Stefan's law for radiation between body and surroundings

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The discussion focuses on Stefan's law regarding radiation heat transfer between two bodies. It clarifies that a single value of emissivity (ε) is applicable for both heat exchange directions due to Kirchhoff's law, which states that absorptivity equals emissivity. The necessity for the body to be small compared to its surroundings is emphasized to simplify calculations. When considering two bodies, the geometry of the second body must be factored into the heat transfer calculations. The conversation also briefly critiques the use of imperial units like ft² and °R.
Remusco
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My PE handbook describes how to calculate the "Net Energy Exchange by Radiation Between Two Bodies", as shown below:

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How can a single value of epsilon be used for heat transfer in both directions? Why does the body need to be small compared to the surroundings?
 
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Remusco said:
My PE handbook describes how to calculate the "Net Energy Exchange by Radiation Between Two Bodies", as shown below:

View attachment 354010



How can a single value of epsilon be used for heat transfer in both directions? Why does the body need to be small compared to the surroundings?
There is one ##\varepsilon## because the absorptivity of the body is the same as its emissivity per Kirchhoff's law of thermal radiation. You are calculating the heat on the body due to surroundings. If the surroundings are instead another body then you have to take into account the geometry of the second body.

Edit: not important, but who in their sane minds uses ft2 and °R?
 

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