The equation mentioned in the previous section for the rate of energy
exchange by radiation between a gray body and the surrounding medium assumed that the
radiating body could see only the large enclosing blackbody (represents the surrounding
medium). Therefore, all energy emitted by the small, radiating body is assumed to be
absorbed by the surrounding medium.
In situations where two radiating bodies are exchanging radiation energy
partially, the radiation view factor (F) should be introduced.
The radiation view factor of surface I with respect to surface j is
defined as the ratio of energy radiated from face I and directly reaching surface j to
the total energy leaving surface I. With that definition in mind, the net radiation heat
exchange between a surface of area AI and temperature TI and a surface of Aj and temperature
Tj is given by:
where:
Fij is the view factor of surface i with
respect to surface j
εi is the emissivity of surface i
σ is the Stefan-Boltzmann constant.
When the two surfaces have different values of emissivity εi and εj, the net radiation heat exchange between
the surfaces is given by:
