PPT: Radiative Heat Transfer | Heat Transfer - Mechanical Engineering PDF Download

 Page 1


Radiation Heat Transfer
Page 2


Radiation Heat Transfer
Contrast in Radiative Mode of Heat Transfer
 Any body (> absolute zero) emits radiation at various
wavelengths.
 Transparent bodies radiate energy in spherical space.
 Non-transparent bodies radiate energy in hemi-spherical
space.
 The radiation energy emitted by a body is distributed in
space at various wavelengths.
 This complex phenomenon requires simplified laws for
engineering use of radiation.
Page 3


Radiation Heat Transfer
Contrast in Radiative Mode of Heat Transfer
 Any body (> absolute zero) emits radiation at various
wavelengths.
 Transparent bodies radiate energy in spherical space.
 Non-transparent bodies radiate energy in hemi-spherical
space.
 The radiation energy emitted by a body is distributed in
space at various wavelengths.
 This complex phenomenon requires simplified laws for
engineering use of radiation.
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.
()
1
1 2
,
5
2
-
=
kT
hc
e
hc
T E
l
l
l
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
Page 4


Radiation Heat Transfer
Contrast in Radiative Mode of Heat Transfer
 Any body (> absolute zero) emits radiation at various
wavelengths.
 Transparent bodies radiate energy in spherical space.
 Non-transparent bodies radiate energy in hemi-spherical
space.
 The radiation energy emitted by a body is distributed in
space at various wavelengths.
 This complex phenomenon requires simplified laws for
engineering use of radiation.
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.
()
1
1 2
,
5
2
-
=
kT
hc
e
hc
T E
l
l
l
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
Monochromatic emissive power E
l
 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 l and l+dl.
( ) l l d T E dE , =
The monochromatic emissive power of a blackbody is given by:
()
1
1 2
,
5
2
-
=
kT
hc
e
hc
T E
l
l
l
Page 5


Radiation Heat Transfer
Contrast in Radiative Mode of Heat Transfer
 Any body (> absolute zero) emits radiation at various
wavelengths.
 Transparent bodies radiate energy in spherical space.
 Non-transparent bodies radiate energy in hemi-spherical
space.
 The radiation energy emitted by a body is distributed in
space at various wavelengths.
 This complex phenomenon requires simplified laws for
engineering use of radiation.
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.
()
1
1 2
,
5
2
-
=
kT
hc
e
hc
T E
l
l
l
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
Monochromatic emissive power E
l
 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 l and l+dl.
( ) l l d T E dE , =
The monochromatic emissive power of a blackbody is given by:
()
1
1 2
,
5
2
-
=
kT
hc
e
hc
T E
l
l
l
Wein’s Displacement Law:
 At any given wavelength, the black body monochromatic
emissive power increases with temperature.
 The wavelength l
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 l.
()
0
1
1 2
,
max
max
5
2
=
ï
þ
ï
ý
ü
ï
î
ï
í
ì
-
=
l
l
l
l
l
l
l
d
e
hc
d
d
T dE
kT
hc
mK T m l 8 . 2897
max
=