Acoustic Properties of Solid-Liquid Mixtures and the Limits of Ultrasound Diagnostics—I: Experiments (Data Bank Contribution)

Ultrasound as a technique for interrogating two-phase mixtures has the advantages of being nonintrusive, it has a very high frequency response, and is able to penetrate typically opaque highly concentrated mixtures. There exists, however, an inherent compromise in the choice of the frequency of the ultrasound between maximizing spatial resolution and ensuring adequate beam penetration. To this end, the propagation of ultrasound in solid-liquid mixtures has been investigated experimentally for a range of frequencies and concentrations of the dispersed phase. The measured attenuation has been shown to depend roughly linearly on frequency for 0.1<kr<0.75 (where the wavenumber k = 2π/λ, and λ and r are the wavelength and particle radius, respectively), and quadratically for kr > 0.75. As a function of solids concentration, the attenuation displays a maximum at a solids fraction of about 30 percent for the present system of silica beads in water. This robust and reproducible result contradicts models of attenuation that rely on linear superposition of single particle effects. The intensity field produced by a circular disk transducer in a two phase medium at kr~1 shows excellent agreement with the Rayleigh integral with a modified wavenumber and attenuation parameter, and it allows for the prediction of the transducer beam geometry in two phase mixtures for a wide range of frequencies and solids fractions. The limitations of ultrasonic wave propagation as a nonintrusive diagnostic technique, in terms of spatial resolution, have been discussed. Acknowledging these limitations, an ultrasonic instrument for determining the velocity of moving particles at or near maximum packing was built. Preliminary results from this prototypical ultrasonic Doppler velocimeter show good agreement with observations of the settling velocity of silica beads at high concentrations.