Step Solar Still by Solar Ray Tracing (Species Transport), ANSYS Fluent CFD Simulation Training

Description

Description

The present study investigates the performance of a step solar desalination unit by ANSYS Fluent software. We perform this CFD project and investigate it by CFD analysis.

The present model consists of a small chamber with a sloping glass surface on both sides and steps within it, where saline water flows on the surface of these steps.

Solar radiation heat transfer passes through the glass to the surface of the water in the enclosure to evaporate the water surface on the steep walls. The resulting vapor meets the cold glass surface and undergoes a distillation process.

Pure water from hot vapor distillation moves down the slope of the glass plate and discharges as pure water.

The present model is designed in three dimensions using Design Modeler software. Since the geometry is symmetric, only one-quarter of the geometry is modeled. This quadrilateral geometry consists of two sloping glass and steps carrying water flow.

The meshing of the model has been done using ANSYS Meshing software. The element number is 809037.

Step Solar Still Methodology

To convert the saline water into fresh water, it should be turned into vapor; then, by condensing it, fresh water is obtained. The distillation devices normally comprise a sloped glass layer placed on top of the device and a stepped platform for storing saline water that will be distilled.

The glass layer acts as a hard substrate and a transparent layer that allows sun rays to enter the device. The heating process for obtaining vapor from saline water is generally done using the heat absorbed from solar rays in the distillation devices.

Next, the generated vapor can be condensed into fresh water by rising and colliding with the glass substrate, which acts as a cold surface. Hence, the solar ray tracing model is enabled to simulate the heat absorbed by the sun.

Also, the species transport model is activated to simulate the two phases of water and vapor inside the designed distillation system. It should be mentioned that this method assumes a mixture of water and vapor inside the chamber interior and does not simulate distilled water flow.

In fact, at the beginning of the simulation, the space inside the chamber is filled with vapor only; the bottom plane of the chamber is considered the surface of the water, and since the water evaporates on the surface, it is assumed that the surface consists of vapor.

Constant values of 1.00314⨯10-5 m²/s and 0.0002 kg/ms are considered for the mass diffusion coefficient and the thermal diffusion coefficient, respectively, to convert water into vapor and vice versa.

It is also assumed that the sloping plate where the steam collides is composed only of vapor. It is also assumed that the ambient air temperature is 311.75K with a heat transfer coefficient of 25W/m²K.

The stepped platform containing saline water is assumed only to absorb heat with no transmissivity, while the glass substrate is assumed to have maximum transmissivity with no absorbance.

Gravity has been enabled in the Y direction, and the incompressible ideal gas model accounts for the density difference between water and vapor.

Step Solar Still Conclusion

At the end of the solution process, three-dimensional contours related to the velocity, temperature, velocity vector, and species mass fraction inside the distillation device are obtained.

The contours related to velocity and its vectors show how the generated vapor rises within the distillation device to hit the top sloped glass surface.

Also, by viewing the 3D temperature contour, it can be easily observed that the temperature of saline water on the stepped platform has increased, making it evaporate.

Meanwhile, the temperature of the glass layer is cold enough to cause the vapor to condense and then slip to the lower part of the distillation device, where the freshwater is collected.

Also, by viewing the species mass fraction contour, it can be seen that the mass fraction of water is equal to one on the stepped platform and glass substrate. At the same time, the water volume fraction value decreases inside the distillation system space, indicating that vapor is forming there.