Heat Transfer Convection is the transfer of heat by the bulk movement of a fluid. Convection can be...
13
Heat Transfer Convection is the transfer of heat by the bulk movement of a fluid. Convection can be natural or forced.
-
Upload
vivien-taylor -
Category
Documents
-
view
220 -
download
1
Transcript of Heat Transfer Convection is the transfer of heat by the bulk movement of a fluid. Convection can be...
Heat TransferConvection is the transfer of heat by the bulk movement of a fluid. Convection can be natural or forced.
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.
Materials that conduct heat well are thermal conductors. Those that conduct heat poorly are thermal insulators.
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.
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°).
Air, like most gases, has a low thermal conductivity and is a good insulator when convection is kept at a minimum.
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.
Absorption of energy is as important as emission. A perfect blackbody is an object that absorbs all the EM waves falling on it.
All objects absorb and emit EM waves simultaneously. If the temperature of the object is room temperature, the absorption and emission must be balanced.
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.
Dark clothing in the summer is uncomfortable because half of the emitted radiation is emitted toward the body.
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)
