Acoustic methods for transport properties measurements in gases

Abstract

Two novel acoustic resonators have been developed, one for measuring the viscosity and a second for measuring the Prandtl number of gases. For viscosity measurements, a double Helmholtz resonator, called the Greenspan viscometer, is used. This resonator consists of two chambers connected by a small duct. The Greenspan viscometer has a single, isolated, low-frequency mode that is fitted to determine the gas’ viscous diffusivity. The Greenspan viscometer may be used for absolute measurements if the dimensions of the duct and chambers are accurately known. Otherwise, the viscometer may be calibrated with argon or helium. For Prandtl number measurements, a cylindrical resonator that has been modified by inserting an array of short, parallel plates halfway between the ends is used. The lower longitudinal modes of this resonator are fitted to determine the Prandtl number. The odd order modes are attenuated primarily by the viscous diffusivity; the even order modes are attenuated primarily by the thermal diffusivity. Detailed acoustic models of both resonators have been developed and are necessary to realize the full precision and accuracy of these techniques. Transport property measurements of several gases will be presented. [This work is supported in part by the Office of Naval Research.]