A solid sphere of radius r1 = 20 mm is placed concentrically

A solid sphere of radius r₁ = 20 mm is placed concentrically inside a hollow sphere of radius r₂ = 30 mm as shown in the figure.

The view factor F₂₁ for radiation heat transfer is

F₁₁ + F₁₂ = 1, here F₁₁ = 0
∴ F₁₂ = 1

Now from reciprocating law
A₁ F₁₂ = A₂ F₂₁
∴ 22π (20)² × 1 = 2π (30)² × F₂₁

⇒ F₂₁ =		10		²	=	4
		30				9