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AN-892
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Temperature Measurement Theory and Practical Techniques
By Donal McNamara
Rev. 0 | Page 1 of 8
WARM AIR
RISES
COOLER AIR DROPS
AND REPLACES THE
WARMER AIR
HEATER
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HEAT TRANSFER THEORY
The transfer of heat is normally from a high temperature object
to a lower temperature object. Heat transfer from a cold region
to a hot region can be done by forcing the system (refrigerators,
for example) to perform the energy transfer.
Heat transfer is accomplished by three basic methods.
• Conduction
• Convection
• Radiation
Figure 2. Convection
Conduction
Radiation
Conduction is the most common means of heat transfer in a
solid. On a microscopic scale, conduction occurs as hot, rapidly
moving or vibrating atoms and molecules interacting with
neighboring atoms and molecules, transferring some of their
energy (heat) to these neighboring atoms.
Radiation is the only form of heat transfer that can occur in the
absence of any form of medium and as such, is the only means
of heat transfer through a vacuum. Thermal radiation is a direct
result of the movements of atoms and molecules in a material.
Because the amount of emitted radiation increases with increasing
temperature, a net transfer of energy from higher temperatures
to lower temperatures results.
HOT COLD
HEAT FLOW
L
CROSS SECTIONAL AREA = A
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Figure 1. Conduction
Convection
Convection is usually the dominant form of heat transfer in
liquids and gases. This is a term used to characterize the
combined effects of conduction and fluid flow. In convection,
heat content transfer occurs by the movement of hot or cold
portions of the fluid together with heat transfer by conduction.
Figure 3. Radiation
