Electronic Thesis and Dissertation Repository


Doctor of Philosophy


Chemical and Biochemical Engineering


Zhu, Jingxu


Knowing the instantaneous flow structure is of great importance for the understanding of gas-particle interaction and for the prediction of reactor performances. In this thesis, a systematic and comprehensive study has been conducted on the instantaneous flow structure in a narrow rectangular riser (19 mm in thickness, 114 mm in width and 7.6 m in height), in a cylindrical riser (76 mm in diameter, 10.2 m in height) and in a cylindrical downer (50 mm in diameter, 4.9 m in height). A wider range of operating conditions has been achieved in risers and downer with superficial gas velocity from 3.0 to 9.0 m/s and solids circulation rate from 50 to 700 .

With high-speed imaging and optical fiber sensing, it has been found that there are crest clusters, coalesced particles, trough clusters and dispersed particles in CFBs. Crest clusters are surrounded by a cloud of coalesced particles, while trough clusters are immersed in dispersed particles. Then, instantaneous flow dynamics are computed with the tracking of image blocks. The existence of aggregations influences both the particle velocity and the solids flux. After that, a physically meaningful threshold was proposed to characterize crest cluster and trough clusters in terms of solids holdup, size and shape.

As the optical fiber probe can be used in high-density conditions, a discrimination method was also proposed for probe signals using wavelet transform. A thorough characterization of crest clusters, coalesced particles, trough clusters and dispersed particles is conducted in the rectangular CFB and their properties are consistent with those originated from the images. Moreover, how particle properties influence phase characteristics was also investigated by comparing phase information formed by FCC and glass beads.

Finally, probe signals captured in the cylindrical riser and cylindrical downer are processed to investigate the phase properties in high-density conditions. Phases, including crest clusters, coalesced particles, trough clusters and dispersed particles, are characterized over the riser and downer in terms of length, frequency and time fraction. With the systematic comparison of flow information between the CFB riser and downer, it has been found that aggregation in the CFB downer is less severe than that in the CFB riser.