6.3 Radiation
6.3.1 General Principles
All matter emits energy as radiation in the form of moving electromagnetic waves.
In contrast with convection and conduction, which require a natural body, radiation
may also be transmitted in a vacuum. Radiation energy is transported by photons,
with properties like particles and waves (Oke 1992).
Photons move at the speed of light which in a vacuum is 3 Â 10
8 ms
−1 . The
ability to emit and absorb radiation is intrinsic to solids, liquids, and gases, associated with changes in the electromagnetic energy state of atoms and molecules.
These changes represent possible changes in the energy state of their electrons,
which are converted into radiation emission at specific or ranges of frequencies.
Molecular changes of the electromagnetic spectrum corresponding to vibration and
rotation of atoms resulting in different energy states, so that radiation can be emitted
or absorbed over a wide frequency range forming spectral bands (Monteith and
Unsworth 1991).
Photons are discrete quantum quantities proportional to radiation frequency m, or
the inverse of the wavelength 1/k. The proportionality constant h, known as
Planck’s constant, is 6.626 Â s 10
−34 Js is a universal property of matter. The
frequency and wavelength of the radiation vary with the state and properties of the
emitting body (solid, liquid, or gaseous), related by the equation
c ¼ k m
ð6:56Þ
where c is the velocity of light (3 Â 10
8 ms
−1 , in a vacuum). A quantum of energy
Q = h m (about 4.2 Â 10
–19 J at k = 0.5 lm) is very small, so that the radiative
energy is expressed in terms of multiples of Avogadro’s number (6 Â 10
23 ),
denoted Einstein or quantum mole.
Surfaces emit radiation proportional to the fourth power of their absolute temperature (Eq. 6.59). The amount and type of radiant energy emitted from a surface
per unit time and per unit surface area depending on the nature of the surface and its
temperature. At low temperatures, surfaces emit radiation in the infrared (IR) range,
and a surface at a higher temperature, such as an incandescent filament of a tungsten
lamp, emits radiation at lower wavelengths such as in the visible spectrum (0.38–
0.76 lm), as well as the near ultraviolet (UV).
Radiative flux emitted by a body is determined by its surface properties. The
radiation is emitted over a wide range of frequencies and wavelengths and the
magnitude of the flow, for a given wavelength is determined by the relative efficiency or e, the emissivity of the radiant surface (Lee 1978). Photons are emitted or
absorbed due to discrete energy transitions in the middle of emission or absorption,
and each transition produces photons at a discrete wavelength (Campbell and
Norman 1998). If there are an infinite number of transitions over the entire electromagnetic spectrum, then a black body will be a perfect emitting or absorbent of
180
6 Heat and Mass Transfer Processes
6.3.1 General Principles
All matter emits energy as radiation in the form of moving electromagnetic waves.
In contrast with convection and conduction, which require a natural body, radiation
may also be transmitted in a vacuum. Radiation energy is transported by photons,
with properties like particles and waves (Oke 1992).
Photons move at the speed of light which in a vacuum is 3 Â 10
8 ms
−1 . The
ability to emit and absorb radiation is intrinsic to solids, liquids, and gases, associated with changes in the electromagnetic energy state of atoms and molecules.
These changes represent possible changes in the energy state of their electrons,
which are converted into radiation emission at specific or ranges of frequencies.
Molecular changes of the electromagnetic spectrum corresponding to vibration and
rotation of atoms resulting in different energy states, so that radiation can be emitted
or absorbed over a wide frequency range forming spectral bands (Monteith and
Unsworth 1991).
Photons are discrete quantum quantities proportional to radiation frequency m, or
the inverse of the wavelength 1/k. The proportionality constant h, known as
Planck’s constant, is 6.626 Â s 10
−34 Js is a universal property of matter. The
frequency and wavelength of the radiation vary with the state and properties of the
emitting body (solid, liquid, or gaseous), related by the equation
c ¼ k m
ð6:56Þ
where c is the velocity of light (3 Â 10
8 ms
−1 , in a vacuum). A quantum of energy
Q = h m (about 4.2 Â 10
–19 J at k = 0.5 lm) is very small, so that the radiative
energy is expressed in terms of multiples of Avogadro’s number (6 Â 10
23 ),
denoted Einstein or quantum mole.
Surfaces emit radiation proportional to the fourth power of their absolute temperature (Eq. 6.59). The amount and type of radiant energy emitted from a surface
per unit time and per unit surface area depending on the nature of the surface and its
temperature. At low temperatures, surfaces emit radiation in the infrared (IR) range,
and a surface at a higher temperature, such as an incandescent filament of a tungsten
lamp, emits radiation at lower wavelengths such as in the visible spectrum (0.38–
0.76 lm), as well as the near ultraviolet (UV).
Radiative flux emitted by a body is determined by its surface properties. The
radiation is emitted over a wide range of frequencies and wavelengths and the
magnitude of the flow, for a given wavelength is determined by the relative efficiency or e, the emissivity of the radiant surface (Lee 1978). Photons are emitted or
absorbed due to discrete energy transitions in the middle of emission or absorption,
and each transition produces photons at a discrete wavelength (Campbell and
Norman 1998). If there are an infinite number of transitions over the entire electromagnetic spectrum, then a black body will be a perfect emitting or absorbent of
180
6 Heat and Mass Transfer Processes