Radiation Heat Transfer (Heat transfer by thermal radiation)

All bodies radiate energy in the form of photons moving in a random direction, with random phase and frequency. When radiated photons reach another surface, they may either be absorbed, reflected or transmitted. The behavior of a surface with radiation incident upon it can be described by the following quantities:

Reflective energy may be either diffuse or specular (mirror-like). Diffuse reflections are independent of the incident radiation angle. For specular reflections, the reflection angle equals the angle of incidence.

Ideal Radiators

An ideal thermal radiator is called a "black body". It has several properties:

1) It has α = 1, and absorbs all radiation incident on it.

2) The energy radiated per unit area is Eb = σT4 where σ is the Stefan-Boltzmann constant,

The distribution of eλ varies with temperature. The quantity λT at the condition where eλ is a maximum is given by ( ) λmax λT e = 0.2898 cm K. As T increases, the wavelength for maximum energy emission shifts to shorter values. The frequency of the radiation, f, is given by f = c/λ so high energy means short wavelengths and high frequency.

A physical realization of a black body is a cavity with a small hole. There are many reflections and absorptions. Very few entering photons (light rays) will get out.

The inside of the cavity has radiation which is homogeneous and isotropic (the same in any direction, uniform everywhere). Suppose we put a small black body inside the cavity as seen in Figure 9.4. The cavity and the black body are both at the same temperature.