A Comprehensive Characterization of Aggregative Flow in a Circulating Fluidized Bed (2): High-Density Downer

In a circulating fluidized bed (CFB) downer, gas and particles flow concurrently downward, and the flow behavior is dependent on two operation parameters, namely, the superficial gas velocity and solid circulation rate. In the literature, the solids circulation rate for a CFB downer is usually below 200 kg/m2s. However, a dense suspension is highly desired to enhance the reactions in a downer. In our laboratory, high solid circulation rates, up to 700 kg/m2s, have been achieved in a large-scale CFB downer for the first time, and the instantaneous solids holdup has been measured over the downer using an optical fiber probe. In this work, aiming to explore the dense suspension, the instantaneous flow structure was thoroughly investigated by analyzing the details of probe signals. According to signal characteristics, essential phases, namely, crest clusters, trough clusters, coalesced particles, and dispersed particles, have been discriminated and quantified in perspectives of solids holdup, length, time fraction, and frequency. Then, phase properties were discussed in terms of radial distributions, axial distributions, and trends with operating conditions. Finally, a comparison has been conducted between the high-density riser and high-density downer.