It is important to understand the flow hydrodynamics behavior of a circulating fluidized bed (CFB) reactor for efficient operation. The objective of this research is to identify and characterize the flow regimes in a fluidized bed with different riser diameters. For this purpose, electrical capacitance tomography (ECT) combined with pressure measurement has been used to investigate the flow characteristics and flow regime transitions. Experiment was carried out in a cold gas solids fluidized bed to investigate the flow regimes and the transition velocities. Three risers of different diameter, 10, 12 and 15 cm, are designed and used for comparison. A twin-plane ECT sensor and a differential pressure transducer are used to obtain the solids volume fraction and differential pressure in the bottom region. Different flow regimes including bubbling, slugging and turbulent flow regime were formed with two distinct transition velocities. The flow characteristics are investigated in terms of solids volume fraction, bubble diameter and bubble rising velocity. The transition velocities are compared based on the ratio of the static bed height to bed diameter (H-st/D) and the measured position (i.e. the axial position and radial position). (C) 2014 Elsevier Ltd. All rights reserved.