Study of granular temperature in dense fluidized beds by diffusing wave spectroscopy

Abstract

Diffusing wave spectroscopy (DWS), a non-intrusive multiple scattering technique, can be used to study the fundamentals of particle motion in dynamic dense granular media and measure the mean of the square of the particle velocity fluctuations about their mean, which is related directly to the so-called ‘granular temperature’ that underpins many theories for dynamic granular processes. An overview of DWS in the context of other techniques for studying the granular temperature and dynamics of particles in granular systems, and its application to a range of fluidized bed configurations is reported in this thesis. The thesis reports how the granular temperature and particle dynamics varies with the level of forcing for a liquid fluidized bed, and both dry and water-immersed vibro-fluidized beds. This data reveals that the granular temperature scales with, and is of the same order as, the forcing velocity (i.e. superficial velocity or peak vibrational velocity) for all except the water-immersed vibro-fluidized bed where it scales with the peak acceleration. The former appear to be in accord with theory, while no theory as yet has predicted the latter. The experimental data was also deconvoluted to reveal how the solid fraction affects the granular temperature in a liquid fluidized bed, which represents the most detailed experimental study in this regard. For the first time is revealed that vertical profiles of the granular temperature are a consequence of observed concentration stratification in a liquid fluidized bed. By way of example of the results that can be obtained using DWS, we have shown how it has provided a basis for us to hypothesise that particles in vibro-fluidized beds (both dry and water-immersed) rattle around in cages formed by their neighbours until their collective re-arrangement occurs at long time scales.