Heat Transfer Convection is the transfer of heat by the bulk movement of a fluid. Convection can be...
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Transcript of Heat Transfer Convection is the transfer of heat by the bulk movement of a fluid. Convection can be...
![Page 1: Heat Transfer Convection is the transfer of heat by the bulk movement of a fluid. Convection can be natural or forced.](https://reader036.fdocuments.in/reader036/viewer/2022082417/56649e1f5503460f94b0ae17/html5/thumbnails/1.jpg)
Heat TransferConvection is the transfer of heat by the bulk movement of a fluid. Convection can be natural or forced.
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Conduction is the transfer of heat through a material, any bulk motion of the material playing no role in the transfer. The free electrons in a metal allow heat energy to be transferred very easily.
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Materials that conduct heat well are thermal conductors. Those that conduct heat poorly are thermal insulators.
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The amount of heat Q conducted through a material depends on four factors:
• 1) Q is proportional to the length of time of conduction.
• 2) Q is proportional to the temperature difference between the two ends.
• 3) Q is proportional to the cross-sectional area.
• 4) Q is inversely proportional to the length of the conductor.
![Page 5: Heat Transfer Convection is the transfer of heat by the bulk movement of a fluid. Convection can be natural or forced.](https://reader036.fdocuments.in/reader036/viewer/2022082417/56649e1f5503460f94b0ae17/html5/thumbnails/5.jpg)
These proportionalities can be stated as Q is proportional to (A ∆T)t/L. This can be made an equality using a proportionality constant k, called the thermal conductivity.
Q = (kA∆T)t/LThe unit of k is J/(s•m•C°). A J/s is a watt, so the unit is also the W/(m•C°).
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Air, like most gases, has a low thermal conductivity and is a good insulator when convection is kept at a minimum.
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Radiation is the transfer of heat by electromagnetic waves. All objects radiate energy in the form of electromagnetic waves, but the temperature must be over 1000 K for the light to be visible.
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Absorption of energy is as important as emission. A perfect blackbody is an object that absorbs all the EM waves falling on it.
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All objects absorb and emit EM waves simultaneously. If the temperature of the object is room temperature, the absorption and emission must be balanced.
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A material that is a good absorber is also a good emitter, and a material that is a poor absorber is also a poor emitter.
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Dark clothing in the summer is uncomfortable because half of the emitted radiation is emitted toward the body.
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Stefan-Boltzman Law of RadiationRadiant energy Q emitted in a time t by an object that has a Kelvin temperature T, a surface area A, and an emissivity e, is given by:
Q = eT4At is the Stefan-Boltzmann constant: 5.67 x 10-8 J/(s•m2•K4)
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