A new method to determine the characteristics of particle clusters in gas-solids circulating fluidized beds by using Phase Doppler Particle Analyzer (PDPA) was proposed. The influence of operating conditions on the structure of particle clusters in the dilute regime of circulating fluidized beds was investigated. Under the operating conditions in this study

both axial and radial distributions of the characteristics of particle clusters

such as time fraction

frequency

transient local voidage and axial average velocity

showed heterogeneous profiles. The radial profile was not affected greatly by solids circulation rate and showed a core-annulus structure. The axial profile changed much more significantly than the core-annulus structure when superficial gas velocity changed.