Introduction to Conduction, Thermal Properties
CONDUCTION HEAT TRANSFER
? Conduction Rate Equation
Fourier’s Law was based on experimental investigation in which it was shown that for a rod of length DX and cross-sectional area A with the lateral surface insulated and the ends having a temperature gradient DT, the conduction heat rate q is proportional to A, DT and DX.
? The proportionality constant k (thermal conductivity, W/m K) is an important property of the material. Thus,
? In the limit DX ® 0, we obtain for the heat rate
? or for the heat flux:
? The minus is used because heat is transferred in the direction of decreasing temperature.
? The above equation (Fourier’s Law) implies that the heat flux q”x or heat rate qx is a directional quantity or a vector.
? q”x is normal to A, the cross-sectional area.
? Conduction heat transfer is based on Fourier’s Law, which is derived from the results of an experimental investigation.
? Fourier’s Law is a vector expression, and the heat flux is normal to an isothermal surface or isotherm.
Fourier’s Law is applicable to all matter: gases, liquids and solids
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