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

Abstract

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-ε 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 near-wall 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° bend with a blind pipe, which played a significant important role in vortex vibrations.