What is Shape Factor ?

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.

F1-2 = Q1-2/Q1

Where,

F1-2 = fraction of energy leaving surface 1 and reaching surface 2.

Q1 = Rate of total energy radiated by surface element ‘1’.

Q1-2 = Fraction of rate of energy leaving surface element ‘1’ and reaching surface element ‘2’.

Reciprocity Theorem

  • A1F1-2 = A2F2-1
  • It indicates that the net radiant interchange may be evaluated by computing one way configuration factor from either surface to the other.

Shape Factor of Some Important Surfaces

  • Plane Surface = 0 (F11­ = 0)
  • Convex Surface = 0 (F11­ = 0)
  • Concave Surface ≠ 0 (F11 ≠ 0)

Important Points to Remember

  • Shape Factor depends only on geometry of a body.
  • 0 ≤ Fm-n ≤ 1
  • When one body is completely enclosed inside the other body, then shape factor of inner body with respect to outer body is equal to 1.
  • A1F1-2 = A3F3-2 + A4F4-2 ; if radiating surface A1 is sub-divided into A3 and A4 .
  • A1F1-2 = A1F1-3 + A1F1-4 ; if receiving surface A2 is sub-divided into A3 and A4 .

Read More – Laws of Radiation and Heat Transfer by Radiation

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