## 4 Results and Discussion  
### Effects of Material Mass and Particle Size on Flow Behavior  
As shown in Figure 5, the 2 nd row and 2 nd column of the data in Table 3 are taken to display the effect of the material mass and the particle size on the flow behavior, and the experimental numbers are 06/07/08/09/10 and 02/07/12/17/22, respectively. A series of vertical dotted lines on the left side of Figure 5 represent \(U_{\text{0 mf}}\) of each experiment, and their colors and markers are the same as those of the curves of the corresponding experiments.  
As shown in Figure 5a, when \(U_{0}<U_{\text{0 mf}}\), with an increase in \(U_{0}\), \(\Delta P\) continues to increase linearly; when \(U_{0}>U_{\text{0 mf}}\), \(\Delta P\) stops growing and enters a stable stage. In addition, the higher the material mass, the greater \(\Delta P_{\text{mf}}\) is. The material mass has little effect on \(U_{\text{0 mf}}\); with an increase in the material mass, \(U_{\text{0 mf}}\) increases slightly. As shown in Figure 5b, when \(U_{0}<U_{\text{0 mf}}\), the particles with a larger particle size have a lower growth rate. When \(U_{0}>U_{\text{0 mf}}\), the experiments have similar \(\Delta P_{\text{mf}}\) values. When \(U_{0}\) reaches a certain velocity, experiments 17/22 show significant \(\Delta P\) growth, while the other experiments show little or no increase. We suppose the possible reason is as follows. When the gas velocity is high enough, the fine particles move to the upper part of the fluidized chamber and collide with the top surface. Assuming that all the particles are a whole body, this whole body is now subjected to the downward gravity force and the downward support force exerted by the top surface, so the pressure drop continues to increase because the required gas flow drag force becomes larger. For experiments with larger particle sizes (experiment 02/07/12), the particles cannot be brought high enough to collide with the top surface. Particle size has a significant influence on \(U_{\text{0 mf}}\); as the particle size increases, \(U_{\text{0 mf}}\) increases significantly.  
\(U_{\text{0 mf}}\) and \(\Delta P_{\text{mf}}\) obtained in all experiments are shown in Table 4. 3D views of the particle size and material mass versus \(U_{\text{0 mf}}\) and \(\Delta P_{\text{mf}}\) are plotted to facilitate the observation of the variation regularity in Figure 6.  
Figure 5: \(\Delta P\) at different \(U_{0}\) values of (**a**) the 2 nd row and (**b**) the 2 nd column of experiments in Table 3.  
Table 4 and Figure 6 show that for \(U_{\rm 0 mf}\), the main influencing factor is the particle size; the larger the particle size, the larger \(U_{\rm 0 mf}\) is. The influence of the material mass is small, and \(U_{\rm 0 mf}\) increases slightly with an increase in the material mass. For \(\Delta P_{\rm mf}\), the main influencing factor is the material mass, and \(\Delta P_{\rm mf}\) is approximately proportional to the material mass. The influence of the particle size is small and complex; for the same material mass, \(\Delta P_{\rm mf}\) of #1-#4 particles is almost the same, and \(\Delta P_{\rm mf}\) of #5 particles is 2-13% larger than the former.