BIRLA VISHVAKARMA MAHAVIDYALAYA, BIRLA VISHVAKARMA MAHAVIDYALAYA, ENGINEERING COLLAGE. ENGINEERING COLLAGE.

SUBJECT :-ENGINEERING THERMODYNAMICS AND SUBJECT :-ENGINEERING THERMODYNAMICS AND HEAT TRANSFER(2132502) HEAT TRANSFER(2132502)

TOPIC :-PRESENTATION ON RADIATIONTOPIC :-PRESENTATION ON RADIATION

GROUP NO.16GROUP NO.16PREPARED BY :-PREPARED BY :-

SAHARSH TANDEL -140080125029SAHARSH TANDEL -140080125029 RUSHABH ZALAWADIAYA -140080125031RUSHABH ZALAWADIAYA -140080125031 PATELIA KRUNAL -15PES301 PATELIA KRUNAL -15PES301 DHARMIK DAVE -15PES302DHARMIK DAVE -15PES302

SHAPE FACTOR SHAPE FACTOR ALGEBRAALGEBRA

PREPARED BY :- SAHARSH TANDELPREPARED BY :- SAHARSH TANDEL

Shape FactorShape Factor• The shape factor may be defind as the fraction of radiative energy that is diffusd from our surface element and strikes the other surfce directly with no intervenctions.

•Equation 31is applicable to black surface only and mut not be used for surfaces having emissivities other than one.

•For calculation of shape factors/geometry factors for specific geometries and for the analiys of radiant heat exchange between surfaces, the following propertis and factors may be useful :

1) The shape factor is purely funcion of geometry perameters and it is also known as geometry factor.2) A1F12=A2F12 (reciprocity theorm) Above relectionship is useful when one of the shape factor is unity.3) For a convex or flat surface, the shape factor with respect to it self is zero (i.e F1-1=0), because one

Cannot see any other part of same surface.

4) For a concave surface, the shape factor with respect to itself is not equal to zero because energy coming out from one part of the surface is intercepted by the another part of the same surface.

5) For the two infinite parallel surfaces the shape factor value is unity.

F12=F21=1[A1=A2]

ELECTRICAL ELECTRICAL ANALOGYANALOGY

PREPARED BY :- RUSHABH ZALAVADIYAPREPARED BY :- RUSHABH ZALAVADIYA

Electrical network Electrical network analogy for thermal analogy for thermal

radiation systemradiation system

An electrical network analogy is an alternative approach for analysing radiation heat exchange between any surface(black or non-black).

Electrical network analogy for two non-Electrical network analogy for two non-black/gray bodies exchanging heat with black/gray bodies exchanging heat with each other each other

(Q1-2)Net =Eb1-Eb2/1-ɛ1/A1ɛ1 +1/A1F12+1-ɛ2/A2ɛ2

=σA1(T14-T2

4 ) / 1-ɛ1/ɛ1 + 1 / F12 + 1-ɛ2/ɛ2*A1/A2

Q12 = 1 / 1-ɛ1/ɛ1 + 1/F12 + 1-ɛ2/ɛ2

Q12 = (fg)1-2σ*A1*(T14-T2

4)

(fg)1-2 is known as gray body factor .

Electrical network for two non-Electrical network for two non-black bodies which are parallel black bodies which are parallel to each otherto each other1)A1=A2,2)F12=1So, the intercharge factor or gray body

factor is reduced to, f1-2=(F)12=1 / 1-ɛ1/ɛ1 + 1 + 1-ɛ2/ɛ2 1 / 1/ɛ1 + 1/ɛ2 - 1

Electrical network for concentric Electrical network for concentric cylinders or spheres:cylinders or spheres:

For concentric cylinders and spheres, F1-2=1

A1 /A2 = πd1L /πd2L = d1 / d2 = r1 /r2

A1 / A2 = 4πr12 / 4πr2

2 = r12 / r2

2

So,

f1-2 = (Fg)1-2 = 1 / 1-ɛ1/ɛ1 + 1/1 + 1-ɛ2/ɛ2*A1/A2

Radiation shieldsRadiation shields

PREPARED BY :- krunal pateliaPREPARED BY :- krunal patelia

•Radiation heat transfer between two surfaces can be reduced greatly by inserting a thin, high-reflectivity (low-emissivity) sheet of material between the two surfaces.

•Such highly reflective thin plates or shells are called radiation shields.

•Multilayer radiation shields constructed of about 20 sheets per cm thickness separated by evacuated space are commonly used in cryogenic and space applications.

•Radiation shields are also used in temperature measurements of fluids to reduce the error caused by the radiation effect when the temperature sensor is exposed to surfaces that are much hotter or colder than the fluid itself.

•The role of the radiation shield is to reduce the rate of radiation heat transfer by placing additional resistances in the path of radiation heat flow.•The lower the emissivity of the shield, the higher the resistance.

•Radiation heat transfer between two large parallel plates

•Radiation heat transfer between two large parallel plates with one shield

•The radiation shield placed between•two parallel plates and the radiation network associated with it.

•14

Introduction Introduction to to

Gas RadiationGas Radiation

PREPARED BY :- DHARMIK DAVEPREPARED BY :- DHARMIK DAVE

Radiation in absorbing- emitting mediaRadiation in absorbing- emitting media

• When a medium is transparent to radiation, radiation propagating through such a media remains unchanged

• However gases such as CO, NO, CO2, SO2, H2O and various hydrocarbons absorb and emit radiation over certain wavelength regions called absorption bands

• We will discuss a very simple analysis of radiation exchange in an absorbing and emitting medium, exchange between a body of hot gas and its black enclosure

Beer’s LawBeer’s Law• If Io is the intensity of radiation at the source and I is the

observed intensity after a given path, then optical depth τ is defined by the following equation:

•s

Characterization of Participating MediaCharacterization of Participating Media :- :- Absorption: attenuation of intensity Emission: augmentation of intensity Scattering

– In-scattering: augmentation of intensity– Out-scattering: attenuation of intensity

Equation of Radiative TranferEquation of Radiative Tranfer• Increase in Intensity of radiation per unit length

along the direction of propagation is

Transmissivity, Absorptivity and Transmissivity, Absorptivity and EmissivityEmissivity

• Solution of Radaitive Transfer Equation with the assumption that and Ib(T) are constant everywhere in the medium, gives

Radiation Exchange between a Gas Radiation Exchange between a Gas Body and its Black EnclosureBody and its Black Enclosure

• Assumption:– Entire gas body is isothermal– Enclosure wall is black

• Consider a hemispherical body of gas at uniform temperature Tg and walls are at temperature Tw

• The intensity of spectral radiation I(L) striking the surface element dA as a result of the emission of radiation by the gas along the path L is determined from

Spectral Emissivity of GasSpectral Emissivity of Gas

Emissivity ChartsEmissivity Charts• Hottel measured gas emissivity g and presented

emissivity charts for gases such as CO2, H20, CO, ammonia, SO2, etc. as a function of temperature and product term PiL, where Pi is the partial pressure (in atmospheres) of gas i in the gas mass and L is the beam length.

Calculation of Radiation exchange Calculation of Radiation exchange between a Gas Body and Its between a Gas Body and Its

enclosureenclosure

• The net radiative heat exchange Q between the gas mass at temperature Tg and its black surroundings at temperature Tw is

THANK YOUTHANK YOU