AHMEDABAD INSTITUTE OF TECHNOLOGY

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

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

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).      

     

ABSORPTIVITY, REFLECTIVITY AND TRANSMISSIVITY      

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.  

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.  

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.

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

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,    

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    

RADIATION HEAT EXCHANGE BETWEEN TWO BLACK BODIES

         

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.  

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,