Flow structure in a fast fluidized bed

Flow structures were characterized in a 97 mm diameter by 3.0 m high circulating fluidized bed based on fluctuation behaviors of solids momentum. Measurements of time series of local solids momentum were carried out along the radial direction at both upper and lower parts of the bed for different operating conditions. By examining the probability distribution, standard deviations and power spectra, flow behavior in the upper dilute region was found to be much different from that in the bottom dense region. The results suggest that a core-annular structure does not always mean the flow with downflow of solids near the bed wall, it also includes the flow in which solids travel, on the time average, upwards rapidly in the dilute core region and much more slowly in the dense annular region. By changing the experimental conditions, both flow patterns were identified based on the measured signals from different radial positions. To further understand the completed hydrodynamics of gas-solids flow in the fast fluidized beds, a novel approach, based on the concept of fractals, was also adopted. Attempts were made to characterize the local flow properties by using the fractal dimension.