Figure 6. Box plots of local and
overall flux. For local flux, the fast fluidized bed had 1320 datasets15, while the turbulent fluidized bed had 255 datasets21. For overall flux, the fast fluidized bed had 1320
datasets 15, while the turbulent fluidized bed had 378
datasets 21.
To understand the relative influence of the variables on overall and
local flux, Figure 7 displays the results from the random forest
analysis. For overall flux, the variables were negatively and positively
correlated in the cases of fast and turbulent beds, respectively. Two
further notes on this are highlighted. Firstly, it should be noted that
the magnitude of the estimates for overall flux in fast fluidization was
approximately half that in turbulent fluidization, which is because the
most dominant influence by far in the former has been identified to be
the pressure at the bottom of the riser 27. Secondly,
the positive relationship between overall flux and particle properties
(namely, dave and ρp ) in
turbulent fluidization is expected, since the overall flux has been
linked to the collisional momentum transfer effects earlier20. Specifically, it was shown for the binary mixture
of narrow PSDs of glass (referred to as ‘fine’ due to the lower terminal
velocity) and PS (referred to as ‘coarse’ due to the lower terminal
velocity) that the overall flux increased then decreased with the
percentage mass of the fines (i.e., the glass species), and the
non-monotonic behavior was tied to a trade-off in which collisional
momentum transfer from fine to coarse particles dominated at lower fine
contents and a decreasing amount of coarse particles dominated at high
fine content. In contrast to overall flux, the relative influences for
local flux were similar between the fast and turbulent beds for local
flux. With respect to local flux, r/R was the most dominant,
followed by h/H , implying the importance of position within the
riser relative to other variables. The radial dependence is expected for
the fast fluidized bed due to the well-acknowledged annulus flow
behavior 19,28,29, but surprising for the turbulent
bed wherein core-annulus flow structures are not generally acknowledged
to exist 4. Notably, the turbulent fluidized bed data
presented here has been shown to display more prominent radial
variations of the local fluxes for some particle systems, namely, the
narrower lognormal PSDs and pure polystyrene systems21, which suggests differences between the core and
annulus flows. More studies are needed to ascertain this.