## 4 Experimental Results and Discussion
### Effect of Initial Bed Height  
Several experiments were carried out using three distributor plates by varying initial depth heights to observe the effect of bed aspect ratio on the hydrodynamics of bubbling fluidization. Bed particle size and distributor plate were kept constant for the set of bed aspect ratios, and variation in pressure drop was observed with increasing superficial velocity. The same had been carried out for the other two plates subsequently. It can be observed from Figure 6a that using perorated distributor plate, with increasing initial bed height, bed pressure drop also increased, and an average increase in bed pressure drop was found to be 36% for a rise in each bed aspect ratio.  
Figure 5: Fluidization phenomena for different distributor plates (1.5*U\({}_{\text{mf}}\), L/D = 0.833).  
Figure 6: _Cont._  
The increase in bed pressure drop was due to an increase in the extra weight of the bed owing to the additional volume of bed material [32]. Hence more pressure and power are required to fluidize a larger solid material. Moreover, minimum fluidization velocity (U\({}_{\text{mf}}\)) had also increased from 1.4 m/s to 1.75 m/s for the lowest and highest bed aspect ratio. The rise in minimum fluidization velocity was due to the additional airflow required to fluidize the deeper bed. Bubbles that rise through the column require more time to travel a longer distance up the column and reach the bed's surface, where they collapse, causing delayed fluidization and increased U\({}_{\text{mf}}\).  
Figure 6: Experimental study of initial bed height ratio on fluidization phenomena.  
A similar trend for a rise in bed pressure drop with increasing initial bed height was observed for the 45\({}^{\circ}\) slotted plate and hybrid plate in Figure 6b and 6c, respectively. However, the maximum pressure drop was found for a 45\({}^{\circ}\) slotted plate for all the bed aspect ratios compared with the other two distributor plates. This effect might be due to the lower open area ratio of the plate and the longer angular slot length causing additional frictional airflow to drag through the angular slots. Thus, additional pressure is required through the distributor plate to fluidize extra bed weight at a higher initial bed height. Moreover, there is a difference in minimum fluidization velocity U\({}_{\text{mf}}\) for shallow and deep beds for 45\({}^{\circ}\) slotted and hybrid plates. The difference in minimum fluidization velocity is lesser than 4% for medium and deep beds compared to the difference between medium and shallow beds (11%). The larger difference in U\({}_{\text{mf}}\) between medium and deep beds is due to the bubbles rising with almost similar upward velocity up to a specific bed height. However, as the bed height was further increased, the bubble rise velocity seemed slightly reduced due to more drag force experienced by all the bubbles hence more delayed bed surface rupturing. It is also pertinent to mention here that angular slots present in a 45\({}^{\circ}\) slotted plate and hybrid plate cause the lateral directional flow causing the reduction in an axial component of flow, hence reducing the upper rise velocity component of bubbles, so maximum U\({}_{\text{mf}}\) was observed for these two plates as compared to the perforated plate where the axial component of flow was maximum. The experimental observation with different initial sand bed heights can be seen in Figure 7.