The behavior of the solid phase in the upper zone of a circulating fluidized bed riser was studied using a phase Doppler anemometer,Glass particles of mean diameter 107 μm and superficial gas velocities Ug covering the turbulent and the beginning of the fast fluidization regime were investigated.Three static bed heights were tested.Ascending and descending particles were found co-existing under all operating conditions tested,and at all measurement locations.Superficial gas velocity proved/happened to have a larger effect on descending particles at the wall and on ascending particles in the central region.Transversal particle velocities in both directions (toward the center and toward the wall) behaved relatively equivalently,with only slight difference observed at the wall.However,observation of the number of particles moving in either transversal direction showed a change in bed structure when increasing Ug.Furthermore,a balance was constantly observed between the core zone and the annulus zone where the mutual mass transfer between these two zones occurred continuously.Transition from a slow to a fast particle motion was accompanied by a transition to high levels of velocity fluctuations,and was found corresponding to the appearance of significant solid particle flow rate.