introduction to thermal radiation and radiation heat transfer
TRANSCRIPT
![Page 1: Introduction to Thermal Radiation and Radiation Heat Transfer](https://reader033.vdocuments.us/reader033/viewer/2022061509/5697bfc01a28abf838ca387b/html5/thumbnails/1.jpg)
Introduction to Thermal Radiation and Radiation Heat Transfer
![Page 2: Introduction to Thermal Radiation and Radiation Heat Transfer](https://reader033.vdocuments.us/reader033/viewer/2022061509/5697bfc01a28abf838ca387b/html5/thumbnails/2.jpg)
Thermal Radiation• Occurs in solids, liquids, and gases• Occurs at the speed of light• Has no attenuation in a vacuum• Can occur between two bodies with a colder
medium in between• Occurs in combination with conduction and
convection, and significant where large temperature differences occur
• Applications: Furnace with boiler tubes, radiant dryers, oven baking, designing heaters for manufacturing, estimating heat gains through windows, infrared cameras, metal cooling during manufacturing, Greenhouse effect, thermos design, among others.
![Page 3: Introduction to Thermal Radiation and Radiation Heat Transfer](https://reader033.vdocuments.us/reader033/viewer/2022061509/5697bfc01a28abf838ca387b/html5/thumbnails/3.jpg)
Background
• Electromagnetic radiation – energy emitted due to changes in electronic configurations of atoms or molecules
• where l=wavelength (usually in mm), n=frequency
• In a vacuum c=co=2.998x108 m/s• Other media: c=co /n where n=index of
refraction
c
![Page 4: Introduction to Thermal Radiation and Radiation Heat Transfer](https://reader033.vdocuments.us/reader033/viewer/2022061509/5697bfc01a28abf838ca387b/html5/thumbnails/4.jpg)
Background, cont.
• Radiation – photons or waves?• Max Planck (1900): each photon has an
energy of • h=Planck’s constant=6.625 x 10-34 Js• Shorter wavelengths have higher energy
hche
![Page 5: Introduction to Thermal Radiation and Radiation Heat Transfer](https://reader033.vdocuments.us/reader033/viewer/2022061509/5697bfc01a28abf838ca387b/html5/thumbnails/5.jpg)
Radiation Spectrum
![Page 6: Introduction to Thermal Radiation and Radiation Heat Transfer](https://reader033.vdocuments.us/reader033/viewer/2022061509/5697bfc01a28abf838ca387b/html5/thumbnails/6.jpg)
Types of Radiation
• Two categories– Volumetric phenomenon – radiation emitted or
absorbed throughout gases, transparent solids, some fluids
– Surface phenomenon – radiation to/from solid or liquid surface
• Thermal radiation – emitted by all substances above absolute zero
• Includes visible & infrared radiation & some UV radiation.
![Page 7: Introduction to Thermal Radiation and Radiation Heat Transfer](https://reader033.vdocuments.us/reader033/viewer/2022061509/5697bfc01a28abf838ca387b/html5/thumbnails/7.jpg)
Radiation Properties
• Magnitude of radiation varies with wavelength – it’s spectral.– The wavelength of the radiation is a major factor in
what its effects will be.– Earth/sun example
• Radiation is made up of a continuous, nonuniform distribution of monochromatic (single-wavelength) components.
• Magnitude & spectral distribution (how the radiation varies with wavelength) vary with temp & type of emitting surface.
![Page 8: Introduction to Thermal Radiation and Radiation Heat Transfer](https://reader033.vdocuments.us/reader033/viewer/2022061509/5697bfc01a28abf838ca387b/html5/thumbnails/8.jpg)
Emission Variation with Wavelength
![Page 9: Introduction to Thermal Radiation and Radiation Heat Transfer](https://reader033.vdocuments.us/reader033/viewer/2022061509/5697bfc01a28abf838ca387b/html5/thumbnails/9.jpg)
Blackbody Radiation• Blackbody – a perfect emitter & absorber of
radiation; it absorbs all incident radiation, and no surface can emit more for a given temperature and wavelength
• Emits radiation uniformly in all directions – no directional distribution – it’s diffuse
• Example of a blackbody: large cavity with a small hole
![Page 10: Introduction to Thermal Radiation and Radiation Heat Transfer](https://reader033.vdocuments.us/reader033/viewer/2022061509/5697bfc01a28abf838ca387b/html5/thumbnails/10.jpg)
Stefan-Boltzmann Law
• Joseph Stefan (1879)– total radiation emission per unit time & area over all wavelengths and in all directions:
s =Stefan-Boltzmann constant
=5.67 x10-8 W/m2K4
T must be in absolute scale.
24 mW TEb
![Page 11: Introduction to Thermal Radiation and Radiation Heat Transfer](https://reader033.vdocuments.us/reader033/viewer/2022061509/5697bfc01a28abf838ca387b/html5/thumbnails/11.jpg)
Planck’s Distribution Law• Sometimes we care about the radiation in a certain
wavelength interval• For a surface in a vacuum or gas
• Integrating this function over all l gives us
constant sBoltzmann'J/K 1038051
Kμm 104391
mμmW 1074232
where
μmmW 1
23
42
24821
2
25
1
-
o
o
b
x.k
x.khcC
x.hcC
TCexp
CTE
4 bE T
![Page 12: Introduction to Thermal Radiation and Radiation Heat Transfer](https://reader033.vdocuments.us/reader033/viewer/2022061509/5697bfc01a28abf838ca387b/html5/thumbnails/12.jpg)
Radiation Distribution
• Radiation is a continuous function of wavelength
• Magnitude increases with temp.
• At higher temps, more radiation is at shorter wavelengths.
• Solar radiation peak is in the visible range.
![Page 13: Introduction to Thermal Radiation and Radiation Heat Transfer](https://reader033.vdocuments.us/reader033/viewer/2022061509/5697bfc01a28abf838ca387b/html5/thumbnails/13.jpg)
Wien’s Displacement Law
• Wavelength of radiation with the largest magnitude can be found for different temps using Wien’s Displacement Law:
• Note that color is a function of absorption & reflection, not emission.
max2897.8 m K
powerT
![Page 14: Introduction to Thermal Radiation and Radiation Heat Transfer](https://reader033.vdocuments.us/reader033/viewer/2022061509/5697bfc01a28abf838ca387b/html5/thumbnails/14.jpg)
What is your favorite wavelength?
![Page 15: Introduction to Thermal Radiation and Radiation Heat Transfer](https://reader033.vdocuments.us/reader033/viewer/2022061509/5697bfc01a28abf838ca387b/html5/thumbnails/15.jpg)
More Radiation Properties
• Directional distribution – a surface doesn’t emit the same in all directions.
• Hemispherical – refers to all directions
![Page 16: Introduction to Thermal Radiation and Radiation Heat Transfer](https://reader033.vdocuments.us/reader033/viewer/2022061509/5697bfc01a28abf838ca387b/html5/thumbnails/16.jpg)
Emissive Power• E: amount of radiation emitted per unit area• Spectral hemispherical emissive power El
(often leave out the word “hemispherical”) W/m2l– Rate of emission per unit area of radiation of a
given wavelength l in all directions per unit wavelength interval
• Total (hemisperical) emissive power E (W/m2)– Rate of emission per unit area of radiation of all
wavelengths and in all directions; this is
emittedq
![Page 17: Introduction to Thermal Radiation and Radiation Heat Transfer](https://reader033.vdocuments.us/reader033/viewer/2022061509/5697bfc01a28abf838ca387b/html5/thumbnails/17.jpg)
Diffuse emitters
• Diffuse emitter: intensity is the same in all directions
![Page 18: Introduction to Thermal Radiation and Radiation Heat Transfer](https://reader033.vdocuments.us/reader033/viewer/2022061509/5697bfc01a28abf838ca387b/html5/thumbnails/18.jpg)
Irradiation
• Irradiation: radiation incident on (hitting) a surface per unit area
![Page 19: Introduction to Thermal Radiation and Radiation Heat Transfer](https://reader033.vdocuments.us/reader033/viewer/2022061509/5697bfc01a28abf838ca387b/html5/thumbnails/19.jpg)
Radiosity
• Radiosity: all radiation leaving a surface per unit area, both emitted and reflected
![Page 20: Introduction to Thermal Radiation and Radiation Heat Transfer](https://reader033.vdocuments.us/reader033/viewer/2022061509/5697bfc01a28abf838ca387b/html5/thumbnails/20.jpg)
![Page 21: Introduction to Thermal Radiation and Radiation Heat Transfer](https://reader033.vdocuments.us/reader033/viewer/2022061509/5697bfc01a28abf838ca387b/html5/thumbnails/21.jpg)
![Page 22: Introduction to Thermal Radiation and Radiation Heat Transfer](https://reader033.vdocuments.us/reader033/viewer/2022061509/5697bfc01a28abf838ca387b/html5/thumbnails/22.jpg)
![Page 23: Introduction to Thermal Radiation and Radiation Heat Transfer](https://reader033.vdocuments.us/reader033/viewer/2022061509/5697bfc01a28abf838ca387b/html5/thumbnails/23.jpg)
Emmisivity and Kirchoff’s law
e = E/Eb
Kirchoffs’ Law
( 1)= ( 1)a T e T
![Page 24: Introduction to Thermal Radiation and Radiation Heat Transfer](https://reader033.vdocuments.us/reader033/viewer/2022061509/5697bfc01a28abf838ca387b/html5/thumbnails/24.jpg)
l
El
T1
T2
T3
Energy
e
Ideal EmitterSchematic
T3> T2> T1
![Page 25: Introduction to Thermal Radiation and Radiation Heat Transfer](https://reader033.vdocuments.us/reader033/viewer/2022061509/5697bfc01a28abf838ca387b/html5/thumbnails/25.jpg)
1 In general:
Opaque material:
1
a = absorptivity
r = reflectivity
t = transmissivity
![Page 26: Introduction to Thermal Radiation and Radiation Heat Transfer](https://reader033.vdocuments.us/reader033/viewer/2022061509/5697bfc01a28abf838ca387b/html5/thumbnails/26.jpg)
Mechanism of Radiation Heat Transfer
• Thermal energy of hot source ( furnace wall at T1) is converted into radiant energy.
• These waves travel through the intervening space in straight lines and strike a cold object at T2 such as a furnace tube
• The electromagnetic waves that strike the body are absorbed and converted back to thermal energy.
![Page 27: Introduction to Thermal Radiation and Radiation Heat Transfer](https://reader033.vdocuments.us/reader033/viewer/2022061509/5697bfc01a28abf838ca387b/html5/thumbnails/27.jpg)
Black Body and Gray Body
• Black Body– absorptivity = =1a– emissivity = =1e– ideal emissive power = Eb
4bE T
1
4grayE T
gray bE E
• Gray Body– absorptivity < 1– emissivity < 1
(independent of wavelength)
– emissive power<1
![Page 28: Introduction to Thermal Radiation and Radiation Heat Transfer](https://reader033.vdocuments.us/reader033/viewer/2022061509/5697bfc01a28abf838ca387b/html5/thumbnails/28.jpg)
![Page 29: Introduction to Thermal Radiation and Radiation Heat Transfer](https://reader033.vdocuments.us/reader033/viewer/2022061509/5697bfc01a28abf838ca387b/html5/thumbnails/29.jpg)
Radiation of a small object from surrounding
A1
4 412 1 1 1 1 12 2q A T A T
T2
T1
T2 > T1
4 412 1 1 2( )q A T T
![Page 30: Introduction to Thermal Radiation and Radiation Heat Transfer](https://reader033.vdocuments.us/reader033/viewer/2022061509/5697bfc01a28abf838ca387b/html5/thumbnails/30.jpg)
Combined radiation and convection heat transfer
1 1 2 1 1 2
convection radiation
c r
q q q
q h A T T h A T T
4 41 2
1 2r
T Th
T T
![Page 31: Introduction to Thermal Radiation and Radiation Heat Transfer](https://reader033.vdocuments.us/reader033/viewer/2022061509/5697bfc01a28abf838ca387b/html5/thumbnails/31.jpg)
View factorView Factor: Fij = fraction of radiation from surface i intercepted by surface j.
1 2
![Page 32: Introduction to Thermal Radiation and Radiation Heat Transfer](https://reader033.vdocuments.us/reader033/viewer/2022061509/5697bfc01a28abf838ca387b/html5/thumbnails/32.jpg)
Summation rule (View factor)
1 j
ijF
1...... 111211 nj FFFF
![Page 33: Introduction to Thermal Radiation and Radiation Heat Transfer](https://reader033.vdocuments.us/reader033/viewer/2022061509/5697bfc01a28abf838ca387b/html5/thumbnails/33.jpg)
Reciprocity rule (View factor)
jijiji FAFA
![Page 34: Introduction to Thermal Radiation and Radiation Heat Transfer](https://reader033.vdocuments.us/reader033/viewer/2022061509/5697bfc01a28abf838ca387b/html5/thumbnails/34.jpg)
Superposition rule
![Page 35: Introduction to Thermal Radiation and Radiation Heat Transfer](https://reader033.vdocuments.us/reader033/viewer/2022061509/5697bfc01a28abf838ca387b/html5/thumbnails/35.jpg)
Symmetry rule
![Page 36: Introduction to Thermal Radiation and Radiation Heat Transfer](https://reader033.vdocuments.us/reader033/viewer/2022061509/5697bfc01a28abf838ca387b/html5/thumbnails/36.jpg)
Radiation heat transfer between black surfaces
![Page 37: Introduction to Thermal Radiation and Radiation Heat Transfer](https://reader033.vdocuments.us/reader033/viewer/2022061509/5697bfc01a28abf838ca387b/html5/thumbnails/37.jpg)
Radiation heat transfer between black surfaces
(surfaces forming anenclosure)
![Page 38: Introduction to Thermal Radiation and Radiation Heat Transfer](https://reader033.vdocuments.us/reader033/viewer/2022061509/5697bfc01a28abf838ca387b/html5/thumbnails/38.jpg)
Net radiation from heat transfer to or from a surface
![Page 39: Introduction to Thermal Radiation and Radiation Heat Transfer](https://reader033.vdocuments.us/reader033/viewer/2022061509/5697bfc01a28abf838ca387b/html5/thumbnails/39.jpg)
Electrical analogy
Surface resistance
![Page 40: Introduction to Thermal Radiation and Radiation Heat Transfer](https://reader033.vdocuments.us/reader033/viewer/2022061509/5697bfc01a28abf838ca387b/html5/thumbnails/40.jpg)
Net Radiation Transfer between two surfaces
space resistance
![Page 41: Introduction to Thermal Radiation and Radiation Heat Transfer](https://reader033.vdocuments.us/reader033/viewer/2022061509/5697bfc01a28abf838ca387b/html5/thumbnails/41.jpg)
Radiation heat transfer in two-surface enclosures
![Page 42: Introduction to Thermal Radiation and Radiation Heat Transfer](https://reader033.vdocuments.us/reader033/viewer/2022061509/5697bfc01a28abf838ca387b/html5/thumbnails/42.jpg)
Consider the radiation heat transfer between
two infinite parallel plates
1 2
netq ,12
2be1be1J 2J
22
21
A
11
11
A
121
1
FA
12112 FF 21 AA
22
2
12111
1
21112 111
AFAA
eeAq bb
1
11
21
2112
bb eeq
1
![Page 43: Introduction to Thermal Radiation and Radiation Heat Transfer](https://reader033.vdocuments.us/reader033/viewer/2022061509/5697bfc01a28abf838ca387b/html5/thumbnails/43.jpg)
![Page 44: Introduction to Thermal Radiation and Radiation Heat Transfer](https://reader033.vdocuments.us/reader033/viewer/2022061509/5697bfc01a28abf838ca387b/html5/thumbnails/44.jpg)
![Page 45: Introduction to Thermal Radiation and Radiation Heat Transfer](https://reader033.vdocuments.us/reader033/viewer/2022061509/5697bfc01a28abf838ca387b/html5/thumbnails/45.jpg)
![Page 46: Introduction to Thermal Radiation and Radiation Heat Transfer](https://reader033.vdocuments.us/reader033/viewer/2022061509/5697bfc01a28abf838ca387b/html5/thumbnails/46.jpg)
![Page 47: Introduction to Thermal Radiation and Radiation Heat Transfer](https://reader033.vdocuments.us/reader033/viewer/2022061509/5697bfc01a28abf838ca387b/html5/thumbnails/47.jpg)
Application:Radiation shield
• Highly reflective thin plate to reduce radiation heat transfer between two surfaces
![Page 48: Introduction to Thermal Radiation and Radiation Heat Transfer](https://reader033.vdocuments.us/reader033/viewer/2022061509/5697bfc01a28abf838ca387b/html5/thumbnails/48.jpg)
![Page 49: Introduction to Thermal Radiation and Radiation Heat Transfer](https://reader033.vdocuments.us/reader033/viewer/2022061509/5697bfc01a28abf838ca387b/html5/thumbnails/49.jpg)
![Page 50: Introduction to Thermal Radiation and Radiation Heat Transfer](https://reader033.vdocuments.us/reader033/viewer/2022061509/5697bfc01a28abf838ca387b/html5/thumbnails/50.jpg)
Radiation effect on temperature measurement
![Page 51: Introduction to Thermal Radiation and Radiation Heat Transfer](https://reader033.vdocuments.us/reader033/viewer/2022061509/5697bfc01a28abf838ca387b/html5/thumbnails/51.jpg)