Reverse Osmosis (RO), CFD Simulation Ansys Fluent Training

Description

This simulation is about reverse osmosis CFD Simulation via ANSYS Fluent software.  Osmosis is a natural phenomenon that shows a fluid tends to move from a solution with a lower concentration to a higher concentration until the concentration on both sides is balanced. Suppose a semi-permeable membrane is placed between pure and impure water.

Then water movement, according to the phenomenon of osmosis, is toward impurity until a pressure difference appears between the two pure and impure parts (on both sides of the membrane). This pressure difference is called the osmotic pressure difference. If a pressure equivalent to the osmotic pressure is applied to the non-specific part of the system, this fluid movement from the pure part to the impure part is interrupted. If this pressure is greater than the osmotic pressure, the direction of the natural movement of water is reversed.

Reverse osmosis water desalination systems work exactly according to this mechanism. In these systems, a pressure beyond the osmotic pressure is applied reverse towards the semipermeable membranes. Passing through the filters separates salt and impurities from the base water.

This project includes two problems and has been simulated twice. The first project only deals with fluid behavior due to osmotic pressure. A closed chamber is modeled, which is divided into two parts. Between these two parts, a barrier is supposed to be removed at once.

There is a saltwater solution on the left side of the chamber, and on the right side, there is pure water. The purpose of this simulation is to investigate the movement of fluid between two parts with different concentrations. This displacement can express the concept of the osmotic phenomenon and osmotic pressure.

If the first project is approved, the water desalination system using the reverse osmosis method will be investigated in the second project. In this simulation, a membrane is used in the middle of the system chamber. A porous medium has been used to define the membrane.

The flow of water and salt enters the chamber from the inlet on the model’s left side and moves toward the membrane. After the solution reaches and hits the membrane, pure water passes through it, and salt or water with a high concentration is trapped behind it.

The geometry of the present project is modeled in two dimensions with Design Modeler software. The model is related to a rectangular chamber with a simple geometry structure; a membrane is placed between its two parts. Then the model meshed with ANSYS Meshing software. The model’s grid is structured, and 44800 cells have been created.

Methodology: Reverse Osmosis (RO) CFD Simulation

In this simulation, an impure solution is used instead of pure fluid. So it is necessary to define the two-phase flow. For this purpose, the multiphase model has been used. One can use VOF, mixture, and Eulerian models to define the multiphase model. In the case of Reverse Osmosis, the Eulerian model has been used, which is the most complex method for defining multiphase flows.

Water is defined as the primary phase, and salt is the secondary phase dissolved in water. The concentration of salt dissolved in water is equal to 0.02. The concentration value of the phases is determined by a parameter called volume fraction, and the solver solves the transport equations related to this volume fraction.

Also, a porous medium has been used to model the membrane between two system parts. The porosity determines the permeability of the porous medium. The porosity equals the volume of empty space to the total volume.

This simulation is done in two steps. In the first model, fluid movement is natural and without external forces, but it is necessary to define a mandatory boundary condition in the second model. Since the project’s goal is to investigate the behavior of the system over time, the solution is time-dependent or unsteady.

Conclusion

After the completion of the solution process, two-dimensional contours related to the pressure and volume fraction of water and salt have been obtained. Since the solution is unsteady, the results have been compared at different times to obtain the system’s behavior over time. Also, the animation related to changes in the volume fraction of salt soluble in water has been obtained. All results are obtained in both simulation modes.

The first simulation has a closed enclosure, and no external boundary conditions are applied to the system. Initially, the model’s left side consists of water and salt, and the right contains only pure water. The results over time show that the fluid moves from the side with a higher concentration to the side with a lower concentration.

This displacement continues until the concentration on both sides of the system reaches equilibrium. This fluid movement happens naturally without the intervention of external forces. This phenomenon indicates the osmotic property of the fluid, which is correctly modeled in the problem.

The second simulation uses a porous membrane or medium in the system’s middle. The water and salt solution moves toward the membrane at a specific speed and pressure (beyond the osmotic pressure). This movement is opposite to the normal direction of fluid movement (according to osmotic pressure).

The results show that pure water passes through the membrane, but the salt dissolved in the water is trapped behind the membrane. This movement continues until a completely concentrated solution is obtained behind the membrane and pure water after the membrane.

These results prove that the reverse osmosis sweetening system works properly and makes the water purification process. Also, pressure changes show that the fluid pressure difference on both sides of the reverse osmosis system increases over time.