4. Conclusion
Using high-speed imaging, the current study provides detailed
experimental data on the effects of non-spherical wood particle counts,
static bed height, and superficial gas velocity on the binary
fluidization behavior of a Geldart-D particle type system. Machine
learning-aided image analysis is applied to extract particles mask of a
specific type followed by PIV and PTV analysis to quantify the LDPE
particle velocity field and the velocity and orientation of wood
particles, respectively.
In general, the fluidization for LDPE particles only and LDPE/wood
particles behave rather differently. While LDPE particles slugging are
achieved at a low gas flow rate of 1.4Umf and
2Umf, the bed transitions to a spouting regime at higher
flow rates of 3Umf and 3.4Umf. This
transition is clearly observed in the mean and standard deviation of the
expanded bed height. To our knowledge, the spouting behavior of Geldart
D type particles has not been reported before under uniform flow
conditions using porous distributor plates without gas jets. The
resultant maximum downward velocity within the fountain regions is
significantly lower than those obtained using a central gas jet even
with lower gas velocity.
With the addition of wood particles, the spouting phenomenon has not
been observed under all tested conditions varying static bed height,
wood particles counts, and fluidization velocity. At a low gas flow rate
(1.5 Umf), the wood and LDPE particles remain perfectly
segregated under different static bed heights and wood particle counts
while the LDPE particles form slugging flow. At 2.0Umf– 3.7Umf, wood and LDPE particles are mixed and binary
slugging flows are formed. At 2Umf, adding wood
particles also causes a 26% increase in the rising speed of the slug
and a 112% increase of the maximum fall speed after the slugs
disintegration. Furthermore, the spatially averaged vertical velocity
shows quasi-periodic fluctuations for all cases except for the spouting
bed for the LDPE only case at 3Umf. At
2Umf, the mean amplitude of the signal almost doubled,
while the mean frequency sees a 38% increase with the addition of wood
particles. The increase of fluidization velocity for the binary
fluidization case to 3Umf causes a 110% increase of the
mean amplitude and a 35% decrease in mean frequency. Furthermore, the
mean bed height seems to collapse at different static bed heights after
nondimenlization. The evolution of the spatially averaged vertical
velocities behaves similarly under different static bed heights. After
using the characteristics falling velocity and time for
nondimenlization, the mean periods seem to collapse reasonably well
between different bed height and fluidization velocities. However, the
scaling only works for mean amplitude under the same static bed height.
Finally, the wood particles preferentially orientate themselves towards
0° (vertical) and 90° (horizontal) for all tested cases at dilute and
dense region of the bed respectively. No significant orientation
distribution differences are identified under tested conditions. The
horizontal wood particle velocities show a Gaussian distribution for all
cases and a varying degree of bimodal distribution for the vertical
velocities. One unresolved issue is the systematic quantification of
distribution. As the result shows, the wood particles demonstrate
varying degrees of segregation with varying superficial gas velocity and
wood particle fraction. The current system has limited wood particle
counts which inherently causes the segregation analysis noisy. A larger
system with a significant number of particles is preferred for
meaningful segregation analysis and is deferred to future studies.