1.3 Radiation

Radiation heat transfer involves the transfer of heat by electromagnetic radiation that arises due to the temperature of the body. Any body above absolute zero temperature (0 K) emits radiation. Radiation does not need matter.

  • Emissive power per unit surface area, is given by Stefan-Boltzmann law, as below: \[q = \sigma \epsilon T^4 \tag*{(3)}\] where

    \(\sigma\) = Stefan-Boltzmann constant = \(5.67\times10^{-8}\) W/m\(^2\).K\(^4\)
    \(\epsilon\) = emissivity, which is a surface property (\(\epsilon=1\) for black body)
    \(T\) = absolute temperature of the surface (K)

    The above equation describes a gross heat emission rather than heat transfer.

  • The rate of radiation heat exchange between a small surface of area \(A\) at \(T_1\) and a large surrounding at \(T_2\) is given by the following expression: \[Q = \sigma\epsilon A (T_1^4-T_2^4) \tag*{(4)}\]

  • Surfaces with emissivities nearly unity are good absorbers and hence poor reflectors of incident radiation. Most highly polished, unoxidized metal surfaces are good reflectors of thermal radiations with emissivities less than 0.1. A roughened or an oxidized surface has correspondingly higher emissivities. This can be verified from the values given in the following table.

    Material Emissivity, \(\epsilon\)
    (dimensionless)
    Carbon 0.85–0.95
    Aluminum 0.11
    Brass (oxidized) 0.61
    Brass (unoxidized) 0.030
    Copper (oxidized) 0.60
    Copper (unoxidized) 0.020
    Black gloss paint 0.90
    Gold (polished) 0.020
    Fire brick 0.75