Lec-1-2 Radiation Processes and Properties Basic Principles and Definitions
Lec-1-2 Radiation Processes and Properties Basic Principles and Definitions
Lec-1-2 Radiation Processes and Properties Basic Principles and Definitions
Properties
Basic Principles and Definitions-
Chapter 12
Sections 12.1 through 12.3
General Considerations
General Considerations
• Attention is focused on thermal radiation, whose origins are associated
with emission from matter at an absolute temperature T 0.
• Emission is due to oscillations and transitions of the many electrons that comprise
matter, which are, in turn, sustained by the thermal energy of the matter.
• Emission corresponds to heat transfer from the matter and hence to a reduction
in thermal energy stored by the matter.
• Absorption results in heat transfer to the matter and hence an increase in thermal
energy stored by the matter.
For an opaque solid or liquid, emission originates from atoms and molecules
within 1 m of the surface.
• The dual nature of radiation:
– In some cases, the physical manifestations of radiation may be explained
by viewing it as particles (eg.: photons or quanta).
– In other cases, radiation behaves as an electromagnetic wave.
General Considerations (cont)
• Thermal radiation is confined to the infrared, visible and ultraviolet regions of the
spectrum 0.1 100 m .
• The amount of radiation emitted by an opaque
surface varies with wavelength, and we may
speak of the spectral distribution over all
wavelengths or of monochromatic/spectral
components associated with particular wavelengths.
Directional Considerations
dAn r 2 sin d d
dA n
d 2
sin d d
r
– The solid angle has units of steradians (sr).
– The solid angle associated with a complete hemisphere is
2 / 2
hemi 0 0 sin d d 2 sr
• Spectral Intensity: A quantity used to specify the radiant heat flux W/m2 within
a unit solid angle about a prescribed direction W/m2 sr and within a unit
wavelength interval about a prescribed wavelength W/m2 sr m .
Emitted intensity
• I,e is the rate at which radiant energy is
emitted at the wavelength in the (,)
direction, per unit area of the emitting
surface normal to this direction, per unit
solid angle about this direction, and per
unit wavelength interval d about
dq
I ,e , ,
dA1 cos d d
Directional Considerations (cont)
• The rationale for defining the radiation flux in terms of the projected surface area
dA1 cos stems from the existence of surfaces for which, to a good approximation,
I ,e is independent of direction. Such surfaces are termed diffuse, and the radiation is
said to be isotropic.
• The total emissive power W/m2 corresponds to emission over all directions
and wavelengths.
E 0 E d
• The spectral irradiation W/m2 m is then:
2 / 2
G 0 0 I ,i , , cos sin d d
and the total irradiation W/m2 is
G 0 G d
• The radiosity of an opaque surface accounts for all of the radiation leaving the
surface in all directions and may include contributions to both reflection and
emission.
Radiation Fluxes (cont)
and the total radiosity W/m2 is
J 0 J d
How may J and J be expressed if the surface emits and reflects
diffusely?
The Blackbody
Blackbody Radiation
• The Blackbody
An idealization providing limits on radiation emission and absorption by matter.
– For a prescribed temperature and wavelength, no surface can emit
more radiation than a blackbody: the ideal emitter.
– A blackbody is a diffuse emitter.
– A blackbody absorbs all incident radiation: the ideal absorber.
(a) After multiple reflections, virtually all radiation entering the cavity is absorbed.
(b) Emission from the aperture is the maximum possible emission achievable for
the temperature associated with the cavity and is diffuse.
The Blackbody (cont)
(c) The cumulative effect of radiation emission from and reflection off
the cavity wall is to provide diffuse irradiation corresponding to
emission from a blackbody G E ,b for any surface in the cavity.
– Does this condition depend on whether the cavity surface is highly
reflecting or absorbing?
Planck Distribution
. . . .
. . . .
. . . .
Band Emission (cont)
Note ability to readily determine I ,b and its relation to the maximum intensity from
the 3rd and 4th columns, respectively.
If emission from the sun may be approximated as that from a blackbody at
5800K, at what wavelength does peak emission occur?
Would you expect radiation emitted by a blackbody at 800K to be discernible
by the naked eye?
As the temperature of a blackbody is increased, what color would be
the first to be discerned by the naked eye?
Problem: Solar Irradiation
dir cos .
Gdir q
or
G 1086 W / m2.
SCHEMATIC:
Problem: Solar/Earth Temperatures (cont)
ASSUMPTIONS: (1) Sun and earth emit as blackbodies, (2) No attenuation of solar
radiation enroute to earth, (3) Earth atmosphere has no effect on earth energy balance.
ANALYSIS: (a) Applying conservation of energy to the solar energy crossing two concentric
spheres, one having the radius of the sun and the other having the radial distance from the edge
of the earth’s atmosphere to the center of the sun, it follows that
2
2 De
Es Ds 4 R se q .
2 s
Hence
Ee De2 qS
De2 / 4 .
Hence, from the Stefan-Boltzmann law,
1/ 4 1/ 4
q 1353 W / m2
Te S 278 K.
4 4 5.67 108 W / m2 K 4
COMMENTS: The average earth temperature is higher than 278 K due to the shielding effect
of the earth’s atmosphere (transparent to solar radiation but not to longer wavelength earth
emission).