Effect of refractory surface on the heat flow between source and sink:
Consider a system in which three surfaces and forms an enclosure as shown in the figure. Each surface is characterized by its own uniform temperature and respectively. Also and surfaces are shaped such that each cannot see itself. The following assumptions are made:
Figure 31.1:
A system formed by three surfaces, source sink and refractory. The arrow shows heat flow path.
and 
forms an enclosure as shown in the figure. Each surface is characterized by its own uniform temperature 
and 
respectively. Also 
and 
surfaces are shaped such that each cannot see itself. The following assumptions are made:
