Laws of readiation
- 1. Prepared by: Chauhan vishal (12me243) Kapadiya nirav (12me242) Laws of radiation
- 2. Heat can be transferred in three basic mode Conduction Convection Radiation All the modes of heat transfer required the existence of a temperature difference .
- 3. - Conduction refers to the transfer of heat from one end to another end of the same substance or between the two substance having physical contact with them via molecular vibration Conduction
- 4. -Convection refers to the mode of heat transfer in which the heat is transfer by the moving fluid particles. Convection Force convection Natural convection
- 5. - Radiation is the transfer of heat through the space or matter by means other than conduction and convection . - Radiation is refers to the transfer of heat from the body or from the fluid for which no medium is required. Radiation
- 6. Heat transfer takes place by electromagnetic waves or by energy pockets (called photons) no medium is required for heat transfer. Vacuum is the best medium for the heat transfer by radiation.
- 7. Governing Laws For Radiant Heat Transfer Kirchhoff's law Stefan Boltzmann's law weins displacement law Planck's law
- 8. Example of heat transfer by radiation -Heat libration during nuclear explosions - Space application. - Heat leakage from furnaces. - Solar energy incident upon the earth. -Combustion chambers
- 9. In short……….. All the substance with body temperature above the absolute zero Kelvin level continuously emits energy in the form of radiation.
- 10. First we see some properties The rate of heat transfer by radiation from the given body depends upon 1 .temp of the body 2 .nature of surface of body 3 .frequency or wavelength of radiation Surface emission properties-
- 11. Total emissive power(E) At a given temp the total amount of heat emitted by a surface in all the direction over entire wavelength per unit area per unit time is called emissive power.
- 12. emissivity(€)- It is define as a ability of the surface to radiant heat. OR It is the ratio of total emissive power of any body to the emissive power of black body of equal temp. €=E/Eb
- 13. The emissivity may vary with variation in temp and wavelength. The value of emissivity for a diff substance ranging 0 to 1. For a white ,emissivity =0. For a black ,emissivity =1.
- 14. Monochromatic emissive power (E) The monochromatic emissive power define as the amount of radiant energy emitted by surface at a given temp per unit area per unit time and per unit wavelength. Radiosity: -it is define as a total energy leaving from the surface from all the direction per unit area per unit time, expressed in W/m^2.
- 15. Incident radiation(G) Reflected radiation(Gr) Material surface Absorbed radiation(Ga) Transmitted radiation(Gt) Absorptivity , reflectivity & transmissivity
- 16. When incident radiation(G) on a material surface ,part of that reflect back(Gr),some part is transmitted(Gt) and remaining amount of energy absorbed(Ga) by the material depending upon the characteristics of the body By conversion of energy principal G=Gr+Ga+Gt G/G=1=Gr/G+Ga/G+Gt /G 1=ƍ+α+Ṭ
- 17. α=Ga/G=fraction of incident radiation absorbed or absorbtivity. Ṭ=Gt/G=fraction of incident radiation transmitted or transmissivity ƍ=Gr/G=fraction of incident of radiation reflected or reflectivity
- 18. Stefan-Boltzmann´s law: The emissive power of black body is proportional to forth power of its absolute temperature. Eb = σ⋅T^4 where Eb = total power radiated per unit area from a blackbody (W/m2) σ = 5.669 ⋅ 10-8 W/(m2⋅K4). (Stefan-Boltzmann constant) T = absolute temperature (K)
- 19. black body is an object that absorbs all the radiant energy reaching its surface from all direction with all the wavelength. It is a perfect absorbing body . For black body:- α=1,ƍ=0,Ṭ=0 Black body if all the incident radiation falling on the body are reflected it is called the white body. For a white body:- α=0,ƍ=1,Ṭ=0 White body when all irradiation is transmitted threw the body it is called….. α=0,ƍ=0,Ṭ=1 Transparent body The concept of different bodies
- 20. PLANK’S LAW Plank’s law describes the electromagnetic radiation emitted by a blackbody in thermal equilibrium at a definite temperature. The law is named after the max plank. Plank radiation is the greatest amount of radiation that any body at thermal equilibrium can emit from it’s surface whatever its chemical composition or surface structure
- 21. 1 12 )( /5 2 kThc e hc I where h=6.62x10^-34 Js is the Planck’s constant. =Wavelength c=velocity of light in vacuum=3x10^8 m/sec K= Boltzmann constant=1.3805x10^-23 j/k T=Absolute temperature The intensity (I) of electromagnetic radiation at a given wavelength () is a complicated function of the wavelength and the temperature (T).
- 23. For the given value of ,the (E)b increases with increases in absolute temp The peak (E)b shifts towards a smaller wavelength at a higher temp. it means at higher temp the energy is emitted in form of smaller wavelength for the maximum value of monochromatic emissive power for the T<=800k.the energy emitted in the infrared region and thus the radiation is not visible to eye. The area under the curve gives the rate of radiant energy emitted within the wavelength interval d.