1 RADIATION HEAT TRANSFER Heat conduction and convection - always a fluid which transfers the heat (gas, liquid, solid) – motion of atoms or molecules Heat conduction and convection is not possible in a vacuum In most practical applications all three modes occur concurrently at varying degrees
32
Embed
RADIATION HEAT TRANSFER - Vysoké učení technické v …ottp.fme.vutbr.cz/vyuka/prenostepla/Radiation_Introduction.pdf · 1 RADIATION HEAT TRANSFER Heat conduction and convection
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
1
RADIATION HEAT TRANSFER
Heat conduction and convection - always a fluid which transfers the heat (gas, liquid, solid) – motion of atoms or moleculesHeat conduction and convection is not possible in a vacuumIn most practical applications all three modes occur concurrently at varying degrees
2
A hot object in a vacuumchamber looses heat byradiation only
Unlike conduction and convection, heattransfer by radiation can occur between two bodies, even when they are separated by a medium colder than both of them
convection
radiation
What will be a final equilibrium temperature of the body surface? Can you write an energy balance equation between the body and surrounding air and the hot source (fire)?
3
Theoretical foundation of radiation was establishedby MaxwellElectromagnetic wave motion or electromagnetic radiationElectromagnetic waves travel at the speed of light c in a vacuumElectromagnetic waves are characterized by their frequency for wavelength λ: λ=c/fc=co/n co light speed in a vacuum
n refraction index of a medium (n=1 for air and most gases, n=1,5 for glass, 1,33 for water)
In all material medium, there is attenuation of the energyIn a vacuum there is no attenuation of the energy
4
Electromagnetic radiation covers a wide range of wavelengths
Radiation that is related toheat transfer –Thermal radiationλ from 0,1μm to 100 μmAs a result of energy transition in molecules, atoms and electrons.
Thermal radiation is emitted by all matter whose temperature is above absolute zero.
Everything around us emits (and absorbs) radiation.
5
• Thermal radiation includes entire visible (0,4 to 0,76 μm)and infrared light and a portion of ultraviolet radiation.
• Bodies start emit visible radiation at 800K (red hot) and tungsten wire in the lightbulb at 2000K (white hot) to emit a significant amount of radiation in the visible range.
• Bodies at room temperature emit radiation in infrared range 0,7 to 100 μm.
• Sun (primary light source) emits solar radiation –0,3 to 3 μm – almost half is visible, remaining is ultraviolet and infrared.
• Body that emits radiation in the visible range is called light source.
6
Spectral and Directional Distribution
Radiation characteristics varywith wavelength and direction
• Monochromatic or spectral: Characteristics at a given λ• Total: Integrated values over all wavelengths• Directional: At a given direction
• Diffuse radiation: Uniform in all directions • Hemispherical: Integrated values over all directions
The assumption of diffuse radiation will be made throughout
7
Emissive Power E, Irradiation G and Radiosity J
• Emissive Power (zářivost):Radiation emitted from a surface
• Spectral emissive power λE :
λEper unit area per unit wavelength,
= rate of emitted radiationmW/m2μ
• Total emissive power E:,E = Integration of λE over all values of λ 2W/m :
( ) ( )∫∞
=
0, λλλ dTETE
∫∞
=0
λλdEE
λ
λE
10.1 Fig.
8
• Irradiation: Radiation energy incident on a surface• Spectral irradiation λG :
λGper unit area per unit wavelength,
= rate of radiation energy incident upon a surfacemμW/m2−
• Total irradiationG:G = integration of λG over all values of λ :
( ) ( )∫∞
=
0 , λλλ dTGTG
• Radiosity: The sum of emitted and reflected radiation • Spectral radiosity λJ :
λJ = rate of radiation leaving a surface per unit area perunit wavelength, mμW/m2−
9
In the above definitions, summation in all directions isimplied although the term hemispherical is not used
• Total radiosity J:
( ) ( )∫∞
=
0 , λλλ dTJTJ
J = integration of λJ over all values of λ :
10
Characteristics of blackbody:(1) It absorbs all radiation incident upon it(2) It emits the maximum energy at a given temperature
and wavelength(3) Its emission is diffuse
Planck's Law λbE = spectral emissive power of a blackbody:
( )1)/exp(
,2
51
−=
−
TCCTEb λλλλ C1 and C2 are constants
Blackbody Radiation
Blackbody: An ideal radiation surface used as standard for describing radiation of real surfaces
11
Planck's Law
Blackbody Radiation
2879,6Tλmax =
Maximum emitted energy atspecific temperatures given byWien law:
Note - by qualitative judgment -energy emitted in visible range for 2000 K – tungsten wire ina light bulb.
Thermal radiation 0,1 to 100 μm
12
Stefan-Boltzmann LawBased on: • Experimental data by Stefan (1879)• Theoretical derivation by Boltzmann (1884)
4TEb σ=
bE = total blackbody emissive power (all wavelengths and all directions), [W/m2]
428- KW/m105.67 −×=σ is the Stefan-Boltzmannconstant
It can also be arrived at using Planck's law
Stefan-Boltzmann law
13
( ) ( )
4
0 2
51
0 bλb
Tσ
dλ1)T/λC(exp
λC
dxλ,TETE
=
∫ =−
=
=∫=
∞ −
∞
• Stefan-Boltzmann law gives the total radiation emitted froma black body at all wavelengths from λ=0 to λ=∞.
• Often an interest in radiation over some wavelength band –light bulb – how much is emitted in the visible range?
• We use a procedure to determine Eb,0-λ
∫=−
λ
0bλλb,0 T)dλ(E(T)E ,λ
14
Define a dimensionless quantity fλ(T):
4
λ0 b,λ
λ σT(T)dλE
(T)f ∫=
15
Want to know how energy is emitted in the visible range 0,40 to 0,76 μm.
λ1T=0,40.2500=1000 ⇒ fλ1 = 0,000321
λ2T=0, 76.2500=1900 ⇒ fλ2 = 0,053035
fλ2 - fλ1 = 0,0527
Only about 5% of radiation is emittedin the visible range. The remaining95% is in the infrared region in theform of heat.
Light bulb.
16
Radiation of Real Surfaces
Objective: Develop a methodology for determining radiation heat exchange between real surfaces.
• Surface radiation properties
• The graybody
• Kirchhoff's law
17
Absorptivity a, Reflectivity r, Transmissivity t
Gα
Gτ
GρG E
J
10.2 Fig.
rG
tG
Irradiation incident on a real surface can be absorbed, reflected and transmitted.
Remind: radiosity J (total radiation leavingthe surface) is a sum of emitted E and reflected rG radiation.
a = total absorptivity = fraction absorbedr = total reflectivity = fraction reflected t = total transmissivity = fraction transmitted