The complicated process of the interplay between thermal radiation and fluid motion in coupled radiative and convective heat transfer in enclosures greatly influences temperature distribution and heat transfer rates. The thermal performance largely depends on the emissivity contrast in situations with an interior heater and enclosure walls with different emissivity. With the guide of a reference paper entitled “ Coupled radiative and convective heat transfer in enclosures: Effect of inner heater–enclosure wall emissivity contrast”, the CFD simulation is carried out. There are two cases:

• Case (i): ϵp = 0.8 and ϵw = 0.1 representing a positive contrast
• Case (ii): ϵp = 0.1 and ϵw = 0.8 representing a negative contrast

• Reference [1]: Saravanan, S., and N. Raja. “Coupled radiative and convective heat transfer in enclosures: effect of inner heater–enclosure wall emissivity contrast.” Physics of Fluids 32.9 (2020).

Figure 1: Physical configuration: horizontally cooled enclosure [1]

Simulation Process

The simplest step is drawing a square domain and generating a structured grid by providing slices. The Surface to Surface (S2S) radiation model can help to consider radiative effects inside the square. The only difference between the two cases which are called positive and negative contrast is emissivity.

Post-processing

The temperature contours tell a fascinating story about how emissivity contrast completely transforms heat transfer behavior in the enclosure. Look at how the positive contrast case (right image) bathes the entire chamber in warmer temperatures (mostly green, around 306K) while the negative contrast case (left) maintains a much cooler environment (predominantly blue, around 300K). This isn’t just a minor difference—it’s proof that swapping emissivity values flips the entire thermal character of the system. When the heater has high emissivity (0.8), it actively radiates heat outward, creating those wide yellow-green regions that dominate the positive contrast case. The walls can’t reflect much heat back (with just 0.1 emissivity), so the warmth spreads everywhere. In contrast, when the heater barely radiates (0.1 emissivity) but the walls reflect efficiently (0.8 emissivity), heat gets trapped near the source and the rest of the enclosure stays cool.

Figure 2: Temperature field  inside the enclosure for both cases

The velocity fields depicts why these temperature patterns form in the first place. Notice how the positive contrast case develops a stronger central plume above the heater but also creates multiple circulation cells with distinctive “T” pattern flow. This happens because the efficient radiative heat transfer drives stronger natural convection, pulling cool air from the bottom while pushing heated air up and outward. Meanwhile, the negative contrast case shows a more traditional two-cell circulation pattern with weaker overall flow intensity. The S2S radiation model captures this interplay perfectly, demonstrating that emissivity isn’t just about surface properties—it fundamentally alters the entire convective flow structure.

Figure 3: Velocity pattern inside the enclosure for both cases