Radiation Exchange Between Surfaces (Surface to Surface Radiation)

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: