Figure 7. Sample instantaneous LDPE velocity in spouting regime
(a) 3.0 Umf (b) 3.4 Umf.
The sample instantaneous velocity field for both 3Umfand 3.4Umf cases are shown in Figure 6. Note, the flow
is highly three-dimensional, so the velocity field provided here
represents only the particle velocity near the bed wall close to the
camera. As it shows, for both cases, the fountain regions at top of the
bed have significantly higher velocity magnitude than the rest of the
bed. With the increase of gas velocity, the peak velocity also
increases. However, the maximum velocity magnitude for both
3.0Umf and 3.4 Umf cases is
significantly lower than that for the slugging bed at
2Umf. Moreover, there seems to be an asymmetry between
the fountain region causing the preferential downward flow at the right
side of the bed. This may be due to the disturbance of flows introduced
during the distributor installation. The bed particle velocity at the
fountain region has been measured previously in the spout
bed36–38. He et al.36 conducted a
comprehensive measurement of profiles of vertical particle velocity in
the 1.41mm glass beads (Umf = 0.54 m/s) spouted bed
using fiber optics probes. The maximum downward velocity
vmax to Umf ratio is measured to be
around 2.8 with a central jet velocity of 1.3Umf. In the
current configuration, the corresponding ratio is only around 0.15 and
0.20 for 3.0Umf and 3.4Umf respectively.
The significant decrease in the maximum downward velocity in the
fountain region is possibly due to that the current setup does not have
the central jet configuration used in the previous study.
The mean and fluctuation of the expanded bed height for all four
fluidization velocities are shown in Figure 8. As expected, the bed
height fluctuates overtime for all fluidization velocities as gas flow
facilitates the formation of moving slugs/fountains. However, the
resulting fluctuation intensity and the mean bed height differ. Both the
mean and the fluctuation of the bed height increases from
1.4Umf to 2.0 Umf. With the increase of
superficial gas velocity from 2Umf to
3Umf, both the mean and the fluctuation of bed height
decreased significantly due to the onset of spout at
3Umf. With a further increase of superficial gas
velocity to 3.4 Umf, the mean bed height increases slightly, whereas the
fluctuation of bed height decreases indicating a more stable spouted
bed.