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RADIATIVE HEAT TRANSFER

Prabal Talukdar

Department of Mechanical Engineering

IIT Delhi

E-mail: prabal@mech.iitd.ac.in

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Introduction

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Thermal Radiation Radiation heat transfer can take lace in a vacuum. It

does not need a medium unlike conduction/convection Thermal radiation is the stream of electromagnetic

ra a on em e y a ma er a en y on accoun o s

finite absolute temperature

Infrared radiation from a common household radiator or

electric heater is an example of thermal radiation, as is

the light emitted by a glowing incandescent light bulb.

erma ra a on s genera e w en ea rom emovement of electrons within atoms is converted to

electromagnetic radiation

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Dominant in high temperature applications

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Spectrum of Electro-magnetic

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Thermal radiation falls in the range of 10-1-102 m of the Electro-magnetic spectrum.

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Emission Process

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Volumetric Phenomenon Surface phenomenon

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Emission b a surface

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Gray Diffuse

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Solid an le

Plane Angle Solid Angle

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Solid an le = 2 = 2 2 =

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Solid An le for a Hemisphere2/ 2/2

srs ns nw0 0h 0

===

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Spectral Intensit I

,e(,,)=dq/(dA1cos .d.d) I

,e

is the rate at which radiant energy is emitted at the wave length

in the (, ) direction, per unit area of emitting surface normal to thisdirection, per unit solid angle about this direction and per unit

wavelength interval d about .

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Heat Flux dq=dq/drate at which radiation of wavelength leaves dA1 and

passes through dAn (unit: W/m)

dq= I,e(,, ) dA1cos d 1

= ddsincos),,(Idq e,"

Spectral heat flux associated with emission into hypothetical

hemisphere above dA1 is

=2/2

" ddsincos,,I

Total heat flux associated with emissions in all directions and

at all wavelengths is then

0

,

0

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=0

"" d)(qq

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Emissive Power

emitted per unit surface area

,

E()(W/m2.m)=

2/

e,

2

ddsincos),,(I

Ebased on actual surface area

I based on projected surface area

00

,

Total hemispherical Emissive power:

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=

d)(EE

0

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Relation between Emissive Power

2/2

E= 0 e,0 ddsincos),,(I

For a diffuse surface, I(,,)= I

()

2/2

E=

00

e, ddsincos)(I

,e

=

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