Exploring mechanism of spout deflection in a spout fluidized bed

Abstract

Spout deflection has been widely encountered in spout fluidized beds and can seriously narrow the operating range of normal spouting. However, the underlying mechanism of spout deflection phenomenon remains unknown. In this study, two hypotheses are proposed to explain the spout deflection mechanisms in spout fluidized beds, and two virtual experiments are designed to verify these two hypotheses respectively using the Computational Fluid Dynamics-Discrete Element Method (CFD-DEM) approach. To verify Hypothesis I that the unsymmetrical particle fountaining, including particle spurting and particle falling, is one main reason for the spout deflection, virtual experiment I is designed where the particles are manually collected from the spurting surface and then symmetrically released to the free space above the two annulus regions to allow them to fall freely. Three symmetrical release patterns are compared, and the results confirm that the spout deflection disappears when the symmetrical particle fountaining is deployed. Moreover, two univariate tests are conducted to check the role of unsymmetrical particle falling and unsymmetrical particle spurting, respectively. The results further confirm the Hypothesis I. To verify Hypothesis II that the rheological properties of the annulus region also play a key role in determining whether the spout deflection happens, virtual experiment II is designed, in which the rheological properties of the annulus region are controlled by changing particle restitution coefficient, particle friction coefficient, background gas velocity, and particle diameter. The simulation results show that spout deflection tends to happen when the rheological properties in the annulus regions get better (i.e., larger particle restitution coefficient, smaller particle friction coefficient, larger particle diameter and larger background gas velocity). This work unveils the mechanism of spout deflection and is useful for process design and control in spout fluidized beds.