1. Module -7
Radiation Heat Transfer
By
Faculty: Mr. LALAN KUMAR
Assistant Professor
Department of Mechanical Engineering
Katihar Engineering College Katihar
2. Any matter with temperature above absolute zero (0 K) emits electromagnetic radiation.
In a simplified picture, radiation comes from the constantly changing electromagnetic fields of the oscillating atoms.
Electromagnetic radiation can be visualized as waves traveling at the speed of light.
The two prominent characters of the wave are the wavelength (λ) and frequency (ν).
The wavelength is the distance between crest to crest on the wave.
The frequency is related to wavelength by the following:
Introduction
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The amount of radiation emitted by a body depends on its temperature, and is proportional to T4.
This relation shows that as the temperature of the object increases, the amount of radiation
emitted increases very rapidly
The emitted radiation will travel at the speed of light until it is absorbed by another body.
The absorbing medium can be gas, liquid, or solid
Radiation does not require a medium to pass through.
This is demonstrated by solar radiation which pass through interplanetary space to reach the earth.
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The Emission Process
For gases and semitransparent solids,
emission is a volumetric phenomenon.
In most solids and liquids the radiation
emitted from interior molecules is
strongly absorbed by adjoining
molecules.
Only the surface molecules can emit
radiation.
Hemispherical Surface Emission
Emissive Intensity
4
Te
I b
b
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The radiation emitted by a body is spatially distributed:
),,( rfIb
Electromagnetic Spectrum
Electromagnetic radiation is categorized into types by their wavelengths.
The types of radiation and the respective wavelength ranges are shown in
Figure.
Radiation with shorter wavelengths are more energetic, evident by the
harmful gamma and x-rays on the shorter end of the spectrum.
Radio waves, which are used to carry radio and TV signals, are much less
energetic; however, they can pass through walls with no difficulty due to
their long wavelengths.
The type of radiation emitted by a body depends on its temperature.
In general, the hotter the object is, the shorter the wavelengths of
emitted radiation, and the greater the amount.
A much hotter body, such as the sun (~5800 K), emits the most radiation
in the visible range.
6. RadiationLaws
The average or bulk properties of electromagnetic radiation interacting with matter are systematized in a simple
set of rules called radiation laws.
These laws apply when the radiating body is what physicists call a blackbody radiator.
Generally, blackbody conditions apply when the radiator has very weak interaction with the surrounding
environment and can be considered to be in a state of equilibrium.
Although stars do not satisfy perfectly the conditions to be blackbody radiators, they do to a sufficiently good
approximation that it is useful to view stars as approximate blackbody radiators.
Planck Radiation Law
The primary law governing blackbody radiation is the Planck Radiation Law.
This law governs the intensity of radiation emitted by unit surface area into a
fixed direction (solid angle) from the blackbody as a function of wavelength for a
fixed temperature.
The Planck Law can be expressed through the following equation.
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1
12
, 5
2
kT
hc
e
hc
TE
h = 6.625 X 10-27 erg-sec (Planck Constant)
K = 1.38 X 10-16 erg/K (Boltzmann Constant)
C = Speed of light in vacuum
Where
8. Planck’s Law
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The behavior is illustrated in the figure.
The Planck Law gives a distribution that;
peaks at a certain wavelength,
the peak shifts to shorter wavelengths for higher
temperatures, and
the area under the curve grows rapidly with increasing
temperature.
10. Monochromatic emissive power Eλ
• All surfaces emit radiation in many wavelengths and some, including black bodies,
over all wavelengths.
• The monochromatic emissive power is defined by:
• dE = emissive power in the wave band in the infinitesimal wave band between
and +d.
dTEdE ,
The monochromatic emissive power of a blackbody is given by:
1
12
, 5
2
kT
hc
e
hc
TE
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Wein’s Displacement Law:
• At any given wavelength, the black body monochromatic emissive power
increases with temperature.
• The wavelength max at which is a maximum decreases as the temperature
increases.
• The wavelength at which the monochromatic emissive power is a
maximum is found by setting the derivative of previous Equation with
respect to .
d
e
hc
d
d
TdE kT
hc
1
12
,
5
2
mKT 8.2897max
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Stefan-Boltzmann Law
• The maximum emissive power at a given temperature is the black body
emissive power (Eb).
• Integrating this over all wavelengths gives Eb.
0
5
2
0
1
12
,
d
e
hc
dTE
kT
hc
44
4
42
15
2
, TT
k
hc
hc
TE