Influence of wall conditions on friction factor for flow of gases under slip condition

In this paper, the measurement of friction factor in gas with slip at the wall is presented; these are made to encompass the following aspects: change in pipe material, pipe wall roughness, and channel geometry, for various gases. Two different wall materials (stainless steel and copper), three different grades of roughness, and two wall geometries (circular and square) are considered in this study. The gases employed in this investigation are nitrogen, oxygen and argon. The ranges of Knudsen number and Reynolds numbers covered in this study are 0.0022-0.024 and 0.54-13.2 respectively. The experimental setup is validated by comparing the friction factor close to the continuum and slip flow regimes for nitrogen against available results in the literature. The experimental results suggest that the friction factor is affected by the wall roughness and geometry of cross-section, in addition to the amount of rarefaction. However, the effect of changing the gas or pipe material is minimal. A correlation for slip flow in square duct is also proposed. This is one of the first such detailed studies and has implications for both gas flow in microchannels and rarefied gas in large pipes

Heat transfer coefficient of gas flowing in a circular tube under rarefied condition

The purpose of this paper is to present heat transfer measurements of gas in a tube under rarefied condition The measurements are made in a circular tube of inner diameter 25 mm for approximately constant wall temperature boundary conditions, with nitrogen, oxygen, argon, and helium as the working fluids The range of Knudsen and Reynolds numbers covered in this study are 0.0022-0 032 and 0 13-14.7. respectively. Whereas the continuum values are correctly reproduced in our setup, the measured values for Nusselt numbers are very small in the slip regime. The measured values are two-five orders of magnitude smaller than the corresponding values in the continuum regime, and suggest that the Nusselt number is a strong function of Reynolds, Knudsen and Brinkmann numbers in the slip flow regime These are among the first heat transfer measurements in the slip flow regime and the current theoretical and simulation models are inadequate to explain such low values of Nusselt number. (C) 2010 Elsevier Masson SAS.

Experimental determination of heat transfer coefficient in the slip regime and its anomalously low value

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