Progress on Acoustic Measurements of the Bulk Viscosity of Near-critical Xenon (bvx)

We plan to determine the bulk viscosity of xenon 10 times closer [in reduced temperature τ = (T-Tc)/Tc] to its liquid-vapor critical point than ever before. (Tc is the critical temperature.) To do so, we must measure the dispersion and attenuation of sound at frequencies 1/100 of those used previously. In general, sound attenuation has contributions from the bulk viscosity acting throughout the volume of the xenon as well as contributions from the thermal conductivity and the shear viscosity acting within thin thermoacoustic boundary layers at the interface between the xenon and the solid walls of the resonator. Thus, we can determine the bulk viscosity only when the boundary layer attenuation is small and well understood. We present a comparison of calculations and measurements of sound attenuation in the acoustic boundary layer of xenon near its liquid-vapor critical point. We used a novel, compact, acoustic resonator designed specifically for these measurements (Fig. 1). We measured the frequency response of this resonator filled with xenon at its critical density ρc in the reduced temperature range 10 < τ < 10. From the frequencyresponse data, we obtained the resonance frequency and the attenuation for six resonant modes in the range 0.10 kHz < f < 7.5 kHz. Using the known thermo-physical properties of xenon, we predict that the attenuation at the boundary first increases and then saturates when the effusivity of the xenon exceeds that of the solid. [The effusivity is ε ≡ (ρCPλT), where CP is the isobaric specific heat and λT is the thermal conductivity.] The model correctly predicts (±1.0 %) the quality factors Q of resonances measured in a steel resonator (εss = 6400 kg·K·s); it also predicts the observed increase of the Q, by up to a factor of 8, when the resonator is coated with a polymer (εpr = 370 kg·K·s). The measured acoustic dissipation in near-critical xenon shows a prominent plateau for the Helmholtz mode and, to a lesser extent, for the longitudinal modes (Fig. 2). These results are the first direct Figure 1. Acoustic resonator