**Shape Factor Definition**

**Shape Factor** is defined as the fraction of radiative energy that is diffused from one surface element and strikes the other surface element directly with no intervening reflections.

- It is a function of geometry only.
- It is also called view factor or surface factor or configuration factor.

F_{1-2} = Q_{1-2}/Q_{1}

Where,

F_{1-2 }= fraction of energy leaving surface 1 and reaching surface 2.

Q_{1} = Rate of total energy radiated by surface element ‘1’.

Q_{1-2} = Fraction of the rate of energy leaving surface element ‘1’ and reaching surface element ‘2’.

**Read More- Surface Tension & Capillary Action**

**Reciprocity Theorem**

- A
_{1}F_{1-2}= A_{2}F_{2-1} - It indicates that the net radiant interchange may be evaluated by computing a one-way configuration factor from either surface to the other.

**Shape Factor of Some Important Surfaces**

- Plane Surface = 0 (F
_{11}= 0) - Convex Surface = 0 (F
_{11}= 0) - Concave Surface ≠ 0 (F
_{11}≠ 0)

**Important Points to Remember**

- Shape Factor depends only on the geometry of a body.
- 0 ≤ F
_{m-n}≤ 1 - When one body is completely enclosed inside the other body, then the shape factor of the inner body to the outer body is equal to 1.
- A
_{1}F_{1-2}= A_{3}F_{3-2}+ A_{4}F_{4-2}; if radiating surface A_{1}is sub-divided into A_{3}and A_{4}. - A
_{1}F_{1-2}= A_{1}F_{1-3}+ A_{1}F_{1-4}; if receiving surface A_{2}is sub-divided into A_{3}and A_{4}.

**Read More- Heat and Mass Transfer: Objective Questions with Answers**