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Common Data for Questions 53 and 54: Radiative heat transfer is intended between the inner surfaces of two very large isothermal parallel metal plates. The upper plate (plate 1) is a black surface and is maintained at 727 °C. The lower plate (plate 2) is a diffuse gray surface with emissivity 0.7 and is kept at 227 °C. Assume that the surfaces are sufficiently large to form a two-surface enclosure and that steady-state conditions exist. Stefan-Boltzmann constant is $5.67\times10^{-8}$ W/m$^2$K$^4$. Q.53 The irradiation (in kW/m$^2$) for the upper plate (plate 1) is

1. 2.5
2. 3.6
3. 17.0
4. 19.5

Correct answer: 19.5

Solution

The upper plate is black, so its irradiation equals the radiation leaving the lower plate in the enclosure. Using the two-surface radiation relations with the given temperatures and emissivity gives an irradiation of about 19.5 kW/m². The black-surface condition simplifies the exchange analysis.

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