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Bertram31.com General Bulletin Board
Andre, is this one of your relatives?
Posted By: bruce
Date: Wednesday, 4 May 2005, at 8:49 p.m.
I was doing some reading on heat transfer and thermal conductivity and came across Fourier's equation below. Any kin to you?
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The rate at which heat is conducted through a material is proportional to the area normal to the heat flow and to the temperature gradient along the heat flow path. For a one dimensional, steady state heat flow the rate is expressed by Fourier’s equation:Where:
k = thermal conductivity, W/m-K
Q = rate of heat flow, W
A = contact area
d = distance of heat flow
T = temperature differenceThermal conductivity, k, is an intrinsic property of a homogeneous material which describes the material’s ability to conduct heat. This property is independent of material size, shape or orientation. For non-homogeneous materials, those having glass mesh or polymer film reinforcement, the term “relative thermal conductivity” is appropriate because the thermal conductivity of these materials depends on the relative thickness of the layers and their orientation with respect to heat flow. Another inherent thermal property of a material is its thermal resistance, R, as defined in Equation 2.
This property is a measure of how a material of a specific thickness resists the flow of heat. The relationship between k and R is shown by substituting Equation (2) into (1) and rearranging to form (3)
Equation 3 shows that for homogeneous materials, thermal resistance is directly proportional to thickness. For non-homogeneous materials, the resistance generally increases with thickness but the relationship may not be linear.
Thermal conductivity and thermal resistance describe heat transfer within a material once heat has entered the material. Because real surfaces are never truly flat or smooth, the contact plane between a surface and a material can also produce a resistance to the flow of heat. This contact plane is depicted in Figure 1.
Actual contact occurs at the high points, leaving air-filled voids where the valleys align. The air voids resist the flow of heat and force more of the heat to flow through the contact points. This constriction resistance is referred to as surface contact resistance and can be a factor at all contacting surfaces.etc.
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