Radiation | Heat & Mass Transfer | Gate

Radiation: Radiation is the third mode of heat transfer the heat transfer takes place due to electromagnetic waves (photons). the transfer of heat known as thermal radiation and it does not require medium to transfer heat. 

Irradiation: The total incident radiation on the surface from all direction per unit time and per unit of surface. it is denoted by ‘G;. 

Absorptivity: The fraction of total incident radiations that are absorbed by the material. denoted by α. 

Reflectivity: The fraction of total incident radiation that is reflected by the material. it is denoted by ‘ρ’.

Transmissivity: The fraction of total incident radiations that are transmitted by the material. it is denoted by ‘?’. 

Emissive power: The energy of thermal radiation emitted by per unit area per unit time from the surface of a body at a certain temperature is called emissive power. it is denoted by ‘E’.

Emissivity: This is the ability of any object to emit heat energy. the ratio of an emissive power of any body to the emissive power of a black body.

See also: This Laws And Theories Are Changed our Vision About Radiation

Shape Factor: The fraction of energy emitted by one surface and directly intercepted by the other surface. it depends on the geometry and orientation.

Radiosity: Total radiation leaving the surface per unit area, is known as radiosity. it is denoted by ‘J’.

Some Basic Formula

1. Energy emitted by surface        Q = σAT4
2. Black Bodies   α = 1,  ρ = 0,  ? = 0
3. Emissive power of black body(Stefan Boltzmann law)               E=σT4
4. For gas            α + ? = 1
5. Intensity of radiation (I)             Eb/ π
6. speed of radiation      wavelength x frequency

Shape Factor

1.  Reciprocity theorem       A1 F12 = A2F21
2. Enclosure theorem     F11+ F12+ F13+…….F1n = 1
3. Condition        F11= F22=0
4. Rule        F12= F13+F14 
5.        F21= A1/A2
6.        F12 = F21 =1
7. Hemispherical shape        F11= 0.5
8. Two concentric cylinders        F22= (1-d1/d2)
9. Equilateral triangle    F12=F13= 0.5,  F21=F23=0.5, F31=F32=0.5


Shape Factor 

this is the general introduction to radiation in next post we post MCQ numerical asked in previous years question. feel free to write your questions in the comment section below. And for similar posts follow us on Facebook

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