1. What is thermal radiation?
Thermal radiation is a form of electromagnetic radiation. Electromagnetic radiation may be classified according to its wavelength.
Thermal radiation is electromagnetic radiation which can be detected as heat and it occurs approximately in the range 0.4 <
< 1000μm, which includes the visible and the near, middle and far IR (infrared) regions.
2. What distinguishes thermal radiation?
Thermal radiation is able to ‘act at a distance’ in the absence of an intervening medium. Heat transport by conduction and convection can only occur in the presence of a medium, while for example, thermal radiation emitted by the sun reaches us through empty space.
3. The importance of thermal radiation.
Thermal radiation is emitted in proportion to the fourth power of the absolute temperature so it is important in combustion processes. Many combustion chambers need to be large to contain a large flame and this normally results in low velocities. So convective heat transfer is usually small in most furnaces. In very large chambers, those of power stations for example, some 95% of the total heat transfer to the walls and tubes may be due to thermal radiation.
4. Why do flames radiate?
Essentially for two reasons:
Because they may contain solid material in the form of particulate matter, e.g. coal/ash particles and carbon/soot. These constituents radiate in a continuous spectrum in the visible and IR regions, and cause the flame to be luminous and visible to the eye.
Certain gaseous products of combustion emit thermal radiation, most notably CO2 and H2O. Radiation from gases is emitted at specific spectral lines and bands corresponding to transitions between energy states in the molecules. Although not generally visible to the eye, the emitted radiation from these gases does contribute significantly to the overall furnace heat transfer.
5. What determines the amount of thermal radiation emitted by a flame?
Obviously the flame temperature, but also its ‘emissivity’. [GLOSS]Emissivity[/GLOSS] is a function of flame ‘optical thickness’, the product of the ‘absorptivity’ and the ‘flame thickness’. So large flames radiate more strongly than small ones. Pressure also augments the optical thickness so the thermal radiation in gas turbine combustion chambers, for example, is significant in spite of their relatively small size.
 Seigel R and Howell J, Thermal Radiation Heat Transfer, McGraw Hill, New York, (1972).