Thermal radiation is one of three modes of heat transfer along with convection and conduction, so the more we know about this mode of heat transfer, the more effective we can use it to optimize our thermal management solutions.

What is thermal radiation?

Black bodies and emissivity

Every surface with a temperature greater than absolute zero emits and absorbs heat. Planck’s Law defines that at any given temperature, there is an ideal surface that can absorb and emit the most possible thermal radiation. We refer to this surface as a black body.

What we’re most interested in with a black body surface is it’s ideal ability to emit heat or its emissivity. Since nothing emits more than a black body as defined by Kirchoff’s Law of Thermal Radiation, we give it an emissivity value of 1. Any other surface cannot emit as much heat as a black body, so we define that surface’s emissivity as a ratio of how much that surface emits compare to a black body at that temperature. The more a surface can emit thermal radiation, the closer it is to an emissivity to 1. For thermal management, we generally want higher emissivities on our external surfaces to radiate as much heat out of the system as possible.

Surface finish and material effect on emissivity

Rougher surfaces are better at emitting thermal radiation. These surfaces are less likely to reflect thermal radiation back into the volume. So the less metallic and duller the material, the better emissivity a surface will have.

Thermal management: using radiation

In thermal management, we typically optimize radiation in natural convection solutions or solutions where we’re not actively pushing fluid through the system. You’ll see many natural convection solutions with anodized or coated surfaces, but you’ll see more unfinished aluminum and copper parts in forced convection cooling solutions. This is because the effect of thermal radiation is relatively small compared to the heat transfer accomplished with effective convection. In some applications where devices must be cooled within a vacuum, radiation is the only option for cooling a heat source.