Thermal contact conductance
The inverse of this property is termed thermal contact resistance.
From experience, the temperature profile along the two bodies varies, approximately, as shown in the figure.
Thermal contact resistance is defined as the ratio between this temperature drop and the average heat flow across the interface.
[1] According to Fourier's law, the heat flow between the bodies is found by the relation: where
, which, as previously noted, is the inverse of the thermal conductance coefficient,
[2] Thermal contact conductance is an important factor in a variety of applications, largely because many physical systems contain a mechanical combination of two materials.
As a result, when two bodies are pressed together, contact is only performed in a finite number of points, separated by relatively large gaps, as can be shown in Fig.
Since the actual contact area is reduced, another resistance for heat flow exists.
The gases/fluids filling these gaps may largely influence the total heat flow across the interface.
The thermal conductivity of the interstitial material and its pressure, examined through reference to the Knudsen number, are the two properties governing its influence on contact conductance, and thermal transport in heterogeneous materials in general.
One can characterise a surface that has undergone certain finishing operations by three main properties of: roughness, waviness, and fractal dimension.
The effect of surface structures on thermal conductivity at interfaces is analogous to the concept of electrical contact resistance, also known as ECR, involving contact patch restricted transport of phonons rather than electrons.
This deformation may either be plastic or elastic, depending on the material properties and the contact pressure.
Unfortunately, a centralized database of contact conductance coefficients does not exist, a situation which sometimes causes companies to use outdated, irrelevant data, or not taking contact conductance as a consideration at all.
CoCoE (Contact Conductance Estimator), a project founded to solve this problem and create a centralized database of contact conductance data and a computer program that uses it, was started in 2006.
While a finite thermal contact conductance is due to voids at the interface, surface waviness, and surface roughness, etc., a finite conductance exists even at near ideal interfaces as well.
