41 - Radiation

The radiation heat transfer between two surfaces can be reduced significantly if a radiation shield made of low-emissivity materials is placed between them.

• Radiation shield (plate 3) between flat plates (plates 1 and 2): Heat transfer rate for this case is given by \[Q = \frac{A\sigma(T_1^4-T_2^4)}{(1/\varepsilon_1+1/\varepsilon_2-1)+(1/\varepsilon_{3,1}+1/\varepsilon_{3,2}-1)}\] where \(\varepsilon_{3,1}\) is the emissivity of the surface of plate 3 (the radiation shield) facing the plate 1; and \(\varepsilon_{3,2}\) is the emissivity of the surface of plate 3 (the radiation shield) facing the plate 2.

• Radiation shield between concentric cylinders or spheres (inner=1; outer=2; shield=3, which is placed between 1 and 2): \[Q = \frac{A_1\sigma(T_1^4-T_2^4)}{1/\varepsilon_1+(A_1/A_2)(1/\varepsilon_2-1)+(A_1/A_3)(1/\varepsilon_{3,1}+1/\varepsilon_{3,2}-1)}\]