Gas-solid hydrodynamics of Geldart D particles in a tapered fluidized bed by ECT measurement

Tapered fluidized beds have gained increasing attention in industrial applications due to their superior ability to process particles with wide size and density distributions, effectively alleviating issues such as incomplete mixing and excessive entrainment of fines. In this study, the gas–solid hydrodynamics of a lab-scale tapered fluidized bed were investigated using rice particles as a representative Geldart D material. A dual-plane Electrical capacitance tomography (ECT) sensor and pressure fluctuation measurements were employed to characterize flow behavior over a range of gas velocities. The results reveal that the tapered geometry induces a distinct flow structure compared to conventional cylindrical beds. Both void fraction and bubble size decreased with height, in contrast to the typical bubble growth observed in straight-sided beds. The most significant reduction occurred at the gas velocity of 3U_mf, with a 47 % decrease in void fraction and an 11.7 % reduction in bubble diameter. Transient slugs occasionally appeared at the lower measurement plane but were absent at the upper plane, indicating effective suppression of slug propagation and improved flow uniformity. As the gas velocity increased from 1.5U_mf to 2.5U_mf, the power spectra of pressure signals shifted from broadband to narrowband with a pronounced peak at 3.8 Hz, then broadened again at 3U_mf, reflecting the attenuation of slugging behavior in the tapered bed. Hilbert-Huang Transform analysis further confirmed intensified gas–solid interactions and flow regime transitions with increasing gas velocity. These findings provide new insights into the hydrodynamics of tapered fluidized beds and offer practical guidance for their design, operation, and industrial-scale application.