Film Boiling CFD Simulation using Ansys Fluent is a critical study for industries dealing with very hot surfaces. This process happens when a surface is so hot that the liquid touching it turns into gas instantly. This creates a continuous layer of vapor, like a blanket, between the wall and the liquid. This vapor film stops heat from moving easily, which can be dangerous for nuclear reactors or metal cooling systems.

Engineers use Film Boiling ANSYS Fluent simulations to predict how this vapor behaves without doing dangerous experiments. In this report, we use a special method. We wrote a computer code called User Defined Function (UDF) to calculate exactly how fast the liquid turns into gas. We also used User Defined Memory (UDM) and User Defined Scalar (UDS) to track the data. This Film Boiling CFD simulation helps us see the bubbles forming and rising, which is essential for designing safe cooling equipment. 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: A conceptual diagram showing the Film Boiling phenomenon, where a stable vapor layer separates the liquid from the heated surface.

Simulation Process: VOF Model and Custom UDF Implementation in Fluent

The simulation process for this Film Boiling CFD project started with a 2D rectangular domain. We used a structured quadrilateral mesh. Using square cells is very important for Film Boiling ANSYS Fluent studies because they keep the interface sharp and stable. A fine mesh is necessary to see the thin vapor film near the hot wall.

We selected the Volume of Fluid (VOF) multiphase model to track the liquid and vapor. Since standard Fluent does not calculate film boiling mass transfer automatically, we wrote a custom UDF code in C language. This code calculates the evaporation rate based on the heat flow at the interface. We used specific tools to manage the data:

1. UDM (User Defined Memory): To store the mass transfer rate and heat energy values in every cell.
2. UDS (User Defined Scalar): To help calculate the volume fraction gradients accurately.

We enabled the Transient solver because boiling changes every second. This setup allows the Film Boiling CFD simulation to mimic the real physics where bubbles grow, pinch off, and rise due to gravity.

According to the reference paper, the mass source term for the vapor phase is:

where and are thermal conductivities of liquid and vapor, represents volume fractions, and is the latent heat of vaporization (1×10⁵ J/kg).

The UDF includes five main functions:

1. Copies VOF to UDS, calculates effective thermal conductivity manually, computes dot product of temperature and VOF gradients, and stores mass transfer rate in UDM
2. Adds positive mass source to vapor phase when evaporation occurs
3. Adds negative mass source to liquid phase (equal and opposite to vapor)
4. Adds latent heat sink to energy equation when liquid evaporates
5. Initializes the wavy vapor-liquid interface and temperature profile

Post-processing: Bubble Dynamics and Interface Mass Transfer Analysis

The post-processing analysis provides a deep look into the invisible physics of boiling. We must analyze the contours to understand how the simulation helps manufacturers. First, we look at the UDM Contours in Figure 3. This is the most technical part of the study. The simulation calculates the Mass Transfer Rate in real-time. The contours show red and orange colors exactly at the edges of the bubbles. The data shows values around 2.0 to 2.26 kg/m³s. This means the liquid is evaporating very fast in these specific spots. For a boiler designer, this is crucial. It identifies exactly where the steam is being generated. The blue areas show zero evaporation, which is correct because phase change only happens at the interface.

Next, we analyze the UDS Contours (Figure 2 and 4). These images confirm that our custom code works. We see a clear separation between liquid (UDS < 0.1) and vapor (UDS > 0.8). The green zones represent the sharp interface. The simulation captures four distinct vapor structures: large mushroom-shaped bubbles at the top and small bubbles forming at the hot wall. This proves the Film Boiling ANSYS Fluent model is replicating the “Rayleigh-Taylor instability,” which is the natural force that breaks the vapor film into bubbles.

Figure 2: Time-sequence contours of Gas Volume Fraction, visualizing the growth and detachment of vapor bubbles during the Film Boiling CFD simulation.

Figure 3: User Defined Memory (UDM) contours displaying the mass transfer rate calculated by the UDF specifically at the liquid-vapor interface.

Figure 4: User Defined Scalar (UDS) contours used to track the exact interface location and volume fraction gradients in the ANSYS Fluent analysis.

Finally, the Velocity and Temperature Contours (Figures 5 and 6) show the system’s movement. The velocity plot reveals speeds up to 0.12 m/s  alongside the rising bubbles. This is caused by buoyancy—the hot vapor is lighter than liquid, so it floats up quickly. The temperature data shows the bottom wall is the hottest point at 510 K, while the top is cooler at 500 K. This smooth gradient confirms that heat is effectively moving from the wall into the fluid, despite the insulating vapor. This Film Boiling CFD simulation successfully validates that the cooling process is active and stable.

Figure 5: Velocity Magnitude contours illustrating the buoyancy-driven natural convection currents rising around the vapor bubbles

Figure 6: Static Temperature contours showing the thermal gradient from the superheated wall through the liquid and vapor phases.