Two very large parallel plane with emissivity’s 0.3 and 0.8 exchange ...
The radiation heat transfer rate without shield
Rate of radiation heat transfer with one shield
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Two very large parallel plane with emissivity’s 0.3 and 0.8 exchange ...
Given:
- Two parallel planes with emissivity of 0.3 and 0.8 exchange radiative energy.
- A polished aluminum radiation shield with emissivity of 0.05 is placed between them.
To find:
- The percentage reduction in radiative energy transfer.
Solution:
1. Let's first calculate the radiative heat transfer between the two planes without the aluminum shield.
- We can use the Stefan-Boltzmann law:
Q = σε₁ε₂A(T₁⁴ - T₂⁴)
where Q is the radiative heat transfer, σ is the Stefan-Boltzmann constant, ε₁ and ε₂ are the emissivities of the two planes, A is the surface area, and T₁ and T₂ are the temperatures of the two planes.
- Since the two planes are parallel and exchange energy by radiation only, we can assume that they are at the same temperature (T).
Q = σε₁ε₂A(T⁴ - T⁴) = σε₁ε₂AT⁴
2. Now, let's calculate the radiative heat transfer when the aluminum shield is placed between the two planes.
- The aluminum shield reduces the radiative heat transfer by absorbing and reflecting some of the radiation. The emissivity of the shield is 0.05, so it reflects 95% of the radiation and absorbs only 5%.
- The radiative heat transfer between the two planes and the shield can be calculated using the same formula as before, but with ε₂ replaced by 0.05:
Q' = σε₁(0.05)A(T⁴ - T'⁴)
where T' is the temperature of the shield.
- The shield is assumed to be at the same temperature as the two planes (T), so T' = T.
Q' = σε₁(0.05)AT⁴
3. The percentage reduction in radiative energy transfer can be calculated as:
% reduction = [(Q - Q') / Q] x 100%
= [(σε₁ε₂AT⁴ - σε₁(0.05)AT⁴) / (σε₁ε₂AT⁴)] x 100%
= [(1 - 0.05) / 1] x 100%
= 95%
Therefore, the percentage reduction in radiative energy transfer when the aluminum shield is placed between the two planes is 95%.
Answer: (A) 91.58