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Heat transferThermal energy is related to the temperature of matter.

For a given material and mass, the higher the temperature, the greaterits thermal energy.

Heat transfer is a study of the exchange of thermal energy through abody or between bodies which occurs when there is a temperaturedifferences.

When two bodies are at different temperatures, thermal energytransfers from the one with higher temperature to the one lowertemperature.

Heat always transfers from hot to cold.

Units for heat measurementSI units

Thermal Energy (Q) 1 JHeat Transfer Rate (q) 1 J/s or wHeat Flux (q'') 1 w/m2

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Application Areas of Heat TransferEnergy production and conversion

For thermal design of boilers, steam turbines, condensers, gasturbines, cooling towers, feed heaters, internal combustionengine

Have to make a detailed heat transfer analysis.

Refrigeration and air conditioning:

The thermal design of compressors, evaporators, incineratorsinvolves an intensive study of heat transfer.

Electric machines:Insulations provide on electric wires need a heat transfer analysisto avoid conditions which will cause overheating.

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Application Areas of Heat TransferCivil Engineering

Design of buildings for the minimization of heat losses need heattransfer study.

Construction of dams and other heavy structures, calculation ofthermal expansion of suspension bridge and railway tracks.

Manufacturing processThe casting of metals, extrusion, metal cutting and heattreatment of metals involve heat transfer study.

Chemical and Petrochemical operations

Environmental Engineering

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Application Areas of Heat TransferEarth Sciences

Astronomy

Agriculture and Food processing

Application:The human body is constantly rejecting heat to its surroundings andhuman comfort is closely tied to the rate of this heat rejection.

We try to control this heat transfer rate by adjusting our clothing to theenvironmental conditions.

Computer, TV, VCR, Freeze, Air-conditioning System

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Application Areas of Heat Transfer

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Engineering Heat Transfer

Heat transfer equipment such as heat exchangers, boilers,condensers, radiators, heaters, furnaces, refrigerators, and solarcollectors are designed primarily on the basis of heat transferanalysis.

The heat transfer problems encountered in practice can beconsidered in two groups: (1) rating and (2) sizing problems.

The rating problems deal with the determination of the heat transferrate for an existing system at a specified temperature difference.

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Engineering Heat TransferThe sizing problems deal with the determination of the size of asystem in order to transfer heat at a specified rate for a specifiedtemperature difference.

An engineering device or process can be studied eitherexperimentally (testing and taking measurements) or analytically (byanalysis or calculations).

The experimental approach has the advantage that we deal with theactual physical system, and the desired quantity is determined bymeasurement, within the limits of experimental error. However, thisapproach is expensive, time consuming, and often impractical.

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Basic modes of heat transferThere are three basic modes of heat transfer

(i) Conduction

(ii) Convection

(iii) Radiation

Exchange of heat between bodies occurs through one of these modesor a combination of them.

They have in common that temperature difference must exist and thatheat is always transferred in the direction of decreasing temperature.

But they differ entirely in the physical mechanism and laws by whichthey are governed.

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Basic modes of heat transferConduction is the transfer of heat through solids or stationary fluids.

Convection uses the movement of fluids to transfer heat.

Radiation require a medium for transferring heat; this mode uses theelectromagnetic radiation emitted by an exchanging heat.

Fourier’s law of heat conduction

Stefan–Boltzmann law

σ = 5.670 × 10−8 W/m2 · K4 Stefan–Boltzmann constant

Newton’s law of cooling

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Basic modes of heat transfer

There are THREE ways heat can move.

¤ Conduction

¤ Convection

¤ Radiation

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ConductionHow are the particles arranged in a solid, a liquid and a gas?

solid liquid gas

Particles that are very close together can transfer heat energy as they vibrate. This type of heat transfer is called conduction.

Conduction is the method of heat transfer in solids but not liquids and gases. Why?

What type of solids are the best conductors?

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ConductionMetals are good conductors of heat. The outer electrons of metal atoms are not attached to any particular atom. They are free to move between the atoms.

When a metal is heated, the freeelectrons gain kinetic energy.

This means that the free electrons movefaster and transfer the energy throughthe metal.

This makes heat transfer in metals veryefficient.

Insulators do not have free electronsand so they do not conduct heat as wellas metals.

heat

How does heat energy get from the Sun to the Earth? There are no particles

between the Sun and theEarth so it CANNOT travel byconduction or by convection.

?RADIATION

Radiation

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RadiationRadiation is a process by which heat flows from a high temperaturebody to a body at lower temperature in the form of electromagneticwaves when the bodies are separated in space even when a vacuumexists between them (when the bodies are not in direct physicalcontact with each other).

No material medium is require for heat transfer.

At ordinary temperature, the energy transferred by radiation can beneglected.