Radiation and law associated with its.

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
LECTUIRE : 01
01 Version: 1, KEC Katihar


c


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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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7. Version: 1, KEC Katihar06
 
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.

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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



09 Version: 1, KEC Katihar

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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 

 




