Consider two infinitely long thin concentric tubes of circular

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Consider two infinitely long thin concentric tubes of circular cross section as shown in figure. If D₁ and D₂ are the diameters of the inner and outer tubes respectively, then the view factor F₂₂ is given by

1. D₂ - 1
  D₁
2. Zero
3. D₁
  D₂
4. 1 - D₁
  D₂

Correct Option: D

F₂₂ = 1 - F₂₁ =

= 1 -	A₁	= 1 -	D₁
	A₂		D₂