The effect of particle mass flow rate and annular channel thickness on the formation and characteristics of the bubbles is investigated experimentally by high-speed digital photography. A criterion is proposed to determine the emergence of the bubbles. The experimental results demonstrate that the particle mass flow rate and the annular channel thickness are both crucial factors for the bubble-formation process. The bubble size, bubble growth rates and bubbling frequency are investigated by analyzing a large number of images. The radial growth rate of the bubble is still governed by the superficial air jet velocity. In addition, the radial velocity difference between the gas and particle phase is confirmed to be the main cause of bubbling. (C) 2013 Elsevier Ltd. All rights reserved.