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Transcript of Radiation
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Radiative Properties • When radiation strikes a surface, a portion of it is
reflected, and the rest enters the surface. • Of the portion that enters the surface, some are
absorbed by the material, and the remaining radiation is transmitted through.
• The ratio of reflected energy to the incident energy is called reflectivity, ρ.
• Transmissivity (τ) is defined as the fraction of the incident energy that is transmitted through the object.
• Absorptivity (α) is defined as the fraction of the incident energy that is absorbed by the object.
• The three radiative properties all have values between zero and 1. • Furthermore, since the reflected, transmitted, and absorbed radiation must
add up to equal the incident energy, the following can be said about the three properties:
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Emissivity
• A black body is an ideal emitter. • The energy emitted by any real surface is less than the
energy emitted by a black body at the same temperature. • At a defined temperature, a black body has the highest
monochromatic emissive power at all wavelengths. • The ratio of the emissive power of real body Eto the
blackbody emissive power bat the same temperature is the hemispherical emissivity of the surface.
bEE
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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.
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Spherical Geometry
dA
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Vectors in Spherical Geometry
Zenith Angle :
zimuthal Angle : (,r)
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Hemispherical Black Surface Emission
4TEI bb
Black body Emissive Intensity
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Real Surface emission
The radiation emitted by a real surface is spatially distributed:
),,( TII real Directional Emissive Intensity:
Directional Emissivity:
bITIT ),,(),,(
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Directional Emissivity
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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
12, 5
2
kThc
e
hcTE
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
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Real Surface Monochromatic emission
The radiation emitted by a real surface is spatially distributed:
),,,(, TII real Directional MonochromaticEmissive Intensity:
Directional Monochromatic Emissivity:
,
),,(),,(bI
TIT
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Monochromatic emissive power E
• 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
kThc
e
hcTE
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Shifting Peak Nature of Radiation
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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
de
hcd
dTdE kT
hc
1
12
,5
2
mKT 8.2897max
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Wien law for three different stars
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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.
05
2
0 1
12,
de
hcdTEkThc
444
42
15
2 TT
khc
hcTEb
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The total (hemispherical) energy emitted by a body, regardless of the wavelengths, is given by:
4ATQemitted
• where ε is the emissivity of the body,• A is the surface area, • T is the temperature, and • σ is the Stefan-Boltzmann constant, equal to 5.67×10-8 W/m2K4. • Emissivity is a material property, ranging from 0 to 1, which
measures how much energy a surface can emit with respect to an ideal emitter (ε = 1) at the same temperature
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The total (hemispherical emissive power is, then, given by
Here, can be interpreted as either the emissivity of a body,
which is wavelength independent, i.e., is constant, or as the
average emissivity of a surface at that temperature.
A surface whose properties are independent of the wavelength is known as a gray surface.
The emissive power of a real surface is given by
00
)( dEdEE b
Define total (hemispherical) emissivity, at a defined temperature
0
0
0
0
)(
dE
dE
dE
dE
b
b
b
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Absorptivity , Reflectivity and Transmissivity
• Consider a semi-transparent sheet that receives incident radiant energy flux, also known as irradiation, G .
• Let dG represent the irradiation in the waveband to d.
• Part of it may be absorbed, part of it reflected at the surface, and the rest transmitted through the sheet.
• We define monochromatic properties,
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Monochromatic Absorptivity :dGdG
Monochromatic reflectivity :dGdG
Monochromatic Transmissivity :dGdG
Total Absorptivity :GGd
0
Total reflectivity :GG
d
0
Total Transmissivity :GGd
0
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Conservation of Irradiation
The total Irradiation = GGGG
GG
GG
GG 1
1
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3..
Blackbody Radiation
• The characteristics of a blackbody are :• It is a perfect emitter.• At any prescribed temperature it has the highest
monochromatic emissive power at all wave lengths.• A blackbody absorbs all the incident energy and there fore
• It is non reflective body (• It is opaque (= 0). • It is a diffuse emitter
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Radiative Heat Transfer Consider the heat transfer between two surfaces, as shown in Figure. What is the rate of heat transfer into Surface B? To find this, we will first look at the emission from A to B. Surface A emits radiation as described in
4, AAAemittedA TAQ
This radiation is emitted in all directions, and only a fraction of it will actually strike Surface B. This fraction is called the shape factor, F.
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The amount of radiation striking Surface B is therefore:
4, AAABAinceidentB TAFQ
The only portion of the incident radiation contributing to heating Surface B is the absorbed portion, given by the absorptivity αB:
4, AAABABabsorbedB TAFQ
Above equation is the amount of radiation gained by Surface B from Surface A. To find the net heat transfer rate at B, we must now subtract the amount of radiation emitted by B:
4, BBBemittedB TAQ
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The net radiative heat transfer (gain) rate at Surface B is
emittedBabsorbedBB QQQ ,,
44BBBAAABABB TATAFQ
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Shape Factors • Shape factor, F, is a geometrical
factor which is determined by the shapes and relative locations of two surfaces.
• Figure illustrates this for a simple case of cylindrical source and planar surface.
• Both the cylinder and the plate are infinite in length.
• In this case, it is easy to see that the shape factor is reduced as the distance between the source and plane increases.
• The shape factor for this simple geometry is simply the cone angle (θ) divided by 2π
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• Shape factors for other simple geometries can be calculated using basic theory of geometry.
• For more complicated geometries, the following two rules must be applied to find shape factors based on simple geometries.
• The first is the summation rule.
122211 FAFA
• This rule says that the shape factor from a surface (1) to another (2) can be expressed as a sum of the shape factors from (1) to (2a), and (1) to (2b).
• The second rule is the reciprocity rule, which relates the shape factors from (1) to (2) and that from (2) to (1) as follows:
ba FFF 212121
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Thus, if the shape factor from (1) to (2) is known, then the shape factor from (2) to (1) can be found by:
121
221 F
AAF
If surface (2) totally encloses the surface 1:
121 F
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Geometric Concepts in Radiation
• Solid Angle:
2rdAd n
sind d d
Emissive intensity SrW/mddS
dqIn
e2 ,
Monochromatic Emissive intensity
mSrW/mdddS
dqIn
e
2
, ,,
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Total emissive power dS
ndS
cosndS dS
ne e
alldirections
dSE I ddS
2 2
,0 0 0
( , , ) cos sine eq E I d d d
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Radiation Heat Transfer• e = emissive power• G = total irradiation• J = total radiosity
1
In general:
Opaque material:
1
= absorptivity = reflectivity = transmissivity
= emissivity
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e
T1
T2
T3
Energy
e
Ideal EmitterSchematicT3> T2> T1
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Radiation Heat Transfer
• Black Body– absorptivity = – emissivity = – ideal emissive power = eb
4Teb 1
f
f
4Tegray
bgray ee
• Gray Body– absorptivity < 1– emissivity < 1– emissive
power<1
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e Gray Body
Black Body
Energy
Real Body
black
gray
ee
f
Schematic
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Radiation Heat Transfer
1 j
ijF 1...... 111211 nj FFFF
jijiji FAFA Thermal Equilibrium
View Factor: Fij = fraction of radiation from surface i intercepted by surface j.
1 2
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Radiation Heat Transfer
1be1J 2JRJ
11
11
A
22
21
ARFA 11
1
121
1FA
RFA 22
1
No net heat flux wall
Analog circuit
12Q11212 AqQ
Find:
2be
X
1 2
R
R
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Radiation Heat Transfer
1be1J 2J
2be
RJ11
11
A
22
21
ARFA 11
1
121
1FA
RFA 22
1
RR FAFA 2211
11
1be1J 2J
2be
11
11
A
22
21
A
121
1FA
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Radiation Heat Transfer
1be1J 2J
2be
11
11
A
22
21
ARR FAFA 2211
11
121
1FA
1be 1J 2J 2be
11
11
A
22
21
A
RR FAFA
FA
2211
121 111
1
RR FAFA
FAFA
2211
121121 111
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Radiation Heat Transfer
1be 1J 2J 2be
11
11
A
22
21
A
121
1FA
RR FAFA
FAFA
2211
121121 111
22
2
12111
1 111
AFAA
1be 2be
22
2
12111
1121 111
1
AFAA
A
F
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1be 2be
121
1FA
42
41112 TTAQ 12F
21112 bb eeAQ 12F
121tan FAceConduc
Radiation Heat Transfer
RR FAFA
FAFA
2211
121121 111
22
2
12111
1121 111
1
AFAA
A
F
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Radiation Heat TransferConsider the radiation heat transfer between two infinite parallel plates.
1 2
netq ,12
2be1be1J 2J
22
21
A
11
11
A
121
1FA
12112 FF 21 AA
22
2
12111
1
21112 111
AFAA
eeAq bb
111
21
2112
bb eeq
1
Special Case
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Radiation Heat TransferConsider the radiation heat transfer between a small object and infinite surroundings.
1
S
netSq ,1 bSe1be1J SJ
SS
S
A 1
11
11
A
SFA 11
1
11 SF 01 SAA
SS
bSbS
AFAA
eeAq
2
2
1111
1
111 111
bSbS eeq 111
10
1 SS
Special Case
44111 SS TTq