lect - 15 radiation basics.pptx

8/17/2019 Lect - 15 Radiation basics.pptx

1/16

Thermal RadiationDr. Senthilmurugan S. Department of Chemical Engineering IIT Guwahati - CL204 - Part !

Black Body and Gray Body Radiation Laws


2/16

5/12/16 | Slide 2

Content

Thermal Radiation Oeriew

Basic conce!ts o" thermal radiation

Basic de"inition

Laws o" thermal radiation


3/16

5/12/16 | Slide #

Thermal Radiation

"a#iation is ener$y that comes "rom a so%rce and traels thro%$h some material or

thro%$h s!ace& Li$ht' heat and so%nd are ty!es o" ra#iation&

Radiation that res%lt o" tem!erat%re in the "orm o" electroma$netic waes only called

(thermal radiation” &

)ll s%*stance/s%r"ace at tem!erat%re a*oe a*sol%te +ero (emit ra#iation$ that

inde!endent o" e,ternal so%rces

-ntrod%ction

Electromagnetic spectrum and thermal radiation range


4/16

5/12/16 | Slide .

Thermal radiationBasic conce!ts 1/#0

The radiation emitted *y any $ien s%r"ace / mass o" material is in#epen#ento" that *ein$ emitted *y other in si$ht o"' or in contact with mass / s%r"aceThermal Radiation does not re%ire any medi%m "or !ro!a$ationContin%es s!ectr%m o" all wae len$th !ro!a$ates the radiationoweer most o" the thermal radiation ind%ced *y electroma$netic waes

ran$in$ "rom 34&1 to 314 µm wae len$th

ner$y leel aries with s%r"ace tem!erat%re


5/16

5/12/16 | Slide 5

Thermal radiation

hen *odies at di""erent tem!erat%res are !laced in si$ht o" one another inside an

enclos%re

-n addition to emittin$ radiation' the s%r"ace o" a *ody has the ca!acity "or

a*sor*in$ all/!art o" the radiation emitted *y s%rro%ndin$ s%r"aces and "allin$ on

it

The hotter *odies lose ener$y *y emission o" radiation "aster than they receieener$y *y a*sor!tion o" radiation "rom the cooler *odies and the tem!erat%res

o" the hotter *odies decrease&

Sim%ltaneo%sly the cooler *odies a*sor* ener$y "rom the hotter ones "aster

than they emit ener$y' and the tem!erat%res o" the cooler *odies increase& The

!rocess reaches e%ili*ri%m when all the *odies reach the same tem!erat%re'

Basic conce!ts 2/#0


6/16

5/12/16 | Slide 6

Thermal radiation

7irectional nat%re o" thermal radiation8 a s%r"ace emits radiation in all directions

encom!assed *y a hemis!here&

Thermal radiation "allin$ on a *ody e,!ected to re"lected *y the *ody' a*sor*ed *y

the *ody and transmitted thro%$h the *ody *ased on its nat%re&

Basic conce!ts #/#0


7/16

5/12/16 | Slide 9

Basic 7e"inition

Irra#iation %0

Rate at which radiation is incident on a

s%r"ace "rom all directions !er %nit area

w/m20

"eflecti&it' % ρ0

The "raction o" the radiation "allin$ on a

*ody that is re"lected is called the re"lectiity : re"lected "l%,

()*orpti&it' %α0

The "raction that is a*sor*ed is called the

a*sor!tiity

: a*sor*ed "l%,

α : 1 "or *lack *ody Tran*mi**i&it' %τ0

The "raction that is transmitted is called the

transmissiity

τ:4 "or o!a%e *ody e&$& metallic0

The s%m o" these "ractions m%st *e %nity

Thermal Radiation

+or monochromatic irra#iation

"a#io*it'

Rate at which radiation leaes a s%r"ace d%e to

emission and re"lection in all directions !er %nit

area o" the s%r"ace&


8/16

5/12/16 | Slide ;

)*sor!tion o" Radiation *y O!a%e Solids

hen radiation "alls on a solid *ody' a de"inite "raction ρ

may *e re"lected and the

remainin$ "raction 1 < ρ enters the solid to *e either transmitted or a*sor*ed

=ost solids other than $lasses' certain !lastics' %art+' and some minerals0 a*sor*

radiation o" all waelen$ths

,ce!t in thin sheets' all others the transmissiity τ is +ero

)ll nonre"lected radiation is com!letely a*sor*ed in a thin s%r"ace layer o" the solid The a*sor!tion o" radiation *y an o!a%e solid is there"ore a s%r"ace !henomenon'

not a ol%me !henomenon' and the interior o" the solid is not o" interest in the

a*sor!tion o" radiation&

The heat $enerated *y the a*sor!tion can "low into or thro%$h the mass o" an

o!a%e solid only *y cond%ction


9/16

5/12/16 | Slide >

Basic 7e"inition

,lac ,o#'

-t is an ideali+ed !hysical *ody that a*sor*s all

incident electroma$netic radiation

The ma,im%m !ossi*le a*sor!tiity is %nity'

attained only i" the *ody a*sor*s all radiation

incident %!on it and re"lects or transmits none

-n !ractice no material has *een "o%nd to a*sor* all

incomin$ radiation

) *lack *ody also' *lack*ody0 is an ideali+ed

!hysical *ody that a*sor*s all incident

electroma$netic radiation' re$ardless o" "re%ency

or an$le o" incidence&

) *lack *ody in thermal e%ili*ri%m that is' at a

constant tem!erat%re0 emits electroma$netic

radiation called *lack *ody radiation&

Black Body radiation is emitted accordin$ to

?lanck@s law

) white )o#' is one with a Aro%$h s%r"ace that

re"lects all incident rays com!letely and %ni"ormly in

all directions

Black Body

)n a!!ro,imate reali+ation o" a *lack *ody as

a tiny hole in an ins%lated enclos%re&


10/16

5/12/16 | Slide 14

Basic 7e"inition

onochromatic emi**i&e/ra#iating

power

The radiation emitted "rom %nit area

o" the *ody in %nit time diided *y

the waelen$th is called the

monochromatic radiatin$ !ower λ The ener$y emitted is A*roadcastA in

all directions thro%$h any

hemis!here centered on the

radiation area

Total emi*pherical Emi**i&e power

The total radiatin$ !ower is the total

radiation o" all waelen$ths emitted

*y %nit area in %nit time in all

directions thro%$h a hemis!here

centered on the area

Thermal Radiation < missie !ower

ae Len$th λ

R a d i a

t i o n i n t e n s i t y 6

λ

Entire area under any of the curves

)t constant s%r"ace tem!erat%re T


11/16

5/12/16 | Slide 11

Basic 7e"inition

=ethod o" de"inin$ emissie !ower o"

any *ody with res!ect to *lock *ody

emissie !ower

=onochromatic missiity ελ0 8 Theratio o" the monochromatic emissie

!ower λ to that o" a *lack*ody * ' λ at the same waelen$th

missiity ε08 The ratio o" the totalemissie !ower o" a *ody to that o" a

*lack*ody *

-" the monochromatic emissiity o" a

*ody is the same "or all waelen$ths'

the *ody is called a $ray *ody&

Thermal Radiation < missiity


12/16

5/12/16 | Slide 12

Basic 7e"inition

missiity %s%ally increases with

tem!erat%re

missiities o" !olished metals are low'

in the ran$e 4&4# to 4&4;&

=ost o,idi+ed metals ran$e "rom 4&6 to

4&;5 Donmetals s%ch as re"ractories' !a!er'

*oards' and *%ildin$ materials' "rom

4&65 to 4&>5

?aints' other than al%min%m !aint' "rom

4&;4 to 4&>6&

ater at #44 E 4&>5

Thermal Radiation < missiity


13/16

5/12/16 | Slide 1#

Laws o" Black Body Radiation

The distri*%tion o" ener$y in the

s!ectr%m o" a *lack*ody is de"ined *y

!lankFs law

: monochromatic emissie !ower o"

*lack*ody h : ?lanck@s constant

c : s!eed o" li$ht

λ : waelen$th o" radiation

k : Bolt+mann@s constant

T : a*sol%te tem!erat%re

:4&5>6 , 14


14/16

5/12/16 | Slide 1.


)t a !artic%lar tem!erat%re' increase

with λ $oes thro%$h a ma,im%m andthen decreases asym!totically to +ero

)t !artic%lar λ' -ncrease withtem!erat%re

The ma,im%m al%e o" occ%rs at asmaller wae len$th as tem!erat%re

increases

To $et ma, al%e o"

ienFs Law

ienFs Law

ae Len$th λ

R a d i a t i o n i n t e n s i t y 6

λ


15/16

5/12/16 | Slide 15


Ste"an


16/16

lect - 15 radiation basics.pptx

Documents