Chapter 4.1-4.2 Radiation
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Transcript of Chapter 4.1-4.2 Radiation
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Ch. 4
Radiation
4.1 Introduction to Radiation Heat Transfer
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OBJECTIVES
understand radiation and it’s
terminology
describe the mechanism of radiant
heat transfer
list applications of radiation
describe radiation properties
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What is radiation?
•The energy emitted by matter in the form of
electromagnetic waves (or photons) as a result of the changes in the
electronic configurations of the atoms or molecules.
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Therefore, HT must occurthrough another mechanism
that involves the emission of the internal energy of the
object
NO HT by conduction or
convection because these mechanism cannot occur in
vacuum.
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Thermal radiation
• The form of radiation emitted by bodies because of their temperature.
• It differs from other forms of
electromagnetic radiation such as x-rays, gamma rays, microwaves, radio waves and television waves that are
not related to temperature.
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The electromagnetic wave spectrum
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• The type of electromagnetic radiation that is applicable to HT is the thermal
radiation emitted as a result of energy transitions of molecules, atoms, and electrons of a substance.
• Temperature is a measure of the strength
of these activities at microscopic level.
• High temp, high thermal radiation emissions.
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•All bodies at a temperature above absolute zero emit thermal radiation.
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• Energy transfer by radiation is fastest (at the speed of light)
• It occurs in solids, liquids and gases-emit, absorb or transmit radiation to varying degrees.
Eg: the solar radiation reach the surface of the earth after passing through a cold air layers
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•RHT can occur between 2
bodies separated by a medium colder
than both bodies
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Examples..
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4.2 Blackbody Radiation
• A body at temp. above 0 emits radiation in all directions over a wide range of wave length
• The amount of radiation energy emitted from a surface at given wavelength depends on:
– Material of the body
– The condition of its surface
– Surface temp
• Therefore, different materials emit diff. amount of radiation even at same temp.
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Blackbody
• Blackbody --- an idealized body- to
serve as a standard against which the
radiation properties of real surfaces may be compared.
• Thus, a blackbody is a perfect emitter
and absorber of radiation
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• A BB absorbs all incident radiation regardless directions and wavelength
• A BB emits radiation energy uniformly per
surface area
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4)( TTEb
428 ./1067.5 KmW Stefan- Boltzmann
Constant
The radiation energy emitted “Emissive Power”
(W/m2)
T is the absolute temperature of the surface in K
Stefan- Boltzmann Law
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Emissivity (ε)
• The emissivity of a surface represents the ratio of the radiation emitted by a surface to the radiation emitted by a BB at the same temp
• Denoted by ε
• 0 < ε < 1
• Measures of how closely a surface to a BB
(ε = 1)
• For a real surface or gray body
= E/Eb and ε < 1.0
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Radiation from Black Body
• Heat transfer by radiation from a perfect black body with ε = 1.0 is:
Where:
q = heat flow in (W)
A = surface area, (m2)( ft2 )
σ = constant 5.676 x 10 -8 W/m2K4 or 0.1714 x 10 -8 Btu/hr.ft2.°R4
T = temperature of black body (K) or (R)
4TAq
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- For non black body (gray body or real
surface) the emissivity, < 1.0
- The emissive power is reduced by
emissivity ().
4TAq
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Absorptivity, Reflectivity and
Transmissivity
• Absorptivity (α)
• 0 < α < 1
• Reflectivity (ρ)
• 0 < ρ < 1
• Transmissivity (τ)
• 0 < τ < 1
• α + ρ + τ = 1
• Both and α of a surface depend on the temperature and the wavelength of the radiation.
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Radiation heat transfer between a surface and the surfaces surrounding it.
•When a surface of emissivity and surface area As at an absolute temperature Ts is completely enclosed by a much larger (or black) surface at absolute temperature Tsurr
separated by a gas (such as air) that does not intervene with radiation, the net rate of radiation heat transfer between these two surfaces is
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• The total heat transfer rate to or from a surface by convection and radiation is expressed as
• Note that the combined heat transfer coefficient is essentially a convection heat transfer coefficient modified to include the effects of radiation.
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• Radiation is usually significant relative to conduction or natural convection but
negligible relative to forced convection.
• Thus radiation in forced convection is usually disregard, especially when the surfaces involved have low emissivities and
low to moderate temperatures.