Figure 16. The distribution of the orientation of wood
particles at 3 Umf, Hb0=15.24 cm, and
varying wood particle counts.
Figure 15 shows the probability density function (PDF) of wood particle
orientation in the fluidized bed with both shallow (Hb0= 7.62 cm) and deep bed (Hb0 = 15.24 cm) configurations
and a superficial gas velocity of 2Umf and
3Umf. The orientation is quantified using the angle
between the detected major axis of the wood particle and z -axis
of the bed, with ranges from 0° (vertical) to 90° (horizontal).
Currently, the measurements are inherently two-dimensional over the x-z
plane which is incapable of measuring accurately all wood particle’s
orientation relative to z-axis. As an alternative, to minimize the
measurement error due to the three-dimensional projection, the
orientation statistics samples only particles with an aspect ratio
larger than 1.8. Namely, the particles that lie almost in the x-z plane
are conditionally sampled in the orientation statistics. As the
distribution shows, for all cases, the distribution has a dip around 45°
and slightly favors both the vertical and horizontal orientation,
respectively. This presumably corresponds to the wood particle
fluidization in the dilute and dense LDPE particle regions of the bed.
Namely, the wood particle tends to be horizontal when reaching the dense
LDPE particle pool while staying vertical when falling after the slug
disintegrates. These results are in qualitative agreement with previous
experimental results by Vollmari et al. 201614 with a
comparable superficial gas velocity range. For example, for the long
cylindrical wood particle case (14 mm × 4 mm wood cylindrical particle)
in the study, the superficial gas velocity varies from around 1.6 to 2.4
Umf, all the orientation statistics show local peaks for
both vertical and horizontal particle orientation. Even though the
experiment was conducted using a single non-spherical particle type.
Figure 16 shows the orientation distribution of the wood particles at a
fixed superficial gas velocity (3Umf) and a deep bed
configuration with varying wood particle counts from 50 - 200. The
results show similar trends as described above without significant
variation between the tested cases.
Apart from the orientation statistics of the wood particles, the
velocity has also been acquired based on the tracked wood particles.
Figure 17 shows both the horizontal and vertical velocity of wood
particles centroid. As expected, for horizontal velocity (Figure 17a),
the distributions are well presented by a normal distribution with a
mean of around zero for all cases. The standard deviation increases
slightly with the increase of superficial gas velocity from
2Umf to 3Umf, and decreases with the
increase of the static bed height from 7.62 cm to 15.24 cm. For the
vertical velocity (Figure 17b), the distributions are non-gaussian and
spread more widely compared to the horizontal velocity from around -3
m/s to 3 m/s. As the superficial gas velocity increases with a fixed
static bed height, the standard deviation also increases. For the
Hb0=15.24cm case, the distribution becomes bimodal. With
the increase of the bed height, the distribution shifts to more negative
values possibly due to the increase of mean bed height (c.f. Figure 13)
and thus an increase in the downward wood particle velocity.
Furthermore, the effects of wood particle counts on the wood particle
velocity are shown in Figure 18. Similarly, the distributions remain a
normal distribution with zero mean for all cases. The standard deviation
increases slightly with the increase of wood particle counts. For the
vertical velocity distribution, all three cases are bimodal. The first
mode is at around zero whereas the second mode is centered at -1.5 m/s,
-1.75 m/s and -1.85 m/s for the 50, 100, and 200 wood pellets cases,
respectively.