1. AHMEDABAD INSTITUTE OF TECHNOLOGY

2. TOPIC :- LAW OF RADIATION
SUB:- HEAT TRANFER
TOPIC :- LAW OF RADIATION

3. HEAT TRANSFERRED BY RADIATION
Radiation heat transfer is defined as the transfer of heat across a system boundary by means of electromagnetic waves which is caused by a temperature difference.
Examples
Heat dissipation from the filament of a vacuum tube
Heat leakage through the evacuated walls of thermos flask
Heat leakage from furnaces, combustion chambers
Solar energy incident upon the earth
Heat liberation during nuclear explosions
Space application

4. SURFACE EMISSION PROPERTIES
i) Total emissive power (E)
At a given temperature, the total amount of heat emitted by a surface in all the directions over entire wavelength per unit area, per unit time is called the emission power (E).

6. ABSORPTIVITY, REFLECTIVITY AND TRANSMISSIVITY

7. CONCEPT OF DIFFERENT BODIES
Black Body:
A black body is an object that absorbs all the radiant energy reaching its surface from all the directions with all wavelengths.
White Body:
If all the incident radiation falling on the body are reflected, it is called a white body.
Gray Body:
A gray body is defined as a body whose absorptivity of a surface does not vary with variation in temperature and wavelength of the incident radiation.

8. Opaque body:
When no irradiation is transmitted through the body, it is called opaque body.
Transparent body:
When all the irradiation is transmitted through the body, it is called transparent body.

9. REGULAR AND DIFFUSE REFLECTION
Regular Reflection:
If the angle between the reflected beam and normal to the surface equals the angle made by the irradiation with the same normal then it is called regular or specular reflection
Diffused Reflection:
In this case the incident radiation beam is reflected in all the directions.

10. LAWS OF RADIATION Stefan Boltzmann law
The emissive power of black body is proportional to fourth power of its absolute temperature. or

11. Kirchhoff’s law
This law states that the ratio of total emissive power to absorptivity is constant for all substances which are in thermal equilibrium with the surroundings.
Planck’s law
The spectral distribution of the radiation intensity of a black body is given by,

12. Wien’s Displacement Law
Wien established a relationship between the wavelength at which the maximum value of monochromatic emissive power occurs and absolute temperature of black body.
It is maxima of the Planck’s law

13. RADIATION HEAT EXCHANGE BETWEEN TWO BLACK BODIES

15. SHAPE FACTOR
The shape factor may be defined as the fraction of radiative energy that is diffused from one surface element and strikes the other surface directly with no intervening reflections.

16. HEAT EXCHANGE BETWEEN TWO INFINITE PARALLEL PLANES OF NON BLACK BODY
Net flow of heat from surface 1 to surface 2 per unit time is given by,