Radiation heat transfer is the movement of thermal energy by electromagnetic waves, mainly in the infrared range for ordinary engineering temperatures. Unlike conduction and convection, it does not need a material medium, so it can occur across a vacuum. This makes radiation essential in furnaces, spacecraft, solar collectors, thermal imaging, and high temperature equipment.
Engineers must predict it accurately because radiative heat flow increases very rapidly with absolute temperature.
The ideal reference surface is a blackbody, which absorbs and emits the maximum possible thermal radiation at a given temperature. Real surfaces emit less energy than a blackbody, and their effectiveness is described by emissivity. For two surfaces facing each other, net radiation depends on their temperatures, emissivities, areas, and geometry through the view factor.
In many engineering problems, radiation is combined with conduction and convection to determine the total heat transfer rate.
Key Facts
- Stefan-Boltzmann law for a blackbody: E_b = sigma T^4
- Real surface emission: E = epsilon sigma T^4
- Net radiation from a small surface to large surroundings: q = epsilon sigma A (T_s^4 - T_sur^4)
- For two large parallel plates: q/A = sigma (T_1^4 - T_2^4) / (1/epsilon_1 + 1/epsilon_2 - 1)
- Stefan-Boltzmann constant: sigma = 5.67 x 10^-8 W/(m^2 K^4)
- All radiation temperature calculations must use kelvin, not degrees Celsius.
Vocabulary
- Thermal radiation
- Thermal radiation is energy emitted by matter as electromagnetic waves because of its temperature.
- Blackbody
- A blackbody is an ideal surface that absorbs all incoming radiation and emits the maximum possible radiation at a given temperature.
- Emissivity
- Emissivity is a dimensionless measure of how effectively a real surface emits radiation compared with a blackbody at the same temperature.
- View factor
- View factor is the fraction of radiation leaving one surface that directly reaches another surface.
- Irradiation
- Irradiation is the rate of incoming radiant energy received by a surface per unit area.
Common Mistakes to Avoid
- Using degrees Celsius in T^4 calculations is wrong because the Stefan-Boltzmann law requires absolute temperature in kelvin.
- Treating every surface as a blackbody is wrong because real materials usually have emissivity less than 1, which can greatly reduce radiative heat transfer.
- Subtracting temperatures before raising to the fourth power is wrong because net radiation depends on T_1^4 - T_2^4, not (T_1 - T_2)^4.
- Ignoring geometry and view factor is wrong because only the radiation that reaches the other surface contributes to exchange between those surfaces.
Practice Questions
- 1 A blackbody surface has area 0.50 m^2 and temperature 600 K. Calculate its emitted radiant power using E_b = sigma T^4.
- 2 Two large parallel plates are at 800 K and 500 K with emissivities 0.80 and 0.60. Calculate the net radiative heat flux between them using q/A = sigma (T_1^4 - T_2^4) / (1/epsilon_1 + 1/epsilon_2 - 1).
- 3 A polished metal shield and a matte black shield are placed between a hot furnace wall and a cooler instrument. Explain which shield better reduces radiative heating of the instrument and why.