We report experimental observations of the dynamic behavior
of single, magnetically tagged 3–4 mm particles varying in
density from 0.55 g/cm3 to 1.2 g/cm3 as they
migrate freely in a bubbling air-fluidized bed of 177–250 μm
glass beads of 2.5 g/cm3 density over a range of air flows.
The densities of the tracer particles (made by imbedding small magnets
in wooden particles) were chosen to span a range typical for many
biomass materials and exhibited both segregated and well-mixed behavior.
Using high-speed measurements from externally mounted magnetic probes,
we were able to reconstruct three-dimensional spatial and temporal
information about the tracers’ trajectories over periods of
five minutes. Based on this information, we describe general trends
in how the tracers moved and redistributed themselves as functions
of their density, fluidization air flow, and the overall concentration
of low density particles present. One key finding was that the time
average vertical probability distribution of the tracer particles
locations is consistent with a Weibull distribution. The effective
Weibull parameters appear to vary systematically with the degree of
fluidization and particle density. Also, we observed that temporal
autocorrelations in the vertical position of the tracer particles
vary systematically with fluidization intensity and reveal important
information about the dominant bed circulation time scales. Our results
suggest that it may be possible to develop relatively simple statistical
models or correlations for describing the spatial distribution and
circulation of mm sized particles in bubbling beds of this type. Such
tools should be useful for simulating some types of fluidized biomass
processing and for validating kinetic-theory models of fluidized bed
systems.
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