Water Surface Evaporation is a natural process where liquid water turns into vapor without boiling. This happens in cooling towers, pools, and industrial dryers. Predicting the exact rate of evaporation is difficult because it depends on air temperature, humidity, and wind speed. Standard software settings often cannot calculate this accurately at a free surface. Therefore, engineers use CFD – Water Surface Evaporation models with custom code.

In this report, we perform a CFD Analysis of Water Surface Evaporation using ANSYS Fluent. To get the correct physics, we wrote a specific User-defined function fluent (UDF). This code adds the missing math to the simulation. It uses the Hertz-Knudsen equation to calculate the mass transfer based on kinetic theory. The governing equation we programmed into the UDF fluent is:

Where is the mass flux, is saturation pressure, and is vapor pressure. This allows the Surface Evaporation fluent simulation to track exactly how much water leaves the liquid phase and enters the air phase. For more details on phase change modeling, please explore our Mass Transfer tutorials: https://cfdland.com/product-category/engineering/mass-transfer-cfd-simulation/

Figure 1: Schematic representation of the Water Surface Evaporation process showing the interaction between the liquid phase and the gas phase.

Simulation Process: VOF Model and UDF Mass Transfer Setup

For this CFD – Water Surface Evaporation study, we created a 2D domain. The bottom part contains liquid water, and the top part contains air. The line between them is the free surface. To ensure high accuracy in ANSYS Fluent, we used a Structured Mesh. We generated 92,000 quadrilateral cells. A structured mesh is superior for surface tracking. This refinement is critical. It allows the Surface Evaporation ANSYS Fluent solver to detect tiny changes in temperature and species concentration right where the evaporation happens.

We used the Volume of Fluid (VOF) multiphase model in ANSYS Fluent. This model tracks the position of the water surface. It assigns a value of 1 to water cells and 0 to air cells. However, standard VOF cannot move mass from water to air automatically. To fix this, we compiled a User-defined function fluent. This C-code calculates the Mass Transfer CFD rate using the Hertz-Knudsen equation. It uses the local temperature to find the saturation pressure. Then, it removes mass from the water phase and adds it to the vapor phase. This custom approach gives us a scientifically accurate evaporation fluent simulation.

Post-processing: Analysis of Phase Change and Thermal Dynamics

This section analyzes the engineering data to explain what is really happening inside the domain. We must look at the contours and graphs to verify the physics and understand how the simulation helps designers. First, we analyze the Liquid Volume Fraction contours (Figure 2). These contours show the physical location of the water. Red is liquid, and blue is air. At the start (1 second), the surface is flat. As the simulation runs, the surface moves down. By t = 2.1 seconds, the water level has dropped by approximately 1.5 mm. This is a major achievement. It proves that the UDF fluent code is working correctly by removing liquid mass. We also see that the surface becomes wavy and irregular. This happens because evaporation is not the same everywhere. For a reservoir designer, this is critical. It allows them to predict the exact water loss over time without needing expensive physical tests.

Figure 2: Contours of Liquid Volume Fraction at different time steps, showing the water surface level dropping by 1.5 mm due to evaporation.

Next, we examine the Velocity Magnitude contours (Figure 3). The air above the water does not stay still. As water turns to vapor, it becomes lighter and rises. The contours show vapor plumes moving up with a speed of 0.08 m/s. This is Natural Convection. This movement is very important. It carries the humid air away from the surface and brings dry air down. If this circulation did not happen, the air near the water would get saturated, and evaporation would stop. This helps manufacturers of drying systems understand that they need good airflow to keep the process efficient.

Figure 3: Velocity magnitude contours illustrating the natural convection flow patterns above the water surface during evaporation simulated in ANSYS Fluent.

Finally, we look at the thermal physics. The Temperature contours (Figure 4) show a clear cooling effect at the surface. The water temperature drops from 300 K to about 297 K. This happens because evaporation uses heat energy. The Mass Transfer Rate plot (Figure 5) confirms this activity. The rate rises to a peak and then stabilizes around 8.28 kg/(m³·s). The fluctuations in the graph are caused by the swirling air currents. This CFD Analysis of Water Surface Evaporation successfully links the airflow, cooling, and mass loss into one accurate prediction.

Figure 4: Temperature Distribution contours illustrating the cooling effect (thermal boundary layer) at the interface as water evaporates.

Figure 5: Plot of the Mass Transfer Rate over time, showing the fluctuation of evaporation intensity calculated by the UDF fluent code.