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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:

Q radiation = e s A ( T s 4 - T a 4 )

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

 

200-600

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

0.52-0.56

0.8

0.08

0.35

Zinc

Oxidized

Unoxidized

 

200-300

300

 

0.04-0.05

0.05



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