Subject : Heat & Mass TransferTopic: ConductionLecture No: 3Date: 8 – Feb - 2013

Presented By:Pritesh K Jaradi

Some Expectations

Respect others Respect the college Humility

Work Ethic

DO participate and ask questions. For order in class, raise your hand when asking a question.

BRING all specified books. HAND IN all homework ON TIME! SHOW CARE and concern for each other.

Help one another in homework. BUT NO COPYING OF ENTIRE ASSIGNMENTS!

Topics Covered

Fourier’s Law of heat conduction Effect of temperature

On thermal conductivity of metal On refractories & building material On gas Liquid General heat conduction equation

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Fundamentals of Heat TransferConduction, Convection, and Radiation Heat Transfer ModeHeat transfer (or heat) is thermal energy in transit due to a temperature difference

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Fourier’s Law of heat conduction

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• Rate of heat flow is directly proportional to area of section at normal to heat flow.

• Rate of heat flow is directly proportional to temperature gradient.

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Fourier’s Law of heat conduction (Contd.)Assumptionssteady state condition.No internal heat generationHomogeneous and isotropic materialUnidirectional heat flow (1-D)

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Conduction Heat Transfer

TkAq

dxdTk

Axq ;

•First mechanism - molecular interaction (e.g. gas)

•Greater motion of molecule at higher energy level (temperature) imparts energy to adjacent molecules at lower energy levels

•Second mechanism – by free electrons (e.g. solid)

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Thermal Conductivity of Gas

Smaller than solid. Intermolecular space is much

large. Continuous random motion. K increases as increase of

temp. At high temp collision

increase.

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Thermal Conductivity of Metal

Due to migration of free electrons and molecular vibration.

At high temp vibration becomes violent and obstruct free flow of electrons.

Thus reduces HT by movement of free electrons.

For alloys TC is less than pure metals.

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Thermal Conductivity of Liquid

Close spacing molecules + strong molecular attraction force.

Heat exchange via collision process.

Lies between gases and solid.

As temp increases reduction in molecular attraction….. Leads to decrease in K

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Conductivity of refractories and building materials

Very low thermal conductivity. Known Insulating materials Porous structure with trapped air / liquid. At increased temp, conductivity of gases increases

………….leads to increase in thermal conductivity of building materials.

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General heat conduction equation:

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General heat conduction equation (Contd.):

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THE END