Clean Water: Reverse Osmosis (RO)

Reverse Osmosis (RO) CFD Simulation, ANSYS Fluent Tutorial

Description

This project simulates reverse osmosis using ANSYS Fluent.

Reverse osmosis is one of the most widely used technologies in clean-water engineering, where it drives desalination and water-purification systems that turn seawater, brackish water, and contaminated supplies into potable water. Understanding how salt and impurities separate across the membrane is central to designing and improving these systems.

Osmosis is a natural phenomenon in which a fluid tends to move from a region of lower concentration to one of higher concentration until the concentration on both sides is balanced. Imagine a semi-permeable membrane placed between pure and impure water. By osmosis, water moves toward the impure side until a pressure difference builds up across the membrane. This difference is called the osmotic pressure. If a pressure equal to the osmotic pressure is applied to the impure side, the fluid movement stops. If the applied pressure exceeds the osmotic pressure, the natural direction of flow reverses.

Reverse osmosis desalination systems work on exactly this principle: a pressure beyond the osmotic pressure is applied across the semi-permeable membrane, and as the water passes through, salt and impurities are separated from it.

This project is simulated in two parts. The first part looks only at fluid behavior driven by osmotic pressure. A closed chamber is modeled and divided into two sections by a barrier that is removed instantaneously. The left side holds a saltwater solution and the right side holds pure water. The goal is to observe how fluid moves between the two sections of different concentration, which illustrates the concepts of osmosis and osmotic pressure.

Building on the first part, the second part studies the reverse osmosis desalination system itself. Here a membrane, modeled as a porous medium, is placed in the middle of the chamber. The water-and-salt mixture enters from the inlet on the left and moves toward the membrane. When the solution reaches the membrane, pure water passes through while the salt (the higher-concentration water) is trapped behind it.

The geometry was built in two dimensions in Design Modeler as a simple rectangular chamber with a membrane between its two sections. The domain was meshed in ANSYS Meshing using a structured grid of 44,800 cells.

Simulation Methodology

Because an impure solution is used instead of a single pure fluid, a two-phase flow must be defined, so a multiphase model is used. Of the available options (VOF, Mixture, and Eulerian), the Eulerian model, which is the most complex of the three, is used here. Water is the primary phase and salt is the secondary phase dissolved in it, with a salt concentration of 0.02. Concentration is tracked through the volume fraction, and the solver handles the transport equations for that volume fraction.

The membrane between the two sections is modeled as a porous medium, where the porosity (the ratio of empty space to total volume) sets its permeability. The simulation runs in two steps: in the first model the fluid moves naturally with no external forces, while the second model applies a driving boundary condition. Since the aim is to study how the system behaves over time, the solution is transient (time-dependent).

Results & Conclusion

After solving, two-dimensional contours of pressure and of the water and salt volume fractions were obtained. Because the solution is transient, the results are compared at different times to capture the system's behavior, and an animation of the change in dissolved-salt volume fraction was produced. Results were obtained for both simulation cases.

In the first case (a closed chamber with no external boundary conditions), the left side initially holds water and salt while the right holds pure water. Over time, fluid moves from the higher-concentration side to the lower-concentration side and continues until both sides reach equilibrium. This movement occurs naturally, without external forces, and correctly reproduces the osmotic behavior of the fluid.

In the second case, a porous membrane sits in the middle of the system. The water-and-salt solution is driven toward the membrane at a set velocity and pressure beyond the osmotic pressure, opposite to the natural osmotic direction. The results show pure water passing through the membrane while the dissolved salt is trapped behind it. This continues until a fully concentrated solution builds up behind the membrane and pure water collects beyond it. The pressure results also show the pressure difference across the system increasing over time. Together, these results confirm that the reverse osmosis system works correctly and successfully purifies the water.