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Heat Transfer
Convection & Radiation
GATE 2008
Medium

A hollow enclosure is formed between two infinitely long concentric cylinders of radii 1m and 2m. Radiative heat exchange takes place between the inner surface of the larger cylinder (surface-2) and the outer surface of the smaller cylinder (surface-1); surfaces are diffuse, medium non-participating. The fraction of thermal radiation leaving the larger surface and striking itself is

  • A0.25
  • 0.5
  • C0.75
  • D1

Explanation

All radiation from surface 1 (inner) reaches surface 2: F12=1. By reciprocity A1F12=A2F21, so F21=(r1/r2)(1)=1/2. Since these are the only two surfaces, F21+F22=1 ⟹ F22=1-0.5=0.5.

Source: GATE ME 2008 (2008)

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