Lect_24_Heat-transfer

Lect_24_Heat-transfer - Lecture 24 Heat transfer Mechanisms...

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Lecture 24 Heat transfer
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Mechanisms of heat transfer Conduction : Two surfaces in contact, or within a body. Convection : Particles move between regions of different temperature and carry energy Radiation : Energy carried by electromagnetic waves. Can happen through vacuum!
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Conduction Metal bar of length L and cross-sectional area A between two sources at T HOT and T COLD . k = thermal conductivity Q t = kA T H C L Also, the rate of transfer of energy decreases with length. What transfers energy are the collisions between faster and slower molecules, and the number of collisions increases with cross-section area, A. This formula “makes sense”: Thermal conductivity is the efficiency of collisions to exchange energy
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Thermal conductors and insulators Conductors (high k) Metals 50-500 W/(m K) Insulators (low k) Wood 0.12-0.04 W/(m K) Air (still) 0.024 W/(m K) Styrofoam 0.01 W/(m K) Of course, there are no perfect thermal conductors or insulators. How low can you go?????…. “Aerogel”: 8x10 -5 W/(m K) What’s aerogel…. a man-made substance formed by specially drying a wet silica gel,
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Lect_24_Heat-transfer - Lecture 24 Heat transfer Mechanisms...

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