Heat Transfer: View Factors
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Radiation View Factors
The above equations for blackbodies and graybodies assumed that the small body could see only the large enclosing body and nothing else. Hence, all radiation leaving the small body would reach the large body. For the case where two objects can see more than just each other, then one must introduce a view factor F and the heat transfer calculations become significantly more involved.

The view factor F12 is used to parameterize the fraction of thermal power leaving object 1 and reaching object 2. Specifically, this quantity is equal to,

Likewise, the fraction of thermal power leaving object 2 and reaching object 1 is given by,
The case of two blackbodies in thermal equilibrium can be used to derive the following reciprocity relationship for view factors,
Thus, once one knows F12, F21 can be calculated immediately.

Radiation view factors can be analytically derived for simple geometries and are tabulated in several references on heat transfer (e.g. Holman, 1986). They range from zero (e.g. two small bodies spaced very far apart) to 1 (e.g. one body is enclosed by the other).

Heat Transfer Between Two Finite Graybodies
The heat flow transfered from Object 1 to Object 2 where the two objects see only a fraction of each other and nothing else is given by,
This equation demonstrates the usage of F12, but it represents a non-physical case since it would be impossible to position two finite objects such that they can see only a portion of each other and "nothing" else. On the contrary, the complementary view factor (1 - F12) cannot be neglected as radiation energy sent in those directions must be accounted for in the thermal bottom line.

A more realistic problem would consider the same two objects surrounded by a third surface that can absorb and readmit thermal radiation yet is non-conducting. In this manner, all thermal energy that is absorbed by this third surface will be readmitted; no energy can be removed from the system through this surface. The equation describing the heat flow from Object 1 to Object 2 for this arrangement is,

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