Measurements of viscosity, velocity slip coefficients and tangential momentum accommodation coefficients for gas mixtures using a spinning rotor gauge

A selection of experimental measurements for He-Ar, He-N2, and He-Ne binary gas mixtures which have been made with a spinning rotor gauge (SRG; MKS Instruments, Inc.) are reported. All of the experiments were conducted in the slip regime. Theoretical results are used to extract values of the viscosity and the velocity slip coefficient from the experimentally obtained data for each of the gas mixtures. Slip coefficients for binary gas mixtures have not previously been reported. An important issue here is whether or not the velocity slip coefficients for binary gas mixtures can be predicted accurately using separately measured tangential momentum accommodation coefficients. Calculation of slip coefficients from theory requires a knowledge of the accommodation coefficients of each constituent of the mixture. The dependence of these coefficients on the gas composition is not known and the simplest assumption is to regard them as being independent of the composition. The slip coefficients computed in conjunction with this work use this simple assumption in a theoretical expression for the slips. These computations led to reasonable agreement of the theoretical results with our data for the He-Ne mixture. Agreement in the cases of the He-Ar and He-N2 mixtures, however, was less than satisfactory.;Also reported are a selection of experimental viscosity and slip measurements for He and Ar made with a modified spinning rotor gauge. Currently available SRGs operate in a horizontal mode where the axis of the tube containing the spherical rotor is orthogonal to the angular momentum vector of the rotor. The theory, however, specifies that the angular momentum vector of the rotor should lie along the cylinder axis. This alignment difference doesn't prohibit the desired measurements from being made but does require that existing SRGs be calibrated against a known standard gas to account for the geometric differences. The geometry factor in the theory is known explicitly, however, when the rotor angular momentum vector is aligned with the cylinder axis. The implication of this is that a properly designed and aligned system can be used to measure viscosities and slips directly without reference to a known standard gas. Measurements made with this modified version of the SRG are compared with previous experimental results from the standard SRG.