1)      The analysis of radiation transfer in enclosures consisting of black surfaces is relatively easy but most enclosures encountered in practice involve nonblack surfaces, which allow multiple reflections to occur

a)      Radiation analysis of such enclosures becomes very complicated unless some simplifying assumptions are made. It is common to assume the surfaces of an enclosure to be opaque, diffuse, and gray

b)      Before proceeding further we have look into Radiosity in detail

i)        Surfaces emit radiation as well as reflect it, and thus the radiation leaving a surface consists of emitted and reflected parts.

ii)      The total radiation energy leaving a surface per unit time and per unit area is the radiosity and is denoted by J

For a surface i that is gray and opaque   the radiosity can be expressed as

iii)    For black body  the radiosity of a blackbody is equal to its emissive power

c)      Net Radiation Heat Transfer to or from a Surface

i)        

ii)      

Applying the reciprocity relation 

iii)    

iv)    Radiation Heat Transfer in Three-Surface Enclosures

The algebraic sum of the currents (net radiation heat transfer) at each node must equal zero