chapter 4.1-4.2 radiation
DESCRIPTION
tutoTRANSCRIPT
Ch. 4
Radiation
4.1 Introduction to Radiation Heat Transfer
OBJECTIVES
understand radiation and it’s
terminology
describe the mechanism of radiant
heat transfer
list applications of radiation
describe radiation properties
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.
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.
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.
The electromagnetic wave spectrum
• 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.
•All bodies at a temperature above absolute zero emit thermal radiation.
• 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
•RHT can occur between 2
bodies separated by a medium colder
than both bodies
Examples..
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.
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
• A BB absorbs all incident radiation regardless directions and wavelength
• A BB emits radiation energy uniformly per
surface area
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
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
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
- For non black body (gray body or real
surface) the emissivity, < 1.0
- The emissive power is reduced by
emissivity ().
4TAq
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.
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
• 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.
• 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.