Experimental and numerical study of secondary flow in a T-type bend of a CFB riser

Periodic flow behavior was investigated in a T-type bend of a circulating fluidized-bed riser. With the geldart group A particle, experimental observations suggested that a periodic flow behavior formed under certain conditions. The gas velocity in the T-type bend was simulated using a renormalization group k-e turbulence model and measured by hot-wire anemometry. Flow patterns in the T-type bend were numerically studied under different superficial gas velocities. Preliminary results suggested that there was a pair of vortices moving periodically in the T-type bend, appearing, increasing, decreasing, and shedding in a cycle, which possessed partial characteristics of a Karman vortex street and Dean vortex. When the superficial gas velocity Vg was low (< 10 m/s), flow patterns presented periodic changes with a stable, symmetrical structure of two vortices. In contrast, when the superficial gas velocity reached a certain magnitude (10 m/s), the stable structure disappeared, switching to a pair of alternately swinging vortices; this process was demonstrated by a series of pressure contour measurements. Fourier transformation of continuous local static pressure data from two nearwall points allowed frequency analysis of static pressure fluctuations under different conditions. It was found that the main pressure frequency in the T-type bend increased with increased superficial velocity in the riser. Complex flow characteristics in the T-type bend were mainly due to its specific structure, including a 90 degrees bend with a blind pipe, which played a significant important role in vortex vibrations.