What Is Radiation Heat Transfer

Radiation heat transfer is basically the energy transfer via electromagnetic waves. Before going into details about radiation heat transfer, it is essential to understand the radiations first.

What Are Radiations?

Radiations are defined as the electromagnetic waves having wavelength of 0.1 to 100 microns, which doesn’t require any medium to travel. Radiation waves are classified into two types; ionizing radiations and non ionizing radiations. Ionizing radiations have the tendency (because of having sufficient energy) to ionize an atom. Non-ionizing radiations cannot ionize an atom (heat waves, radio waves and light waves are the examples of non-ionizing radiations).  Hence, in physics on nuclear engineering, we deal with the ionizing radiations, while here; non-ionizing radiations are of our main interest.

What Is Radiation Heat Transfer?

According to the quantum theory, radiations consist of energy packets (named as photons), that has no rest mass and move at the velocity of light. So, radiation heat transfer basically deals with the exchange or transfer of that energy between the bodies.  Every object, having temperature greater then absolute zero (0 K) emits radiations. Mostly, the solids are considered as the radiation emitters, because the energy emitted by the fluid particles is usually absorbed by the nearby molecules, and thus this energy cannot reach the surface. These emissions are directly proportional to the temperature of the body; the higher the temperature, higher will be the radiations emissions. 

Hence:

Ever object, above absolute zero temperature emits energy carrying electromagnetic radiations. When these radiations fall on the other object, some energy is transferred from the radiation waves to the object. This transferred energy is known as the radiation heat transfer. 

Emissivity:

Emissivity is the tendency of an object to release electromagnetic radiations per unit area and per unit time. In order to calculate the radiation emissive power, we assume an ideal surface, which can absorb and radiate all wavelength radiations. This ideal surface is named as the black body, or the ideal radiator. However, the heat flux of the real surface is less than that of the black body. According to Stefan Boltzmann’s law:

E = ԑσ T_s⁴

Where:

E             =             Emissive power of real surface;  (W/m²)

ԑ             =             Radiative property of the surface.

                σ             =             Stefan Boltzmann constant;          (5.67 x 10^-8 W/m².K⁴)

T_s               =             Absolute temperature;                  (K)

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