Description

This simulation is about surface evaporation in solar desalination via ANSYS Fluent software. We perform this CFD project and investigate it by CFD analysis.

A simple solar still is designed in this simulation, consisting of inclined glass plates. Solar rays pass through the glass and reach the water's surface inside the system. Heating the water surface causes surface evaporation.

The produced steam is pure and impure materials settled at the bottom of the system. This pure steam hits the inner surface of the inclined glass and causes condensation by losing heat. The slope used for the glasses causes the drops of distilled water to slide on the surface of the glass and be directed to the system's outlet.

The geometry of the present project is modeled in three dimensions with Design Modeler software. Then the model meshed with ANSYS Meshing software. The model's grid is structured, and 938,174 cells have been created.

Methodology: Surface Evaporation considering Solar Desalination

In this simulation, three different phases are used. In the initial state, the system is filled with air, and water is up to a certain level. After the solution process starts, the water gradually evaporates from the surface and creates vapor. So, to define the three phases simultaneously, it is necessary to use the Multiphase Model.

The VOF (volume of fluid) model has been used to define the multiphase model. This multiphase model can completely separate different phases from each other and display a distinct boundary between phases. Also, since the separation boundary between the two phases does not have a layer transition, the Sharp option has been used to interface between the phases.

Water liquid is defined as the primary phase of the model, and air and water vapor are defined as the secondary phases of the model. When the multiphase model is used, a parameter called volume fraction is provided for the secondary phases of the model to solve their transport equations.

Since a phase change occurs between the water and vapor phases, a Mass Transfer between these two phases is defined. This mass transfer is defined based on the Evaporation-Condensation mechanism. This mechanism deals with the phase change process between liquid and vapor.

A UDF function defines the rate of evaporation (water to steam). The equation used in this function is equivalent to the surface evaporation rate. The characteristic of surface evaporation is that it only happens on the surface of the water's level, and it is possible at a temperature lower than the saturation temperature. At the same time, boiling is a volumetric phenomenon and only happens at the saturation temperature.

Conclusion

After solving, the contours of temperature, pressure, and volume fraction of each water, air, and vapor phase have been obtained. The results correctly show that the water starts to evaporate from the surface with time. For this reason, the water level decreases over time.

In addition, the results show that the mass transfer does not necessarily occur at the saturation temperature. So, it can be said that the surface evaporation mechanism based on the definition of the UDF has worked correctly.