Radiative exchange in a hollow enclosure with two zones

3 Radiative exchange in a hollow enclosure with two zones 

The relationships derived in the last section for the heat flow, Qx, transferred from a radiation emitter 1 to a receiver 2, are also valid for an enclosure that is only bounded by these two zones. As no reradiating zone is present, with 1R = 0 and F2R = 0 from (5.149), F12 = F12 is obtained. The heat flow transferred from 1 to 2 is 

These equations hold for several important, practical cases  

The area 1 is completely enclosed by the area 2, so that Fu = 1. For the radiative exchange factor we have now 

This result holds in particular for concentric spheres and very long concentric cylinders as here the assumption of isothermal surfaces applies more easily. If, however, body 1 lies eccentric in the enclosure surrounded by body 2, Fig. 5.64, then the two surfaces will generally not be isothermal, as the radiation flow is much higher in the regions where the two surfaces are close to each other than where a large distance exists between them. 

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