Natural Convection With Surface Radiation CFD Simulation: A Fluent Validation Study

Testing a simulation against real data is the only way to trust it. In the world of engineering, a classic test is the Square Cavity problem. This involves a box full of air with one hot wall and one cold wall. When we add radiation to this, it becomes a Natural Convection With Surface Radiation CFD simulation. The goal of this report is to perform a rigorous Surface Radiation CFD Validation.

We compare our results with the famous benchmark paper by Colomer et al. [1]. They used the “Discrete Ordinates Method,” but we will use ANSYS Fluent and the “Surface-to-Surface (S2S)” model. By comparing our numbers, we can prove that our Natural Convection With Surface Radiation fluent setup is correct. For more lessons on heat physics, please visit our Radiation tutorials.

• Reference [1]: Colomer, G., et al. “Three-dimensional numerical simulation of convection and radiation in a differentially heated cavity using the discrete ordinates method.” International Journal of Heat and Mass Transfer 47.2 (2004): 257-269
  

Figure 1: Schematic of the 3D differentially heated cavity used for this benchmark study. [1].

Simulation Process: S2S Radiation Model Setup

To start this Natural Convection With Surface Radiation ANSYS fluent study, we built a 3D cubic cavity. The physics here involves two things happening at once: airflow and radiation. We set one vertical wall to be hot and the opposite wall to be cold. The air inside is transparent, meaning it does not block radiation. Therefore, the heat jumps directly from the hot wall to the cold wall. To model this, we selected the Surface Radiation Simulation model, specifically the Surface-to-Surface (S2S) method. This method calculates how much of the hot wall “sees” the cold wall.

At the same time, we solved for “Natural Convection.” The air near the hot wall gets light and floats up. The air near the cold wall gets heavy and sinks. This creates a loop of wind inside the box. The simulation must solve both the wind and the radiation together to get the correct Natural Convection With Surface Radiation fluent simulation results.

Post-processing: Analysis of Coupled Physics

To understand if this Surface Radiation CFD Validation is successful, we must tell the story of the heat and the flow. The story starts with the “Cause”: the temperature difference. The hot wall heats the air. As the air rises, it creates a “Vortex” or a spinning circle of wind. We measured the speed of this wind. In the horizontal direction (Vx), our simulation error is 5.42% compared to the reference. In the vertical direction (Vy), the error is even lower at 4.52%. These low errors prove that our model correctly captures the movement of the air caused by buoyancy.

The next part of the story is the “Effect” on the temperature. If there were no radiation, the temperature lines (isotherms) would look very different. However, Figure 2 shows that the lines are curved. This curvature happens because the Surface Radiation is moving heat across the box independently of the air. The hot wall radiates energy to the floor and ceiling, which then heat the air near them. This complex interaction is difficult to simulate, but our numbers prove we did it right.

Table1: Validation study for natural convection with surface radiation CFD simulation problem

The final proof is in the “Heat Flux” (Q). This measures the total energy moving through the system. The table below shows that for “Radiative Heat Flux” (Qr), our error is extremely small, only 3.02%. This means the S2S model is almost perfect. When we combine the radiation and the airflow, the “Total Heat Flux” (Qt) has an error of just 5.33%. Because all these errors are around 5% or less, we can confidently say that this Natural Convection With Surface Radiation CFD simulation is valid and accurate.

Figure 2: Isotherms showing the curved temperature pattern caused by the interaction of convection and radiation.

Key Takeaways & FAQ

• Q: Why use the S2S Radiation Model?
  • A: It is efficient for enclosures where the air does not absorb heat. This Surface Radiation Simulation accurately calculates wall-to-wall energy transfer.
• Q: How accurate is the simulation?
  • A: Very accurate. The Natural Convection With Surface Radiation fluent results show a Total Heat Flux error of only 5.33%.
• Q: What causes the curved temperature lines?
  • A: The combination of hot air rising (convection) and heat rays traveling across the box (radiation) bends the isotherms.