Solutions Manual For Thermal Radiation Heat Transfer 6th Edition By John R. Howell , M. Pinar Mengüç

NATIONAL INSTITUTE OF TECHNOLOGY, WARANGAL
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DEPARTMENT OF MECHANICAL ENGINEERING
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ME 351 - HEAT AND MASS TRANSFER
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PART 11 – RADIATION HEAT TRANSFER
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3rd Year, 2nd Sem, B.Tech. – Section B
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Dr. CHANDRAMOHAN V.P.
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Assistant Professor b



Dept. of Mechanical Engg., NIT Warangal
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, RADIATION HEAT TRANSFER - Introduction b b b b




❖ 3rd mechanism of heat transfer: radiation, which is characteristically different from
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the other two.
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❖ Radiation is emitted by every point on a plane surface in all directions into the hemisph
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ere above the surface.
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❖ The quantity that describes the magnitude of radiation emitted or incident in a specifie
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d direction in space is the radiation intensity.
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❖ Various radiation fluxes such as emissive power, irradiation, and radiosity are expresse
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d in terms of intensity.
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❖ The other radiative properties of materials are emissivity, absorptivity, reflectivity, and
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transmissivity and their dependence parameters of wavelength, direction, and temper
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ature.




ChandramohanbV.P./AsstbProf./MED/NITbWarangal

,❑ Consider a hot object that is suspended in an evacuated cha
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mber whose walls are at room temperature (Fig.).
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❑ The hot object will eventually cool down and reach thermal e
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quilibrium with its surrounding wall.
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❑ Heat transfer between the object and the chamber could not
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have taken place by conduction or convection, because thes
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e two mechanisms cannot occur in a vacuum.
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❑ Therefore, heat transfer must have occurred through anothe
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r mechanism that involves the emission of the internal energ
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y of the object. This mechanism is radiation.
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❑ Radiation is a volumetric phenomenon.
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ChandramohanbV.P./AsstbProf./MED/NITbWarangal

, ➢ Energy transfer by radiation is fastest (at the speed of light) and it suffers no atte
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nuation in a vacuum. b b b



➢ Also, radiation transfer occurs in solids as well as liquids and gases.
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➢ In most practical applications, all three modes of heat transfer occur concurrently at var
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ying degrees. b



➢ But heat transfer through an evacuated space can occur only by radiation.
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➢ For example, the energy of the sun reaches the earth by radiation.
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It is interesting that radiation heat transfer can o
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ccur between two bodies separated by a mediu
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m colder than both bodies
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➢ Physicist James Clerk Maxwell (1864), who postulated that accelerated charges or
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changing electric currents give rise to electric and magnetic fields.
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➢ These rapidly moving fields are called electromagnetic waves or electromagnetic radia
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tion, and they represent the energy emitted by matter as a result of the changes in the
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electronic configurations of the atoms or molecules. b b b b b b




➢ Electromagnetic wave depends only on the source and is of
independent the medium through which the
Chandramohan waveProf./MED/NIT
V.P./Asst travels. Warangal b b b b b b b b b b