Radiation heat transfer

Radiation-

The radiation is the emission of energy as electromagnetic waves. It carries heat energy in the form of packets of energy.
These packets are called photons.  Each photon is a discrete entity of electromagnetic radiation

Blackbody-

A blackbody is a hypothetical/idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence.

Emissivity 

The emissivity of a surface is defined as the ratio of the radiation emitted by the surface to the radiation emitted by a blackbody at the same temperature. Thus, 

0 ≤ ε ≤ 1

The total emissivity of a surface is the average emissivity of a surface over all direction and wavelengths:

Total Emissivity-

It is defined as the ratio between the total hemispherical power of the non-black body and the total emissive power of the black body at the same temperature.

Type of surfaces

• White body

The surface that reflects all the incident rays completely and uniformly in all directions.

• Black body

The body which absorbs all the incident radiation on it.

• Gray body-

 it is a surface which its properties are independent of wavelength. 

• Opaque/Diffuse body-

it is a surface which its properties are independent of direction.

Absorptivity α: it is the fraction of irradiation absorbed by the surface. 

Reflectivity ρ: it is the fraction of irradiation reflected by the surface. 

Transmissivity τ: it is the fraction of irradiation transmitted through the surface. 

α+ρ+τ=1

Surface roughness decreases= Reflectivity decreases

Planks law of thermal radiation-

This law states that monochromatic emissive power of a body depends on both temperature and Web length of radiation.

Wins displacement law-

After a long experiment, Wien found that the radiative energy dw per wavelength interval dλ has a maximum at a certain wavelength λm and that the maximum shifts to shorter wavelengths as the temperature T is increased

λmT= constant= 2898 micrometer kelvin

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Stefan Boltzmann law