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RADIATIVE HEAT TRANSFER
Prabal Talukdar
Department of Mechanical Engineering
IIT Delhi
E-mail: [email protected]
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Introduction
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Thermal Radiation Radiation heat transfer can take lace in a vacuum. It
does not need a medium unlike conduction/convection Thermal radiation is the stream of electromagnetic
ra a on em e y a ma er a en y on accoun o s
finite absolute temperature
Infrared radiation from a common household radiator or
electric heater is an example of thermal radiation, as is
the light emitted by a glowing incandescent light bulb.
erma ra a on s genera e w en ea rom emovement of electrons within atoms is converted to
electromagnetic radiation
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Dominant in high temperature applications
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Spectrum of Electro-magnetic
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Thermal radiation falls in the range of 10-1-102 m of the Electro-magnetic spectrum.
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Emission Process
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Volumetric Phenomenon Surface phenomenon
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Emission b a surface
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Gray Diffuse
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Solid an le
Plane Angle Solid Angle
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Solid an le = 2 = 2 2 =
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Solid An le for a Hemisphere2/ 2/2
srs ns nw0 0h 0
===
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Spectral Intensit I
,e(,,)=dq/(dA1cos .d.d) I
,e
is the rate at which radiant energy is emitted at the wave length
in the (, ) direction, per unit area of emitting surface normal to thisdirection, per unit solid angle about this direction and per unit
wavelength interval d about .
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Heat Flux dq=dq/drate at which radiation of wavelength leaves dA1 and
passes through dAn (unit: W/m)
dq= I,e(,, ) dA1cos d 1
= ddsincos),,(Idq e,"
Spectral heat flux associated with emission into hypothetical
hemisphere above dA1 is
=2/2
" ddsincos,,I
Total heat flux associated with emissions in all directions and
at all wavelengths is then
0
,
0
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=0
"" d)(qq
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Emissive Power
emitted per unit surface area
,
E()(W/m2.m)=
2/
e,
2
ddsincos),,(I
Ebased on actual surface area
I based on projected surface area
00
,
Total hemispherical Emissive power:
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=
d)(EE
0
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Relation between Emissive Power
2/2
E= 0 e,0 ddsincos),,(I
For a diffuse surface, I(,,)= I
()
2/2
E=
00
e, ddsincos)(I
,e
=
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