Radiation is the most frequently neglected mechanism in thermal analysis, and the one whose neglect causes the most error. The reason is that, unlike convection, it is hard to picture. Yet the decision is not a matter of feel; it takes a few lines of arithmetic [1].
A simple test
Radiative heat flux rises with the fourth power of absolute temperature. In practice:
- Below 60 °C with forced convection, it can usually be neglected
- Between 100 and 200 °C it reaches the same order as natural convection; neglecting it means a 20–40% error
- Above 300 °C it dominates, and neglecting it invalidates the analysis
The practical way to decide is to hand-calculate the radiative and convective fluxes from the surface and take their ratio. Above 0.1, radiation goes into the model.
Forced convection does not always save you
"There is a fan, so radiation does not matter" is a common and dangerous assumption. In the dead zones the fan does not reach, the local convection coefficient drops and radiation becomes dominant there. Hot spots usually appear in exactly those zones.
Emissivity matters as much as the model
Surface emissivity is often left at its default, and the result scales directly with it.
| Surface | Approximate emissivity |
|---|---|
| Polished aluminium | 0.05 |
| Oxidised aluminium | 0.2 – 0.3 |
| Painted surface (colour is irrelevant) | 0.85 – 0.95 |
| Anodised surface | 0.8 – 0.9 |
There is a factor of fifteen between bright metal and a painted surface. Painting a heat sink can be more effective than adding fins.
Choosing a model
Surface-to-surface radiation (transparent fluid): view-factor based approaches are appropriate and necessary in enclosures.
Participating medium (combustion products, water vapour, soot): gas radiation must be accounted for, and it governs wall heat flux in combustion chambers.
Solar load: for outdoor equipment, the direct and diffuse components should be defined separately.
What belongs in the report
Whether radiation was included, the justification if it was not, and the emissivity values used. "Radiation was neglected" is not enough on its own; why it could be neglected has to be stated.
References
- Modest, M. F. & Mazumder, S. (2021). Radiative Heat Transfer, 4th ed. Academic Press. ISBN 978-0-323-98406-5.
- ASHRAE Technical Committee 9.9 (2021). Thermal Guidelines for Data Processing Environments, 5th ed. ASHRAE, Atlanta.