Thermal engineers are radiant about the effects of thermal radiation. 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?
Thermal Radiation refers to the electromagnetic waves that escape a given volume from its surface. While all of the molecules within a given volume give off radiation, internal molecules generally absorb the radiation from their internal neighbors. Thermal energy leaves from the volume in question when surface molecules radiate heat. This is why in thermal management, we’re so concerned about the surface of our heat sinks and cooling solutions.
Thermal radiation is a portion of the electromagnetic section that ranges between 0.1 um to 100 um in wavelength. This includes the high end of UV radiation, visible light, and infrared radiation. You’ve certainly seen radiation when you’ve seen someone working red hot metal or glass. That visible light you’re seeing 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, so 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
The primary determining factors of emissivity are material composition and surface finish. Metallic surfaces generally have a low emissivity, which includes our typical heat sink materials copper and aluminum. While we prefer these metals for their high thermal conductivities, they’re poor materials when it comes to emitting radiation. This is because these materials are highly reflective, and not just in the visible spectrum, but all electromagnetic waves. The implication of this inherent shininess of every particle is that the surface molecules are more likely to reflect any thermal radiation from internal molecules back into the volume.
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.
Radiation in Aavid Genie
Aavid Genie includes the effects of thermal radiation for natural convection solutions by default. In case you’d like to see the estimated effect on thermal performance without radiation, Aavid Genie has a check box on the Project Conditions page to disable radiation calculations.
Forced convection simulations in Aavid Genie do not account for the effects of heat transfer by thermal radiation because of the minimal impact it has on the final performance. In this way, results for these forced convection will be a tad conservative.
Try out your own natural convection heat sink in Aavid Genie and see how thermal radiation helps your solution.