Lesson 1 Principles of Radiation

8/3/2019 Lesson 1 Principles of Radiation

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Heat Transfer by Radiation

Ch.E. 206


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Lecture Objectives To introduce the basic facts about heat

transfer by radiation

To define the following: Special Types of Bodies

Emissive Power or Total Radiating Power

Emissivity

Kirchoffs Law

Stefan Boltzmann Law

To understand Net Transfer by Radiation Use of View Factors

Use of Radiant Heat Transfer Coefficient


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Definition Radiation Heat Transfer

The transfer of energy through space by

electromagnetic waves Radiation Wavelengths range from 0.5 to 50

microns

Examples: Rays emitting from the sun, hot stoves,glowing furnaces, nuclear explosion, X-ray.

Sun source of thermal energy.

We relate RADIATION with something hot.


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Basic Facts About Radiation Thermal radiation results from the temperature of a

body.

All bodies above absolute zero temperature emitthermal rays.

Thermal rays travel in straight lines from source toreceiver.

Only bodies within sight of each other can exchangeradiation.

One with flu or fever will emit more rays.


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Basic Facts About Radiation The amount of emitted radiation from a body

is independent of the amount emitted by

another body within sight.

Incident Radiation (I) represents the totalradiation from a source striking a receiver.


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I

Incident Radiation (I)

Source

Receiver


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Basic Facts About RadiationQuestion: What will happen to the rays

once it strikes receiver?

(I) has the following effects:

Absorptivity (): fraction of I absorbed

Reflectivity (): fraction of I reflected

Transmissivity (): fraction of I transmitted

through


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I

I I

I

, ,

= I + I + I

+ + = 1


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Basic Facts About RadiationFactors affecting ,,: Temperature

bodies at low temp. more rays areabsorbed.

bodies at high temp. increase reflectivityrather than absorptivity.

Nature of Surface

rough surface - is bigger

smooth surface reflects thermal rays


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Factors affecting ,,: Wavelength

strong wavelength high transmissivity

weak wavelength will be reflected Angle of incidence

direct hit ray will be absorbed

with an angle

more reflectionHigher angle of incidence:

Reflectivity > Absorbtivity


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Basic Facts About Radiation At a given temperature, the rate of thermal emission

varies directly with the state of aggregation of thebody.

State of Aggregation

compactness of the material

how compact the molecules are of a body.

Liquids and gases molecules not closelyaggregated, amount of thermal emission notsignificant.

When in space radiation coming from gases will besignificant.

Radiation mostly comes from SOLID.


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Basic Facts About Radiation Conduction and convection usually take place

simultaneously with radiation.

The net transfer rate by radiation is the differencebetween the emission rate and the absorbance rate.

Emission rate dependent on your temp.

Absorbance rate dependent on the other body.

qr = qre qra

Net transfer rate= emitted absorbed

re = radiation emitted ra = radiation absorbedWhere:


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Special Types of Bodies Black Body: = 1.0 or , = 0

just an ideal concept

Real bodies are non- black bodies Gray Body: = constant

ex. = 0.2, independent of source, consistent,behavior is constant.

Opaque Body: = 0 or + = 1.0Most bodies are opaque, = 0, few bodies will havea significant value of.

Non-Black Body: + + = 1.0


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Total Radiating PowerW = total radiation of all wavelengths emitted per

area per time (Watts/m2, BTU/hr-ft2)

W = W d0

W = emissive power of single wavelength

= wavelength

W

Area under the curve = W

W = heat flux = q/A or q = W A


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Emissive PowerEmissivity () = ratio of emissive power of a body to

that of a black body

WB= Emissive power of a black body

BWW

For a a black body: = 1.0

W= Emissive power of a body

BWW

and =


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Basic Laws of Radiation Kirchoffs Law

At thermal equilibrium : tconsW

tan

2

2

1

1 WW

if body2 is black: W2 = WB &2 = 1

1

WW B

1

1

1B

1

W

W

11

Therefore, if body2 is black


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Basic Laws of Radiation Stefan Boltzmann Law

The emissive power of a black body is directlyproportional to the 4th power of absolute

temperature

4

B TW Where: = 5.67 x 10-8 W/m2-K

T in Kelvin

2

48B

m

WT10x675W .

2

48

m

WT10x675W .

BWWBut


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Basic Laws of Radiation

2

4

m

W

100

T675W

.

AWqre

2

4

re m

W

100

T

A675q

.

re = radiation emitted

qre

= heat transfer rate of emitted radiation


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Net Transfer Rate by Radiationqr = qre qra

Net transfer rate= emitted absorbed

Consider only the net transfer rate between 2 bodies at a time1- source and the 2 - receiver

1

2


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Net Transfer Rate by Radiation

Between two gray bodies 1 and 2:

2

42

4111221rm

WTTAq )(

A1 = surface area of source

2to1frfactorerchangeoverall12 _____int_

1

2

A1T1

A2T2


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Overall Interchange Factor

F12 = View factor from 1 to 2 (see handouts)

1

1

A

A

1

1

F

1

1

22

1

112

12

4

2

4

111221r

100

T

100

TA6725q .)(

Lesson 1 Principles of Radiation

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