Two parallel metal walls of an oven, separated 12 mm apart, have temperatures of

Question

Two parallel metal walls of an oven, separated 12 mm apart, have temperatures of T1 = 232 C and T3 = 27 C, and emissivity's epsilon1 = epsilon2 = 0.30, where the subscripts denote the inner and outer walls, respectively. The designer has a choice of filling the space between with nothing (air) (=low cost), one radiation shield with aluminum foil having an epsilon2 = 0.09 (=higher cost, light weight), or a high temperature insulation material with a conductivity of 0.05 W/m-C (=lower cost, heavier weight). Calculate the heat transfer rate between the two walls for each case as an aid to selection. (15%)

Explanation / Answer

WHEN AIR IS FILLED

q/A)=sigma(T1^4 - T3^4)/(1/e1 +1/e2 -1) sigma=5.67*10^-8 Js-1m-2K-4

q/A=569 Js-1m-2 (ans)

when shield is applied

q/A)=sigma(T1^4 - T3^4)/(1/e1 +1/e2 +2/e3 -2) sigma=5.67*10^-8 Js-1m-2K-4

q/A=120.06 Js-1m-2 (ans)

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