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Radiation Heat Transfer EGR 4345 Heat Transfer. Blackbody Radiation Blackbody – a perfect emitter...

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Radiation Heat Transfer EGR 4345 Heat Transfer
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Page 1: Radiation Heat Transfer EGR 4345 Heat Transfer. Blackbody Radiation Blackbody – a perfect emitter & absorber of radiation Emits radiation uniformly in.

Radiation Heat Transfer

EGR 4345

Heat Transfer

Page 2: Radiation Heat Transfer EGR 4345 Heat Transfer. Blackbody Radiation Blackbody – a perfect emitter & absorber of radiation Emits radiation uniformly in.

Blackbody Radiation

Blackbody – a perfect emitter & absorber of radiation Emits radiation uniformly in all directions – no

directional distribution – it’s diffuse Joseph Stefan (1879)– total radiation emission per

unit time & area over all wavelengths and in all directions:

=Stefan-Boltzmann constant =5.67 x10-8 W/m2K4

24 mW TEb

Page 3: Radiation Heat Transfer EGR 4345 Heat Transfer. Blackbody Radiation Blackbody – a perfect emitter & absorber of radiation Emits radiation uniformly in.

Planck’s Distribution Law

Sometimes we’re interested in radiation at a certain wavelength

Spectral blackbody emissive power (Eb) = “amount of radiation energy emitted by a blackbody at an absolute temperature T per unit time, per unit surface area, and per unit wavelength about the wavelength .”

Page 4: Radiation Heat Transfer EGR 4345 Heat Transfer. Blackbody Radiation Blackbody – a perfect emitter & absorber of radiation Emits radiation uniformly in.

Planck’s Distribution Law

For a surface in a vacuum or gas

Other media: replace C1 with C1/n2

Integrating this function over all gives us the equation for Eb.

constant sBoltzmann'J/K 1038051

Kμm 104391

mμmW 1074232

where

μmmW 1

23

42

24821

2

25

1

-

o

o

b

x.k

x.khcC

x.hcC

TCexp

CTE

Page 5: Radiation Heat Transfer EGR 4345 Heat Transfer. Blackbody Radiation Blackbody – a perfect emitter & absorber of radiation Emits radiation uniformly in.

Radiation Distribution

Radiation is a continuous function of wavelength

Radiation increases with temp.

At higher temps, more radiation is at shorter wavelengths.

Solar radiation peak is in the visible range.

Page 6: Radiation Heat Transfer EGR 4345 Heat Transfer. Blackbody Radiation Blackbody – a perfect emitter & absorber of radiation Emits radiation uniformly in.

Wien’s Displacement Law

Peak can be found for different temps using Wien’s Displacement Law:

Note that color is a function of absorption & reflection, not emission

Km 52897 .T powermax

Page 7: Radiation Heat Transfer EGR 4345 Heat Transfer. Blackbody Radiation Blackbody – a perfect emitter & absorber of radiation Emits radiation uniformly in.

Blackbody Radiation Function

We often are interested in radiation energy emitted over a certain wavelength.

This is a tough integral to do!

Page 8: Radiation Heat Transfer EGR 4345 Heat Transfer. Blackbody Radiation Blackbody – a perfect emitter & absorber of radiation Emits radiation uniformly in.

Blackbody Radiation Function

Use blackbody radiation function, F

If we want radiation between 1 & 2,

40

T

dTE

TF

b

TFTFTF1221

Page 9: Radiation Heat Transfer EGR 4345 Heat Transfer. Blackbody Radiation Blackbody – a perfect emitter & absorber of radiation Emits radiation uniformly in.

Surface Emission

Page 10: Radiation Heat Transfer EGR 4345 Heat Transfer. Blackbody Radiation Blackbody – a perfect emitter & absorber of radiation Emits radiation uniformly in.

Surface Emission

Spectral, directional emissivity

Total, directional emissivity

Spectral, hemispherical emissivity

Page 11: Radiation Heat Transfer EGR 4345 Heat Transfer. Blackbody Radiation Blackbody – a perfect emitter & absorber of radiation Emits radiation uniformly in.

Surface Emission

Spectral, hemispherical emissivity

Substituting spectral emissive power

Page 12: Radiation Heat Transfer EGR 4345 Heat Transfer. Blackbody Radiation Blackbody – a perfect emitter & absorber of radiation Emits radiation uniformly in.

Surface Emission

Total, hemispherical emissivity

Total, directional emissivityNormal emissivity predictable

Page 13: Radiation Heat Transfer EGR 4345 Heat Transfer. Blackbody Radiation Blackbody – a perfect emitter & absorber of radiation Emits radiation uniformly in.

Spectral, normal emissivity

Page 14: Radiation Heat Transfer EGR 4345 Heat Transfer. Blackbody Radiation Blackbody – a perfect emitter & absorber of radiation Emits radiation uniformly in.

Total, normal emissivity

Page 15: Radiation Heat Transfer EGR 4345 Heat Transfer. Blackbody Radiation Blackbody – a perfect emitter & absorber of radiation Emits radiation uniformly in.

Total, normal emissivity

Page 16: Radiation Heat Transfer EGR 4345 Heat Transfer. Blackbody Radiation Blackbody – a perfect emitter & absorber of radiation Emits radiation uniformly in.

Absorption, Reflection, and Transmission

Page 17: Radiation Heat Transfer EGR 4345 Heat Transfer. Blackbody Radiation Blackbody – a perfect emitter & absorber of radiation Emits radiation uniformly in.

Absorptivity

Spectral, directional absorptivity

Spectral, hemispherical absorptivity

Total, hemispherical absorptivity

Page 18: Radiation Heat Transfer EGR 4345 Heat Transfer. Blackbody Radiation Blackbody – a perfect emitter & absorber of radiation Emits radiation uniformly in.

Reflectivity

Spectral, directional reflectivity

Spectral, hemispherical reflectivity

Total, hemispherical reflectivity

Page 19: Radiation Heat Transfer EGR 4345 Heat Transfer. Blackbody Radiation Blackbody – a perfect emitter & absorber of radiation Emits radiation uniformly in.

Reflectivity

Page 20: Radiation Heat Transfer EGR 4345 Heat Transfer. Blackbody Radiation Blackbody – a perfect emitter & absorber of radiation Emits radiation uniformly in.

Transmissivity

Spectral, hemispherical transmissivity

Total, hemispherical reflectivity

Page 21: Radiation Heat Transfer EGR 4345 Heat Transfer. Blackbody Radiation Blackbody – a perfect emitter & absorber of radiation Emits radiation uniformly in.

Special Considerations

Semitransparent medium

+ + = 1

Opaque

+ = 1

Page 22: Radiation Heat Transfer EGR 4345 Heat Transfer. Blackbody Radiation Blackbody – a perfect emitter & absorber of radiation Emits radiation uniformly in.
Page 23: Radiation Heat Transfer EGR 4345 Heat Transfer. Blackbody Radiation Blackbody – a perfect emitter & absorber of radiation Emits radiation uniformly in.
Page 24: Radiation Heat Transfer EGR 4345 Heat Transfer. Blackbody Radiation Blackbody – a perfect emitter & absorber of radiation Emits radiation uniformly in.

Kirchhoff’s Law

=

No real surface can have an emissive power exceeding that of a black surface at the same temperature.

sbs TETE

1

1


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