Radiation Fundamental Concepts EGR 4345 Heat Transfer.
-
Upload
lindsay-thornton -
Category
Documents
-
view
222 -
download
1
Transcript of Radiation Fundamental Concepts EGR 4345 Heat Transfer.
![Page 1: Radiation Fundamental Concepts EGR 4345 Heat Transfer.](https://reader036.fdocuments.in/reader036/viewer/2022062408/56649f1b5503460f94c302f6/html5/thumbnails/1.jpg)
RadiationFundamental Concepts
EGR 4345
Heat Transfer
![Page 2: Radiation Fundamental Concepts EGR 4345 Heat Transfer.](https://reader036.fdocuments.in/reader036/viewer/2022062408/56649f1b5503460f94c302f6/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
![Page 3: Radiation Fundamental Concepts EGR 4345 Heat Transfer.](https://reader036.fdocuments.in/reader036/viewer/2022062408/56649f1b5503460f94c302f6/html5/thumbnails/3.jpg)
Fundamental Concepts
Ts > Tsur
q rad,net
![Page 4: Radiation Fundamental Concepts EGR 4345 Heat Transfer.](https://reader036.fdocuments.in/reader036/viewer/2022062408/56649f1b5503460f94c302f6/html5/thumbnails/4.jpg)
Types of Radiation
Two categories– Volumetric phenomenon – gases, transparent solids– Surface phenomenon – most solids and liquids
Thermal radiation – emitted by all substances above absolute zero
Includes visible & infrared radiation & some UV radiation.
![Page 5: Radiation Fundamental Concepts EGR 4345 Heat Transfer.](https://reader036.fdocuments.in/reader036/viewer/2022062408/56649f1b5503460f94c302f6/html5/thumbnails/5.jpg)
Fundamental Concepts
![Page 6: Radiation Fundamental Concepts EGR 4345 Heat Transfer.](https://reader036.fdocuments.in/reader036/viewer/2022062408/56649f1b5503460f94c302f6/html5/thumbnails/6.jpg)
Background
Electromagnetic radiation – energy emitted due to changes in electronic configurations of atoms or molecules
where =wavelength (usually in m), =frequency In a vacuum c=co=2.998x108 m/s
Other media: c= co /n where n=index of refraction
c
![Page 7: Radiation Fundamental Concepts EGR 4345 Heat Transfer.](https://reader036.fdocuments.in/reader036/viewer/2022062408/56649f1b5503460f94c302f6/html5/thumbnails/7.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 8: Radiation Fundamental Concepts EGR 4345 Heat Transfer.](https://reader036.fdocuments.in/reader036/viewer/2022062408/56649f1b5503460f94c302f6/html5/thumbnails/8.jpg)
Radiation Spectrum
![Page 9: Radiation Fundamental Concepts EGR 4345 Heat Transfer.](https://reader036.fdocuments.in/reader036/viewer/2022062408/56649f1b5503460f94c302f6/html5/thumbnails/9.jpg)
Radiation Properties
Magnitude of radiation varies with wavelength – it’s spectral.
Radiation is made up of a continuous, nonuniform distribution of monochromatic (single-wavelength) components.
Magnitude & spectral distribution vary with temp & type of emitting surface.
![Page 10: Radiation Fundamental Concepts EGR 4345 Heat Transfer.](https://reader036.fdocuments.in/reader036/viewer/2022062408/56649f1b5503460f94c302f6/html5/thumbnails/10.jpg)
Emission Variation with Wavelength
![Page 11: Radiation Fundamental Concepts EGR 4345 Heat Transfer.](https://reader036.fdocuments.in/reader036/viewer/2022062408/56649f1b5503460f94c302f6/html5/thumbnails/11.jpg)
Radiation Properties
Directional distribution – a surface doesn’t emit the same in all directions.
![Page 12: Radiation Fundamental Concepts EGR 4345 Heat Transfer.](https://reader036.fdocuments.in/reader036/viewer/2022062408/56649f1b5503460f94c302f6/html5/thumbnails/12.jpg)
Nomenclature
![Page 13: Radiation Fundamental Concepts EGR 4345 Heat Transfer.](https://reader036.fdocuments.in/reader036/viewer/2022062408/56649f1b5503460f94c302f6/html5/thumbnails/13.jpg)
Nomenclature
![Page 14: Radiation Fundamental Concepts EGR 4345 Heat Transfer.](https://reader036.fdocuments.in/reader036/viewer/2022062408/56649f1b5503460f94c302f6/html5/thumbnails/14.jpg)
Nomenclature
![Page 15: Radiation Fundamental Concepts EGR 4345 Heat Transfer.](https://reader036.fdocuments.in/reader036/viewer/2022062408/56649f1b5503460f94c302f6/html5/thumbnails/15.jpg)
Spectral Intensity
Spectral Intensity of the Emitted Radiation, I,e
– Rate at which radiant energy is emitted at the wavelength, , in the (, ) direction, per unit solid angle about this direction, and per unit wavelength interval d about
![Page 16: Radiation Fundamental Concepts EGR 4345 Heat Transfer.](https://reader036.fdocuments.in/reader036/viewer/2022062408/56649f1b5503460f94c302f6/html5/thumbnails/16.jpg)
Spectral Intensity
![Page 17: Radiation Fundamental Concepts EGR 4345 Heat Transfer.](https://reader036.fdocuments.in/reader036/viewer/2022062408/56649f1b5503460f94c302f6/html5/thumbnails/17.jpg)
Spectral Intensity
![Page 18: Radiation Fundamental Concepts EGR 4345 Heat Transfer.](https://reader036.fdocuments.in/reader036/viewer/2022062408/56649f1b5503460f94c302f6/html5/thumbnails/18.jpg)
Heat Flux
![Page 19: Radiation Fundamental Concepts EGR 4345 Heat Transfer.](https://reader036.fdocuments.in/reader036/viewer/2022062408/56649f1b5503460f94c302f6/html5/thumbnails/19.jpg)
Emissive Power
Spectral, hemispherical emissive power – E
– Rate at which radiation of wavelength is emitted in all directions from a surface per unit wavelength d about and per unit surface area
– Flux based on actual surface area (not projected)– Hemispherical often not used as emission is in all
directions from surface
Total emissive power, E
![Page 20: Radiation Fundamental Concepts EGR 4345 Heat Transfer.](https://reader036.fdocuments.in/reader036/viewer/2022062408/56649f1b5503460f94c302f6/html5/thumbnails/20.jpg)
Emissive Power
![Page 21: Radiation Fundamental Concepts EGR 4345 Heat Transfer.](https://reader036.fdocuments.in/reader036/viewer/2022062408/56649f1b5503460f94c302f6/html5/thumbnails/21.jpg)
Emissive Power
Diffuse emitter – intensity of the emitted radiation independent of direction
ee II ,, ,,
![Page 22: Radiation Fundamental Concepts EGR 4345 Heat Transfer.](https://reader036.fdocuments.in/reader036/viewer/2022062408/56649f1b5503460f94c302f6/html5/thumbnails/22.jpg)
Spectral Irradiation
Spectral Irradiation, G – Rate at which radiation of wavelength is incident
on a surface per unit area of the surface and per unit wavelength d about
Total Irradiation, G
![Page 23: Radiation Fundamental Concepts EGR 4345 Heat Transfer.](https://reader036.fdocuments.in/reader036/viewer/2022062408/56649f1b5503460f94c302f6/html5/thumbnails/23.jpg)
Spectral Irradiation
![Page 24: Radiation Fundamental Concepts EGR 4345 Heat Transfer.](https://reader036.fdocuments.in/reader036/viewer/2022062408/56649f1b5503460f94c302f6/html5/thumbnails/24.jpg)
Spectral Radiosity
Spectral Radiosity, J – Rate at which radiation of wavelength is leaves a
unit area of surface, per unit wavelength interval d about
Total Radiosity, J
![Page 25: Radiation Fundamental Concepts EGR 4345 Heat Transfer.](https://reader036.fdocuments.in/reader036/viewer/2022062408/56649f1b5503460f94c302f6/html5/thumbnails/25.jpg)
Spectral Radiosity
![Page 26: Radiation Fundamental Concepts EGR 4345 Heat Transfer.](https://reader036.fdocuments.in/reader036/viewer/2022062408/56649f1b5503460f94c302f6/html5/thumbnails/26.jpg)
Blackbody Radiation
Blackbody – a perfect emitter & absorber of radiation Emits radiation uniformly in all directions – no
directional distribution – it’s diffuse Joseph Stefan (1879)– total radiation emission per
unit time & area over all wavelengths and in all directions:
=Stefan-Boltzmann constant =5.67 x10-8 W/m2K4
24 mW TEb
![Page 27: Radiation Fundamental Concepts EGR 4345 Heat Transfer.](https://reader036.fdocuments.in/reader036/viewer/2022062408/56649f1b5503460f94c302f6/html5/thumbnails/27.jpg)
Planck’s Distribution Law
Sometimes we’re interested in radiation at a certain wavelength
Spectral blackbody emissive power (Eb) = “amount of radiation energy emitted by a blackbody at an absolute temperature T per unit time, per unit surface area, and per unit wavelength about the wavelength .”
![Page 28: Radiation Fundamental Concepts EGR 4345 Heat Transfer.](https://reader036.fdocuments.in/reader036/viewer/2022062408/56649f1b5503460f94c302f6/html5/thumbnails/28.jpg)
Planck’s Distribution Law
Emitted radiation varies continuously with At any the magnitude of the emitted radiation
increases with increasing temperature The spectral region in which the radiation is
concentrated depends on temperature, with comparatively more radiation appearing at the shorter as the temperature increases
Sun – approximated as a blackbody at 5800 K, radiation is mostly in the visible region
![Page 29: Radiation Fundamental Concepts EGR 4345 Heat Transfer.](https://reader036.fdocuments.in/reader036/viewer/2022062408/56649f1b5503460f94c302f6/html5/thumbnails/29.jpg)
Planck’s Distribution Law
For a surface in a vacuum or gas
Other media: replace C1 with C1/n2
Integrating this function over all gives us the equation for Eb.
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
![Page 30: Radiation Fundamental Concepts EGR 4345 Heat Transfer.](https://reader036.fdocuments.in/reader036/viewer/2022062408/56649f1b5503460f94c302f6/html5/thumbnails/30.jpg)
Radiation Distribution
Radiation is a continuous function of wavelength
Radiation increases with temp.
At higher temps, more radiation is at shorter wavelengths.
Solar radiation peak is in the visible range.
![Page 31: Radiation Fundamental Concepts EGR 4345 Heat Transfer.](https://reader036.fdocuments.in/reader036/viewer/2022062408/56649f1b5503460f94c302f6/html5/thumbnails/31.jpg)
Wien’s Displacement Law
Peak can be found for different temps using Wien’s Displacement Law:
Note that color is a function of absorption & reflection, not emission
Km 52897 .T powermax
![Page 32: Radiation Fundamental Concepts EGR 4345 Heat Transfer.](https://reader036.fdocuments.in/reader036/viewer/2022062408/56649f1b5503460f94c302f6/html5/thumbnails/32.jpg)
Blackbody Radiation Function
We often are interested in radiation energy emitted over a certain wavelength.
This is a tough integral to do!
![Page 33: Radiation Fundamental Concepts EGR 4345 Heat Transfer.](https://reader036.fdocuments.in/reader036/viewer/2022062408/56649f1b5503460f94c302f6/html5/thumbnails/33.jpg)
Blackbody Radiation Function
Use blackbody radiation function, F
If we want radiation between 1 & 2,
40
T
dTE
TF
b
TFTFTF1221