Heat Transfer Miscellaneous
 The radiative heat transfer rate per unit area (W/m^{2}) between two plane parallel gray surfaces (emissivity = 0.9) maintained at 400 K and 300 K is (σ = Stefan Boltzmann constant = 5.67 × 10^{–8} W/m^{2}K^{4})

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For plane surfaces
Q = σ(T_{1}^{4}  T_{2}^{4}) A 1 + 1  1 ε_{1} ε_{2}
Here , ε_{1} = ε_{2} = 0.9Q = 5.67 × 10^{8}(400^{4}  300^{4}) A 1 + 1  1 0.9 0.9 = 8 ∥ .84 w ≈ 812 w m^{2} m^{2} Correct Option: B
For plane surfaces
Q = σ(T_{1}^{4}  T_{2}^{4}) A 1 + 1  1 ε_{1} ε_{2}
Here , ε_{1} = ε_{2} = 0.9Q = 5.67 × 10^{8}(400^{4}  300^{4}) A 1 + 1  1 0.9 0.9 = 8 ∥ .84 w ≈ 812 w m^{2} m^{2}
 For an opaque surface, the absorptivity (α), transmissivity (τ) and reflectivity ρ are related by the equation

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α + τ + ρ = 1
for opaque surface, τ = 0
∴ α + ρ = 1Correct Option: C
α + τ + ρ = 1
for opaque surface, τ = 0
∴ α + ρ = 1
 The following figure was generated from experimental data relating spectral black body emissive power to wave length at the three temperatures T_{1}, T_{2} and T_{3} (T_{1} > T_{2} >T_{3}).
The conclusion in that the measurements are

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Wien's displacement law is not satisfied, i.e., λ_{max} T = C
which tells lower λ_{max} for higher temperature.Correct Option: D
Wien's displacement law is not satisfied, i.e., λ_{max} T = C
which tells lower λ_{max} for higher temperature.
 What is the value of the view factor for two inclined flat plates having common edge of equal length and wlth an angle of 20 degrees?

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F_{12} = F_{21} = 1  sin θ 2 = 1  sin 20° = 0.826 2 Correct Option: A
F_{12} = F_{21} = 1  sin θ 2 = 1  sin 20° = 0.826 2
 A diffuse radiation surface has

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Radiation intensity is independent of angle in diffuse radiation.
Correct Option: A
Radiation intensity is independent of angle in diffuse radiation.