Everything around us gives off invisible heat rays called radiation. However, not all materials radiate heat perfectly. A surface might emit a lot of heat at one temperature, but very little at another. This changing behavior is called “Variable Emissivity.” In engineering, if we ignore this, our designs for solar panels or ovens will be wrong. To understand this complex physics, engineers use a Variable Emissivity CFD simulation. This report is a Variable Emissivity fluent tutorial. We will look at a special case: air trapped inside a triangular box with a hot pipe at the bottom. We use ANSYS Fluent to see how the heat rays bounce around the angled walls. By performing this Variable Emissivity ANSYS fluent analysis, we can learn how shape and radiation control the wind inside enclosed spaces. For more lessons on modeling heat rays, please visit our Radiation tutorials.

Figure 1: The schematic of the triangular cavity used in the Variable Emissivity simulation.

Simulation Process: Discrete Ordinates and Non-Gray Models

To begin this Variable Emissivity fluent simulation, we created a 2D triangle with a circle (the heater) at the bottom. We used a Structured Grid mesh to divide the air into small calculation squares. The most important part of this Radiation fluent setup is the physics model. We turned on the Discrete Ordinates (DO) radiation model. This model calculates how heat rays shoot in all directions. To capture the true physics, we added the Non-gray sub-model. This tells the software that the walls have Variable Emissivity. It means the walls will absorb and reflect different types of heat rays (different wavelengths) in different amounts. Because this creates a very chaotic environment, we used a Pseudo-transient solver to keep the math stable while the air figures out how to move.

Post-processing: How Uneven Radiation Destroys Symmetry

To truly understand this Triangle Radiation case, we must follow the energy. The story starts at the circular source at the bottom. This heater is glowing hot at 979 Kelvin. It is blasting thermal energy into the cavity. If the walls had normal, constant emissivity, they would heat up perfectly evenly. But because we used the Variable Emissivity radiation model, something different happens. The left and right angled walls absorb the heat rays differently. Look at the Temperature Contour (Figure 2). You can see the dark red heat rising from the 979K source, but the colors on the left wall do not perfectly match the colors on the right wall. The heat distribution is lopsided. This uneven temperature field is the direct “Cause” of the strange airflow.

Now, look at the Velocity Streamlines (Figure 3). Hot air is lighter, so the air above the 979K heater shoots straight up. But because the left and right walls are at slightly different temperatures (due to the variable radiation), they push and pull the rising air unevenly. This completely destroys the balance of the room.

Figure 2: Temperature contour from the Variable Emissivity CFD simulation, showing the 979K heat source and the uneven thermal distribution.

Figure 3: Velocity streamlines revealing the complex, asymmetric, and multi-vortex flow pattern trapped in the triangular cavity corners.

The result is a highly complex, Asymmetric wind. Instead of two neat circles of air (vortices) perfectly mirroring each other, the flow breaks apart. The air gets “trapped” in the sharp, narrow corners of the triangle, spinning into multiple, messy secondary vortices. One side of the triangle has a larger swirling zone than the other. This proves a very important engineering lesson: in this Emissivity fluent simulation, the invisible, uneven bouncing of heat rays completely controls the physical wind. Without the advanced non-gray radiation model, we would have predicted a perfectly symmetrical—and completely wrong—airflow.

Key Takeaways & FAQ

• Q: What is Variable Emissivity?
  • A: It means a surface’s ability to emit radiation changes depending on the wavelength or temperature. We model this using the non-gray DO model in this Variable Emissivity CFD simulation.
• Q: Why is the flow pattern asymmetric (lopsided)?
  • A: Even though the triangle is perfectly symmetrical, the Variable Emissivity radiation causes uneven heating on the walls, which creates unbalanced buoyancy forces that push the air into a chaotic, non-symmetrical pattern.
• Q: Why does the air get trapped in the corners?
  • A: The sharp angles of the Triangle Radiation geometry restrict the natural rising and falling loops, forcing the air to spin in tiny, trapped secondary vortices.