In this paper, the measurement of the heat transfer coefficient in rarefied gases is presented; these are among the first heat transfer measurements in the slip flow regime. The experimental setup is validated by comparing friction factor in the slip regime and heat transfer coefficient in the continuum regime. Experimental results suggest that the Nusselt number is a function of Reynolds and Knudsen numbers in the slip flow regime. The measured values for Nusselt numbers are smaller than that predicted by theoretical or simulation results, and can become a few orders of magnitude smaller than the theoretical values in the continuum regime. The results are repeatable and expected to be useful for further experimentation and modeling of flow in the slip and transition regimes.

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This article focuses on determining the friction factor from experimental results available in literature for gas flow in a smooth tube. The friction factor (f), Reynolds number (Re), and Knudsen number (Kn) are calculated from the reported pressure drop and mass flow rate. The variation in friction factor with Knudsen number and Reynolds number is analyzed in the Kn range of 0.0004–2.77 and Re range of 0.001–227. It is found that f Re is independent of gas and surface material and can be estimated by a simple semiempirical correlation within ±20% and a mean absolute error of 8%. The correlation gives friction factor for a particular value of Knudsen number and Reynolds number, irrespective of the type of gas or surface material.

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