Ultrasonic velocimetry for the in situ characterisation of particulate settling and sedimentation

This paper reports on the development of an in situ ultrasonic velocimetry technique, to study the settling and sedimentation behaviour of particle dispersions. Specifically, the technique utilises a commercial ultrasonic velocity profiler (UVP) equipped with a 1 MHz transducer-receiver, to measure both particle velocities in the dispersion and the evolution of the sediment bed interface with time. It was found in systems of bi-modal non-coagulated glass particles (with a major size-peak of ∼10 μm) that measured velocities suggested dispersion segregation, although generally values were not reliable as particle settling velocities were below the instrument's threshold. For particle systems coagulated in 1 M KCl, measured dispersion velocities were within the machine's resolution and a high level of system detail could be extracted from the velocity profile maps, such as the development of hindered settling above the bed and movement of the cloud-front. For both coagulated and non-coagulated dispersions, the evolution of the sediment bed height with time could be measured, by analysing particle velocities in the near-bed region. Bed profiles indicated the non-coagulated particles settled slowly into a compact bed, while the coagulated particle-aggregates initially settled faster into a loosely packed bed that compressed over-time.