Figure 9 . Sample images of spherical LDPE particles
fluidization and 100 cylindrical wood pellets with a superficial gas
velocity of (a) 1.5Umf, (b) 2.0 Umf,(c)
3.0 Umf, (d) 3.7 Umf.
Figure 10 and 11 shows the temporal evolution of the LDPE particle
velocity field at 2.0 Umf and 3.0 Umfrespectively. The general slugging behavior resembles that observed for
the LDPE only case. The process generally involves (1) the slug
formation and rising at an almost constant speed (Figure 10: t=0 - 0.313
s, and Figure 11: t=0 - 0.213s) and (2) disintegration of the slug by
raining down the particles (Figure 10: t=0.313 -0.763s, and Figure 11:
t= 0.213 - 0.606s). However, the characteristic size and velocity of
these processes differ significantly between each case. For example, at
2Umf , both LDPE only case and binary mixture case form
a slug of around 5 cm. At 3Umf the slug length of the
binary mixture case increases to 14 cm. As for the slug rising speed, at
2Umf, the rising speed is 35 cm/s and 44 cm/s for LDPE
only case and the binary fluidization case respectively. At
3Umf, the rising speed of the slug increases to 101
cm/s. Similarly, at 2Umf, the maximum falling speed is
56 cm/s for the LDPE only case, and increases to 119 cm/s for the binary
fluidization case. At 3Umf, it reaches as high as 224
cm/s.
To compare the general trends of fluidization with and without
non-spherical wood pellets, the spatially averaged vertical velocity is
calculated using all available velocity vectors in the field of view,
denoted as <v >FOV. The
comparison of the temporal evolution of
<v >FOV for LDPE only and
binary fluidization case at both 2Umf and
3Umf is shown in Figure 12. As expected, the LDPE only
case at 3Umf remains almost constant due to the
transition from the slugging to the spouting regime. Except for the LDPE
only case at 3Umf, the
<v >FOV varies quasi
periodically with varying amplitude and frequency. To quantify the
amplitude and frequency, the local troughs and peaks are detected for
the whole duration of the available dataset. The corresponding amplitude
and period are measured and the mean values are calculated across all
detected local troughs and peaks. At 2Umf, the mean
amplitude of the <v >FOVsignal increases significantly with the addition of wood pellets from
29.4 cm/s to 58.7 cm/s. At 3Umf the mean amplitude for
the binary fluidization case increases to 123.0 cm/s. The mean frequency
of the signal is 1.72 Hz for LDPE only case at 2Umf and
increases to 2.37 Hz with the addition of wood pellets at the same
superficial gas velocity. At 3Umf, the mean frequency decreases
significantly to 1.54 Hz, presumably due to the longer traveling range
of the slugs in the bed.