Medium-Frequency Acoustic Attenuation and Natural Convection in Nanofluids: Effect of Aggregate Size
Résumé
Acoustic attenuation is gaining traction in industry for nondestructive nanoparticle aggregate sizing in concentrated and opaque suspensions. However, medium frequencies, in the kHz range, are rarely explored for this application, despite being simple to implement in large volumes. In this study, 300 kHz signals were transmitted through nanofluid samples at temperatures from 20 to 80 degrees C. Aqueous nanofluids with carbon black and ultrasound-fragmented coffee nanoparticles (up to 5 wt %) were studied with and without an SDS surfactant. Through these experiments, an existing theory of acoustic attenuation in multiphase systems was verified, and new information was provided on carbon nanofluid acoustic attenuation and convection effects. At 20 degrees C, acoustic attenuation in carbon black and low concentrations of coffee-based nanofluids followed expectations. Moreover, model predictions at 20 degrees C were within 20 nm of DLS measurements in carbon black regardless of the presence of a surfactant. Attenuative effects were diminished as temperature increased, but, at high concentrations, the signal became amplified. As a temperature gradient was imposed across the sample, natural convection currents formed. At the Rayleigh numbers reached in the system (107 order of magnitude), acoustic pressures are enhanced at the walls to create an amplificatory regime. In addition, more frequent aggregate collisions with the wall may become part of the received signal. Therefore, aggregate size predictions were inaccurate at elevated temperatures.
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