GATE2016-2-16

Consider the radiation heat exchange inside an annulus between two very long concentric cylinders. The radius of the outer cylinder is $R_o$ and that of the inner cylinder is $R_i$ . The radiation view factor of the outer cylinder onto itself is

1. $1-\sqrt{\dfrac{R_i}{R_o}}\\$
2. $\sqrt{1-\dfrac{R_i}{R_o}} \\$
3. $1-\dfrac{R_i}{R_o}^{1/3} \\$
4. $1-\dfrac{R_i}{R_o}$

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