Heat transfer by radiation usually takes place simultaneously with heat transfer by convection and conduction. The heat transfer by radiation is of much more importance at high-temperature levels as compared to the other two mechanisms.
Direct-fired kettles, electric heaters, steam boilers, rotary kilns, etc. are examples of chemical process equipment where radiation is a major energy transfer mechanism.
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What is Radiation?
It refers to the transport of energy through space by electromagnetic waves. It depends upon electromagnetic waves as a means for the transfer of energy from a source to a receiver.
Radiation energy is of the same nature as ordinary visible light. It travels in a straight line and it may be reflected from a surface. The electromagnetic waves with wavelengths ranging from 0.50 to 50 μm (microns, 1 μm = 10-6m) are of importance to radiant heat transfer.Â
Radiation of a single wavelength is called monochromatic. Thermal radiation is the energy emitted by a body due to its temperature and we restrict our discussion to this type of radiation.
5 examples of heat transfer by radiation
(i) Transfer of heat from the sun to the earth.
(ii) Heat loss from an unlagged steam pipe.
(iv) Use of energy from the sun in solar heaters.
(v) Heating of a cold room by a radiant electric heater.
Unlike conduction and convection, radiation heat transfer does not require an intervening medium (solid or liquid) and heat can be transmitted by radiation mode across an absolute vacuum.
Radiation is the only significant mode of energy/heat transfer when no medium is present (e.g. the heat leakage through the evacuated walls of a thermos flask)
A body having emissivity 'e' at temperature T1 emits the radiant energy equal to eσT1^4 per unit area. If the surroundings are black, none of this radiation will be reflected by them and if the surroundings are temperature T2, they will emit radiation equal to σT2^4. If a body is grey, it will absorb a fraction 'e' of this energy, so that the net rate of radiant energy flow from the grey body to the black surrounding is given by the expression.
Q/A = e.σ(T1^4 - T2^4)
Wher, e = the emissivity of grey body
T1 = absolute temperature of grey body
T2 = absolute temperature of surrounding of grey body.
Heat transfer coefficient for radiation
The net heat transfer by radiation from a unit surface area of a grey body at temperature T1 to the black surroundings at temperature T2 may be expressed as
Qr = hr(T1 - T2)
hr = Qr/(T1 - T2)
hr = e.σ/(T1 - T2)*(T1^4 - T2^4)
Where hr is a radiation heat transfer coefficient.Â
Absorptivity, Reflectivity, and Transmissivity
Any substance receives and gives off/emits energy in the form of electromagnetic waves. When energy emitted by a heated body falls on a second body, it will be partly absorbed, partly reflected, and partly transmitted. It is only the absorbed energy that appears as heat in the body and this transformation is quantitative.
The proportions of the incident energy that is absorbed, reflected, and transmitted depend mainly on the characteristics of a receiver and the temperature of the receiver.
AbsorptivityÂ
The fraction of the incident radiation on a body that is absorbed by the body is known as Absorptivity and may be denoted by the letter ' a.
ReflectivityÂ
The fraction of the incident radiation on a body that is reflected by the body is known as the reflectivity and may be donated by the letter 'r'.
TransmissivityÂ
The fraction of the incident radiation on a body that is transmitted through the body is known as Transmissivity and may be denoted by the letter 'τ'.
Energy balance in radiationÂ
The energy balance about a body on which the total incident energy falling is unity is given as:
a+r+Ï„ = 1.0
A majority of engineering materials are opaque and in such cases Equation is simplified to:
a+r = 1.0...... for an opaque material/surface
If Ï„ = 1, a = r = 0 all incident energy passes through the body, and it is called perfectly transparent, e.g. rock salt (NaCl), quartz, and fluorite.
If r = 1, a=Ï„ =0, all the incident energy is supposed to be entirely reflected by the body and is specular. If the surface gives a diffused reflection, the body is called a white body, e.g. a piece of white chalk (white body).
(i) r = 0 represents a nonreflecting surface.
(ii) a = 1 represents a perfectly absorbing surface or a black surface.
(iii) Ï„ = 1 represents a perfectly transparent surface.
(iv) r = 1 represents a perfect reflector.Â
Also Read: Kirchhoff's law engineering
Black Body Radiation Class 11 Chemistry
Plate Type Heat Exchanger Working
Take these Notes is, Orginal Sources:Â Unit Operations-II, KA Gavhane