the present model, several assumptions are considered:

  • We perform a pressure-based solver.
  • The simulation is steady.
  • The gravity effect on the fluid is equal to -9.81 m.s-2 along the Y-axis.

A summary of the defining steps of the problem and its solution is given in the following table:

Models
Viscous modelLaminar
EnergyOn
Boundary conditions
Inlet – FeedVelocity Inlet
velocity magnitude0.1 m.s-1
temperature348.15 K
Inlet – AirVelocity Inlet
velocity magnitude0.1 m.s-1
temperature298.15 K
Inlet – PermeateVelocity Inlet
velocity magnitude0.1 m.s-1
temperature298.15 K
OutletsPressure Outlet
gauge pressure0 pascal
Wall between feed channel & membraneWall
wall motionstationary wall
wall thickness0.0001 m
heat generation rate for coupled wall-62122.815 W.m-3
Wall between membrane & air gapWall
wall motionstationary wall
wall thickness0.0001 m
heat generation rate for coupled wall+62122.815 W.m-3
Solution Methods
Pressure-velocity coupling SIMPLE
Spatial discretizationpressuresecond order
momentumsecond order upwind
energysecond order upwind
Initialization
Initialization method Standard
gauge pressure0 Pascal
velocity (x,y)0 m.s-1
temperature300 K

Paper Validation

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 temperature of the fluid bulk 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 line and point 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.

sectionpresent simulation (K)paper results (K)difference (%)
top membranetop bulktop membranetop bulktop membranetop bulk
0.0570.3102474.6651269.31774.9861.430.42
0.169.9293874.4033967.86774.7223.030.426
0.1569.6201974.142566.68173.6674.40.645

Results

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

All files, including Geometry, Mesh, Case & Data, are available in Simulation File. By the way, Training File presents how to solve the problem and extract all desired results.

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