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Radiation

Danar - PTM UNS 1

ea rans er

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Conduction, Convection, & Radiation

Danar - PTM UNS 2

Convection: The transfer of heat by the actual motion of a fluid(liquid or gas) in the form of currents Conduction: The transfer of heat by direct contact of particles

of matter

Radiation: Heat transfer by electromagnetic waves

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Conduction, Convection, & Radiation Conduction

Convection Radition

Danar - PTM UNS 3

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Tiga cara heat transfer:Tiga cara heat transfer: konduksi, konveksi, radiasikonduksi, konveksi, radiasiKita telah mempelajari konduksi dan konveksiKita telah mempelajari konduksi dan konveksi

Thermal radiation Electromagnetic radiation which

Radiation: Heat transferred byelectromagnetic radiation

Danar - PTM UNS 4

temperature differenceExamples: Sun, boiler

Black body An ideal thermal radiator emitting energy

at a rate proportional to the fourth powerof the absolute temperature of the bodyand directly proportional to its surface

4emitted T A =

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Radiasi tidak memerlukan sesuatu di antara

dua benda untuk memindahkan energi!

Humongoid Ball of Fire(a star, our sun)

Danar - PTM UNS 5

Note: Not to Scale

Puny ball of mostly water(Earth)

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PerpindahanPerpindahan PanasPanas RadiasiRadiasi

RadiasiRadiasi aadalahdalah prosesproses transporttransport panaspanas daridari bendabendabersuhubersuhu tinggitinggi keke bendabenda yangyang bersuhubersuhu lebihlebih rendahrendah, , bilabilabendabenda bendabenda ituitu terpisahterpisah didi dalamdalam ruangruang ((bahkanbahkandalamdalam ruangruang hampahampa sekalipunsekalipun) )

q =q = A (TA (T 11 44 TT22 44 )) Dimana :Dimana : = Konstanta Stefan= Konstanta Stefan- -BoltzmanBoltzman

5,669 x105,669 x10 --88 w/mw/m 22 kk44A = Luas penampangA = Luas penampangT = TemperaturT = Temperatur

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ContohContoh: :DuaDua platplat hitamhitam taktak berhinggaberhingga yangyang suhunyasuhunyamasingmasing masingmasing 800800 00 CC dandan 300300 00 CC salingsalingbertukarbertukar kalorkalor melaluimelalui radiasiradiasi. . HitunglahHitunglahperpindahanperpindahan kalorkalor persatuanpersatuan luasluas. .

Perpindahan Panas RadiasiPerpindahan Panas Radiasi

PenyelesaianPenyelesaianDari persamaan:Dari persamaan:

q =q = A (T1A (T144

T2T244

))q/A =q/A = (T1(T1 44 T2T2 44 ))q/A = (5,669 x 10q/A = (5,669 x 10- -8)(10738)(1073 44 573573 44 ))q/A = 69,03 kW/mq/A = 69,03 kW/m 22

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RADIASI TERMALRADIASI TERMALJikaJika suatusuatu bendabenda ditempatkanditempatkan dalamdalam

pengurungpengurung, , dandan suhusuhu pengurungpengurung lebihlebihrendahrendah daridari padapada suhusuhu bendabenda, , makamaka suhusuhubendabenda tersebuttersebut akanakan turunturun, , sekalipunsekalipun

ruangruang dalamdalam pengurungpengurung tersebuttersebut hampahampa. .ProsesProses pemindahanpemindahan panaspanas yangyang terjaditerjadihanyahanya sematasemata karenakarena bendabenda suhusuhu dandantanpatanpa bantuanbantuan zatzat perantaraperantara (medium),(medium),disebutdisebut perpindahanperpindahan panas panas radiasiradiasi

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SedangSedang radiasiradiasi termaltermal, , energienergi pancarannyapancarannyaditentukanditentukan berdasarkanberdasarkan suhusuhu bendabendatersebuttersebut. .

DaerahDaerah spektrumspektrum panjangpanjang gelombanggelombang

RADIASI TERMALRADIASI TERMAL

ra iasira iasi termaterma a a aa a a ariari ,1,1 sampaisampaidengandengan 100100 mikronmikron

RadiasiRadiasi mataharimatahari juga juga merupakanmerupakan radiasiradiasitermaltermal dengandengan daerahdaerah panjangpanjang gelombanggelombangkhususkhusus yaituyaitu 0, 250, 25 s.d.s.d. 33 mikronmikron

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electromagnetic spectrum

microwaves,cell phones

TV thermalradiation

Danar - PTM UNS 14

radiowaves visible

x-rays

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visible electromagnetic waves: LIGHTshorter wavelength more energy

Danar - PTM UNS 15

visible lightthermalradiation UV radiation

produces sunburn

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Thermal radiation spectrum The intensity of

radiation increaseswith temperature the color shifts toward

Danar - PTM UNS 16

the blue at highertemperatures

The UV radiation from

the sun is just beyondthe violet (11,000 F)

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RADIASI BENDA HITAMRADIASI BENDA HITAM

BendaBenda hitamhitam adalahadalah idealisasiidealisasi bendabenda yangyangpadapada suhusuhu berapapunberapapun, , memancarkanmemancarkan atau ataumenyerapmenyerap seluruhseluruh radiasiradiasi padapada panjangpanjang

((disebutdisebut RADIATOR SEMPURNARADIATOR SEMPURNA ).).

DayaDaya pancarpancar bendabenda hitamhitam tergantungtergantung daridari

suhusuhu dandan panjangpanjang gelombangnyagelombangnya. .

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Irradiation, G

Radiation energyincident on a surfaceper unit surface area

per unit time

Fractionabsorbed by

Fractionreflected by

Danar - PTM UNS 22

surface sur ace

Fractiontransmitted by

surface

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The First Law of Thermodynamics :

Danar - PTM UNS 23

The above is for total hemispherical properties, i.e. for all directions andwavelengths incident.

Can also consider the same definitions for specific wavelength or direction:

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Radiation

Danar - PTM UNS 24

The absorption of radiation incident on an opaque surface ofabsorptivity . Kirchhoffs law states that emissivity=absorptivityat a given temperature and wavelength.

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QQQQ ++=

1 / / / =++ QQQQQQ

Absorbed Q Reflected Q Transmitted Q

Heat balance

or

Q

Radiation properties

Danar - PTM UNS 25

QQ =

1=++

SetQQ =

QQ =

absorptivity

reflectivity

transmissivity

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Emission and Absorptionare balanced

Danar - PTM UNS 26

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Practical considerations wear light clothing

in summer lightclothing absorbsless sunlight

Danar - PTM UNS 27

cover all bodyparts in winterwarm body parts(like your head)emit radiation

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STEFAN-BOLTZMANN LAWThe maximum rate of radiation that can be emitted from a surfaceat an absolute temperature is;

Stefan-Boltzman

constant=5.6710 -8 W/m 2.K 4

Black body : an idealized surface that emits radiation at this maximum

4max, S S emit T AQ =

Danar - PTM UNS 29

Black body radiation : radiation emitted by blackbodies

Real surfaces emit less radiation

1=

10 For real bodies

For blackbodies

Emissivity of the surface

4

S S emit T AQ =

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RadiationStefan-Boltzman law

[ W / m 2 ]( )44 sur s T T q =

Danar - PTM UNS 30

q rad = h r A (T s T sur ) [ W ]

radiation heat transfer coefficient,Stefan Boltzman const. , = 5.67 x 10 -8 [ W/m 2.K4 ]

= emissivity

( )( )22

sur ssur sr T T T T h ++=

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STEFAN-BOLTZMANN LAW Stefan showed that the total power emitted

per unit area, R, called the total emissivepower or total emittance is given by the

Danar - PTM UNS 31

4 R e T =

Emissivity,characterisicof surface, 1 e

Constant independentof surface

Temperature onabsolute scale

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STEFAN-BOLTZMANN LAW If a body is in thermal equilibrium with its

surroundings, it must absorb and admit thesame amount of radiant energy(otherwise

Danar - PTM UNS 32

A blackbody is a perfect absorber so if it is

emitting thermal radiation we must have

e=1

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STEFAN-BOLTZMANN LAW Stefan-Boltzmann law: the maximum rate

of radiation that can be emitted from asurface at an absolute temperature T s is

Danar - PTM UNS 33

: Stefan-Boltzmann constant=5.67x10 -8 W/m 2K4

)(4max, W AT Q semit =&

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STEFAN-BOLTZMANN LAW The radiation emitted by all real surfaces

is less than the radiation emitted by ablackbody at the same temperature, and isexpressed as

Danar - PTM UNS 34

: the emissivity of the surface ( ) =1 for blackbody

)(4 W AT Q semit =&10

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STEFAN-BOLTZMANN LAW Absorptivity, : the fraction of the radiation

energy incident on a surface that isabsorbed by the surface. 10

Danar - PTM UNS 35

A blackbody absorbs the entire radiationincident on it, i.e., =1

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Radiation heat transfer between a surface

and the surfaces surrrounding it

Danar - PTM UNS 37

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Heat loss from a person

Danar - PTM UNS 38

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)( 44 surr S S rad T T AQ =

)(

= T T AhQ S S combined total

Danar - PTM UNS 39

Combined heat transfer coefficient includes effects of bothconvection and radiation in such an example and conductionheat transfer may be neglected.

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Blackbody radiation

Danar - PTM UNS 41

Blackbody radiation represents the maximum amountof radiation that can be emitted from a surface at aspecified temperature. Stefan-Boltzmann law.

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Blackbody Radiation "Blackbody radiation" refers to an object or

system which absorbs all radiation incidentupon it and re-radiates energy which is

Danar - PTM UNS 42

,not dependent upon the type of radiationwhich is incident upon it. The radiatedenergy can be considered to be producedby standing wave or resonant modes ofthe cavity which is radiating.

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Cavity = Blackbody Radiation The radiation emitted from a cavity through a small hole

("cavity radiation") is very close to the theoretical

blackbody curve for the same temperature. In the cavity,the radiation is in equilibrium with the material - most ofthe radiation stays inside the cavity, being continuallyemitted and re-absorbed by the walls.

Danar - PTM UNS 43

Radiation emitted from the outer surface of a materialwill not necessarily be fully thermalized - somefrequencies corresponding to certain transitions of thematerial, will be emitted preferentially. So, the blackbodycurve is not material-specific, but the actual emissionfrom an object will be.

Cavity radiation will depend less on the material, and thesmaller the hole, the closer it will correspond to thetheoretical blackbody curve.

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Black Body Radiation A black body: Is a model of a perfect radiator.

Absorbs all energy that reaches it; reflects nothing. Therefore

The energy emitted by a black body is the theoretical.0,1 ===

Danar - PTM UNS 44

maximum: This is Stefan-Boltzmann law; is the Stefan-Boltzmannconstant (5.6697E-8 W/m 2K4).

The wavelength at which the maximum amount ofradiation occurs is given by Wiens law:

Typical wavelengths are max = 10 m (far infrared) atroom temperature and max = 0.5 m (green) at 6000K.

4

T q =

[mK]32.898E =T max

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Blackbody Radiation

Danar - PTM UNS 45

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Real body4T =

The net radiant exchange is

)( 424

1emittednet, T T

A

Danar - PTM UNS 46

Radiation in an enclosure

)( 424

111 T T A =

The Radiation Shape Factor (View Factor)

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The Radiation Shape Factor (View Factor)

Danar - PTM UNS 47

2122121121

212212

121121

b b

b

b

E F A E F AQ

E F AQ E F AQ

=

=

=

&

&&

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Although there are three mechanisms of

heat transfer, a medium may involve onlytwo of them simultaneously

Solids: conduction and radiationFluids:

conduction and radiation no motion

Danar - PTM UNS 48

Heat transfer through a vacuumis by radiation only

convection and radiation (in motion)conduction and convection (no radiation)

Blacken of Common Engineering Materials

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material T,[ ]

red brick 20 0.93

fire brick 0.8 0.9

steel (oxidized) 200 600 0.8

steel (polished) 940 1100 0.55 0.61

Blacken of Common Engineering Materials

Danar - PTM UNS 49

aluminum (oxidized) 200 600 0.11 0.19

aluminum (polished) 225 575 0.039 0.057

copper (oxidized) 200 600 0.57 0.87

copper (polished) 0.03

casting iron (oxidized) 200 600 0.64 0.78

casting iron (polished) 330 910 0.6 0.7

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IR images

Danar - PTM UNS 50

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IR Images

Some materials aretransparent to infrared light,while opaque to visible light(note the plastic bag).

Danar - PTM UNS 51

Other materials aretransparent to visible light,while opaque or reflective to

the infrared (note the man'sglasses)

IR images

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IR images

Danar - PTM UNS 52

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IR images

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IR images

Danar - PTM UNS 54

Shown above is an Infrared Map of the Earth. Red areasrepresent regions of high heat retention in the atmosphere.

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IR images

Danar - PTM UNS 55

Thermal Radiation

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Thermal Radiation

Danar - PTM UNS 56

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Thermal Radiation

Danar - PTM UNS 57

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Thermal Radiation

Danar - PTM UNS 58

Different ways of reducing heat transferb t t i th l l t d th i

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between two isothermal plates, and theireffectivenesses

Danar - PTM UNS 59

Different ways of reducing heat transferbet een t o isothermal plates and their

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between two isothermal plates, and theireffectivenesses

Danar - PTM UNS 60

Thermal radiationThermal radiation is one kinds of the electromagnetic

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=c

Thermal radiation is one kinds of the electromagnetic

radiation, so it is propagated at the speed of light, 3 10m/s.The following relation holds

The propagation of thermal radiation takes place in the form ofdiscrete quanta, each quantum having an energy of

where c speed of light wavelength frequency

61

E = = . - .s s anc s constant

hmc E == 2relativistic relation between mass and energy

2 / chm = ch

ch

c ==

2momentum

Stefan-Boltzmann Law4T E b =

E b is called the emissive power of a blackbody , the energyradiated per unit time and per unit area by a black body

T is the surface absolute temperature

K) W/(m10669.5 28=

Electromagnetic Spectrum

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g p

62

Cosmic rays up to 4 10 -7 mGamma rays 4 10 -7 to 1.4 10 -4 mX-rays 1 10 -5 to 2 10 -2 mUltraviolet rays 1 10 -2 to 3.9 10 -4 m

Visible light 0.38 to 0.76 mInfrared rays 0.76 to 1000 mRadio and 1000 to 2 10 10 mHertzian wavesHeat rays 0.1 to 100 m

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Thermal radiation 0.1100mVisible light 0.35-0.75(0.380.76 )m

most body on the earth

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Danar - PTM UNS 64