The dynamic data of particle concentration in the jet mixing zone of FCC risers was obtained by a PV-6D optical fiber probe in a large scale cold model device. To describe the cluster characteristic, the time fraction of cluster was defined.It is a ratio of cluster existence time with total sampling time. The distributions of cluster time fraction in the jet mixing zone were obtained by using the method of aggregate identification in the fast fluidized bed. Comparison between concurrent and counter-current feed injection structures was made. Results show that the fluctuations of solids hold-up is very large in the main flow influence region and secondary flow influence region of concurrent feed injection structure. Besides, a high time fraction of cluster phase can be obtained. This is mainly because of the effect of jet secondary flow. On the contrary, in the counter-current feed injection scheme, there is no significant fluctuation of solids hold-up, indicating a uniform mixing of feed jets with catalyst particles. Moreover, the time fraction of cluster decreases obviously, which will promote to the contact and reaction between feed injection and catalyst particles.
