Radiation heat transfer is the transfer of heat from one body to another in the form of electromagnetic waves. Most of these waves lies in the infrared region. Unlike Conduction and Convection, Radiation heat transfer does not require any medium. All objects above absolute zero temperature emit radiation energy.
Radiation Heat Transfer is more efficient in vacuum because any media between heat transfer bodies can absorb electromagnetic waves.
For example, transfer of heat from sun to earth takes place due to electromagnetic waves through vacuum space. But as soon the sun radiation enter earth its radiation starts reducing.
All objects (solid, liquid or gas) can emit electromagnetic radiation because of thermal agitation inside their molecules. Rate of heat flux emitted by a body depends on its temperature and type of surface (color/finish etc.)
Radiation heat Transfer Calculation (Stefan-Boltzmann Law)
Stefan-Boltzmann Law of thermal radiation states that: The rate of radiation heat transfer per unit area in black body is directly proportional to the fourth power of the body temperature.
T = absolute temperature in Kelvin
A = area of the emitting body in square meter
ε = emissivity coefficient of the body
What is Black Body?
A Black body absorbs all electromagnetic radiation in all frequency ranges falling on it. And in thermal equilibrium black body emits all the radiations. Therefore black body is a perfect radiator.
But in real world black body does not exists. Real bodies emit radiations at lower rate. To take this into account emissivity (ε) is used. Emissivity value for black body is “one”.
What is Emissivity?
Mathematically emissivity is the ratio of the emission from a real surface to the emission from perfect black surface at the same temperature and wavelength. It’s value varies from 0 (perfect reflector) to 1 (perfect emitter).
Value of emissivity for a material depends on it’s material and surface properties. For example, reflective surfaces have low emissivity whereas rough and oxidized material surface have high emissivity.
|Material||Temperature||Emissivity (For Refrence Only)|
|Ceramic Reflectory Black||93||0.93|
|Ceramic Reflectory White||93||0.90|
Radiation Heat Transfer Calculator
To sum up, Convective heat transfer occurs in liquid or air because of movement of molecules from a hotter place to colder place. Read this article to know various modes of heat transfer.
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