Abstract:
Helium--xenon and helium--argon mixtures have been proposed as good choices for working fluids in thermoacoustic refrigerators, because the Prandtl number of the mixture is lower than that of a pure noble gas. Accurate measurements of the viscosity and the Prandtl number, obtained with acoustic resonance techniques, will be presented for three mixtures: He(25%)/Xe(75%), He(50%)/Xe(50%), and He(75%)/Xe(25%). The measurements span temperatures from -20 to 30 (degrees)C and pressures up to 2 MPa. The viscosity measurements are performed in a calibrated Greenspan acoustic viscometer [K. A. Gillis, J. B. Mehl, and M. R. Moldover, Rev. Sci. Instrum. 67, 1850--1857 (1996)]. The Prandtl number measurements are performed in a cylindrical resonator that was modified to have a large surface area near the center [K. A. Gillis and M. R. Moldover, J. Acoust. Soc. Am. 97, 3376(A) (1995)]. In this modified cylindrical resonator, damping of the odd-numbered longitudinal modes is primarily due to viscous boundary effects, whereas the even-numbered modes are damped primarily by thermal boundary effects. With the interaction parameters deduced from these measurements, the transport properties of any helium--xenon mixture may be determined. [Work supported by the Office of Naval Research.]