Measured dependence of the attenuation of audio-frequency sound on concentration in flowing particulate suspensions

Abstract

The relationship between the attenuation of sound at frequencies between 500 and 3500 Hz and the concentration of flowing suspensions of various powdered solids has been explored. The suspensions were conveyed in a pipeline of 53 mm diameter and the particle concentration was varied up to a maximum of 2.8 kg/kg. Three materials were tested: flour (mean radius, 27 mm), olivine sand (mean radius, 278 mm) and barytes. The feasibility of measuring the acoustic attenuation of particulate suspensions at audio-frequencies and relating the measured attenuation to concentration has been demonstrated. Although the audio-frequency attenuation is smaller at a given concentration than at ultrasonic frequencies, audio-frequency measurements are less susceptible to inhomogeneity of flow and to in-pipe turbulence than ultrasonic measurements.

Particle Irregularity and Aggregation Effects in Airborne Suspensions at Audio- And Low Ultrasonic Frequencies

“…Previous data [9,10,17] and the new data considered here are consistent with a power law relationship between the speci"c acoustic attenuation and the particle concentration. AIRBORNE SUSPENSION ACOUSTICS Figure 2.…”

“…Figure 1 shows that there is negligible di!erence between predictions of sound attenuation in airborne suspensions of smooth spheres with volume concentrations much less than 0)01 m of particles per m of air, in the long wavelength regime, using the multiple-scattering theory [equation (1)] and the coupled phase theory [equation (10)]. It should be remarked that neither of the predictions compares well with the data for attenuation in airborne suspensions of olivine sand [9,10] also shown in this "gure. Given the close agreement between the predictions for smooth spherical particles with relevant properties, only the multiple-scattering theory of Lloyd and Berry will be used in subsequent comparisons of predictions of attenuation in suspensions of smooth spherical particles.…”

“…Attenuation versus mass concentration for a suspension of olivine sand in air. ᭺, data [9,10]; **, prediction of the multiple scattering theory using equation (1) with spheres; ............, prediction of the coupled phase theory using equation (10) with spheres; ) } ) }, ---, predictions of the coupled phase theory [7] with spheroids and cubes respectively.…”

“…Moss [9] and Moss et al [10] have reported signi"cant discrepancies between predictions assuming smooth spherical particles and measurements on #owing suspensions of rough olivine sand. In these measurements, the particles were quite large compared to the viscous or thermal boundary layer thickness.…”

“…In section 3, new experiments designed to test these e!ects are described. In Section 4, the data from these experiments and other data obtained by Moss and co-workers [9,10,17] are compared with the predictions of the fractal modi"cation. Finally, some conclusions are given.…”