Heat Transfer · Radiation · 2 marks · Multiple choice
A hollow cylinder of length L = D, its inner diameter, is sealed by flat end plates. Inner surfaces A₁ (top), A₂ (curved wall), A₃ (bottom) exchange radiation. View factor Fᵢⱼ is the fraction from Aᵢ reaching Aⱼ. Given $F_{13}=3-2\sqrt2$, find F₂₁.
The flat top cannot view itself, so $F_{12}=1-F_{13}=2\sqrt2-2$. Reciprocity gives $A_1F_{12}=A_2F_{21}$. With L = D, $A_1=\pi D^2/4$ and $A_2=\pi DL=\pi D^2$, so $F_{21}=F_{12}/4=(\sqrt2-1)/2$, B.
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