The concrete wall of a building is 0.121 m thick. The temperature inside the bui

Question

The concrete wall of a building is 0.121 m thick. The temperature inside the building is 23.1 °C, while the temperature outside is 0.0 °C. Heat is conducted through the wall. The outside of the wall loses the energy only due to radiation. What is the emissivity of the wall?

I looked up this question on the Chegg site and tried to do the problem from the other examples but have not been successful. If possible, please help do this problem with the above figures so i can have a better understanding of how to solve and what the answer is. Thanks!

Explanation / Answer

Heat radiated/area/sec, Qr = e * sigma*(T^4)
where
sigma = stefan's constant,
A =cross-section area

heat conducted thro wall per sec = k A (23.1 - 0) /x
so per unit area, Qc = k (23.1 - 0) /x
Qc = k (23.1 - 0) /0.121
Qc = 190.91 k

Given Qr = Qc
e * sigma*(T^4) = 200 k
e = 190.91 k / sigma*(T^4) = 200 k / sigma*(273^4)

sigma = 5.67*10^-8 W/m^2-K^4
k = conductivity of concrete = k concrete = 0.872 W/m.K

Substituing Values -
e = (190.91 *0.872) / 5.67*10^-8*(273^4)
e = 0.529
emissivity of the wall, e = 0.529

Related Questions

Navigate

• Browse (All)
• Browse T
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.