Question

A convex grey body denoted by surface is having surface area of $4m^2$ has emissivity 0.35 and surface temperature 680 K completely enclosed by another gray surface denoted by surface 2 having an area of $36m^2$ has emissivity 0.75 and surface temperature 310 K.

The value of $F_{22}$ is

• A. 0.44
• B. 0.89
• C. 0.69
• D. 0.11

Answer 1

The correct option is B 0.89

$T_1=680K,{\epsilon }_1=0.35,A_1=4m^2$

$T_2=310K,{\epsilon }_2=0.75,A_2=36m^2$

$F_{11}+F_{12}=1,F_{11}=0,F_{12}=1A_1{F}_{12}=A_2{F}_{21}$

$F_{21}=\left(A_1/A_2\right)F_{12}=A_1/A_2.$

$F_{21}+F_{22}=1\Rightarrow F_{22}=1-F_{21}$

$F_{22}=1-\frac{4}{36}=0.89$