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.