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Glossary
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Radiation Emission from Real Surfaces

The Stefan-Boltzmann law for the net rate of heat exchange by radiation between a blackbody and a surrounding medium should be modified for real surfaces. For non-blackbody surfaces, the spectral radiation intensity does not obey the Planck distribution and the emitted radiation has preferred direction for emission.

The modified Stefan-Boltzmann law for a non-blackbody is given by:

Qradiation = ε σ A ( Ts4 - Ta4 )

where e is the emissivity of the radiating surface defined as the ratio of the emissive power of the surface to the emissive power of a blackbody at the same temperature. Materials are assigned an emissivity value between 0 and 1.0. A blackbody, therefore, has an emissivity of 1.0 and a perfect reflector has an emissivity of 0.

The emissivity is a material property that is dependent on the surface temperature and the surface finish. The following table lists the emissivity values for some materials:

Metal Temperature (ºC) Emissivity

Aluminum

Polished

Rough surface

Oxidized

50 - 500

20 - 50

200

0.04 - 0.06

0.06 - 0.07

0.11

Brass

Polished

Rolled Sheet

Unoxidized

Oxidized

200

20

25

200

0.03

0.06

0.035

0.61

Bronze

Polished

50

0.1

Carbon

Filament

Graphite

Unoxidized

1000 - 1400

0 - 3600

100

0.53

0.7 - 0.8

0.81

Copper

Calorized

Commercial

Polished

Unoxidized

Oxidized

100

20

50 - 100

100

50

0.26

0.07

0.02

0.02

0.6 - 0.7

Glass

  

20 - 100

0.94 - 0.91

Cast Iron

Rusted

Oxidized

Unoxidized

25

200

100

0.65

0.64

0.21

Lead

Oxidized

Unoxidized

200

200

0.96

0.63

Nickel

Polished

Oxidized

Unoxidized

200 = 400

200

25

0.05 - 0.1

0.37

0.045

Platinum

Polished

Unoxidized

200 - 600

25

0.05 - 0.1

0.037

Porcelain

  

20

0.92

Quartz

  

----

0.92

Rubber

Hard

Soft

20

20

0.95

0.86

Silicon

  

----

0.92

Silver

Polished

Unoxidized

20 - 600

100

0.02 - 0.03

0.02

Steel

Rough Surface

Polished

Oxidized

Unoxidized

Alloyed

50

750 - 1050

200 - 600

100

500

0.95 - 0.98

052 - 0.56

0.8

0.08

0.35

Zinc

Oxidized

Unoxidized

200 - 300

300

0.04 - 0.05

0.05


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