Heat transfer through gas layers of contact interfaces formed by two microscopically rough surfaces is studied. Rarefied gas conduction between smooth parallel plates is examined with data obtained from the literature. Two important dimensionless parameters are introduced; one representing the ratio of the rarefied gas resistance to the continuum gas resistance, and the other representing gas rarefaction effects. Effects of gas rarefaction and surface roughness are studied in relation to the parallel plates case. It is proposed that the effective gap thickness at light loads may be estimate by a roughness parameter, the maximum peak height Rp. Experiments were performed to measure gap conductance for a number of Stainless Steel 304 pairs and Nickel 200 pairs over a range of roughnesses and gas pressures. Three different types of gases, helium, argon, and nitrogen, were employed as the interstitial gas. The comparison between the theory and the measured values of gap conductance shows excellent agreement.

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