03_basic concepts of heat transfer_new

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Basic Concepts of Heat

Transfer


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Thermal Science

This is the combined study of

thermodynamics, fluid mechanics and heat

transfer.


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Heat transfer

Heat transfer is the passage of thermal

energy from hot to cold body


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Modes of Heat Transfer

Conduction

Convection

Radiation


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Conduction

Conduction in a solid, a liquid, or a gas is themovement of heat through a material by thetransfer of kinetic energy between atoms ormolecules

The transfer of heat without the molecules of thematerial changing their basic position,

Drift of free electrons in metals; vibrational

waves (phonons) in non-metals.The rate of conduction of heat is dependent onthe material conductivity.


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Convection

Convection in a gas or a liquid is the bulk movement offluid caused by the tendency for hot areas to rise due totheir lower density.

The transfer of heat by the bodily movement of particles.

It only occurs in liquids and gases, not in solids.Natural convection: air expands when it is heated. This isless dense than the surrounding cooler air, causing it torise.

Examples are a convector heater in a room, heating ofwater in a storage tank and the stack effect in buildings(the transfer of heat from lower to higher levels).


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Radiation

Radiation is the dissemination ofelectromagnetic energy from a source. Thusdoes not require any intervening medium.

The transfer of heat by electromagnetic waves.The heat transfer rate depends upon thetemperature of the surface as well as the natureof the surface (roughness, color).

Dull black surfaces have high absorption andemission rates, shiny silver surfaces have lowabsorption and emission rates.


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Radiation

Applications are solar energy collectors whichhave blackened collector surfaces.

Heat will be exchanged by any two surfaces that

are in view of one another, if they are at differenttemperatures.

The frequency of the electromagnetic radiationis also dependent on temperature. Hot bodies

become visible (dull red) at about 500C, atmuch higher temperatures, bodies appearwhite.


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Microscopic reasons why a conductor is a conductor


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In contrast to conductors,insulators

are materials which impedethe free flow of electrons from atom to atom and molecule to

molecule. If charge is transferred to an insulator at a given

location, the excess charge will remain at the initial location of

charging.


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In a conductor, electric current can flow freely, in an

insulatorit cannot. Metals such as copper typify conductors,

while most non-metallic solids are said to be good insulators,

having extremely high resistance to the flow of charge

through them. "Conductor" implies that the outer electrons of

the atoms are loosely bound and free to move through the

material. Most atoms hold on to their electrons tightly andare insulators. In copper, the valence electrons are essentially

free and strongly repel each other. Any external influence

which moves one of them will cause a repulsion of other

electrons which propagates, "domino fashion" through the

conductor. Simply stated, most metals are good electrical

conductors, most nonmetals are not. Metals are also

generally good heat conductors while nonmetals are not.
http://hyperphysics.phy-astr.gsu.edu/hbase/solids/sili.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/pertab/metal.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/thermo/thercond.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/thermo/thercond.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/thermo/thercond.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/thermo/thercond.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/pertab/metal.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/solids/sili.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/solids/sili.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/solids/sili.html


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Black Body Radiation

Any object heated to a temperature T (on an absolute scale) radiatesElectromagnetic Energy (light) with total power:

P = e sA T4

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