Mass Transfer Macro,UDF, Evaporation and Condensation Rate CFD Simulation

Description

In this project, we performed a numerical simulation using ANSYS Fluent software’s User-Defined Function (UDF). For this CFD product, we used MASS_TRANSFER Macro to write UDF programming. We considered a chamber inside which several rows of tubes are placed. Water vapor enters from the chamber’s top and exits from the bottom. Liquid water also passes through the internal tubes of the chamber.

The temperature difference between the water vapor inside the chamber and the liquid water inside the pipes leads to heat transfer. This heat transfer can lead to phase exchange. If water vapor reaches below the saturation temperature, condensation occurs. If the liquid water reaches above the saturation temperature, evaporation occurs.

In this project, we intend to estimate the evaporation and condensation rates. In the first step, we modeled the geometry in 3D with Design Modeler software. Then, we meshed the model with ANSYS Meshing software. The Mesh is unstructured, and 1,574,100 cells are created. Finally, we numerically simulated the current model based on the CFD method by ANSYS Fluent software.

Methodology (UDF)

In this project, we define two fluids: liquid water and water vapor. So, we used the Multiphase model. We use the Volume of Fluid (VOF) multiphase model since we intend to prevent two fluids from mixing.

Since we aim to model the phenomenon of evaporation and condensation, we need to define a Mass Transfer between liquid water and water vapor. So, we used a User-Defined Function (UDF) to define the mass transfer rate.

We need to use the Mass Transfer Macro (DEFINE_MASS_TRANSFER macro) for this UDF. According to this UDF, we considered the temperature of 373.15 equivalent to saturation temperature. If the temperature is lower than the saturation temperature, the condensation relation is defined; if the temperature is higher than the saturation temperature, the evaporation relation is defined.

Conclusion (UDF)

After completing the calculation, we will review the results. To analyze the results, we obtain some contours. We obtain the volume fraction of water (liquid), the volume fraction of water (vapor), and temperature contours. The contours show that heat transfer leads to evaporation and condensation.

Inside the chamber, the water vapor reaches below the saturation temperature, resulting in condensation. Inside the pipes, the water vapor reaches above the saturation temperature, resulting in evaporation. We conclude that we performed the current numerical simulation correctly, and our UDF worked correctly.