Two distinctive variational regions of radial particle concentration profiles in circulating fluidized bed risers

The radial profiles of particle concentration in circulating fluidized bed (CFB) risers experience actually a sequential variation as 'uniform (dense) -nonuniform-uniform (dilute), in response to the flow evolution from minimu m fluidization to pneumatic transport. Through measuring the lateral distri butions of solid concentration in the range epsilon(mf) similar to 1.0 of t he cross-sectionally averaged voidage, it was found that the first variatio nal stage, from dense uniform (minimum fluidization) to nonuniform, is comp leted via the rapid decrease of the particle concentration in the bed core, and the succeeding second stage, from nonuniform to dilute uniform (pneuma tic transport), is implemented via the quick decrease of the particle conce ntration near the bed wall. Based on this observation, general correlations were proposed to predict the local voidages on the bed wall (specifically, at the point where the dimensionless radius is equal to 0.99) and at the b ed center, which can be used for the entire spectrum of gas-solid fluidizat ion regimes prevailing in CFB risers. Further analyses demonstrated that su ch two variational regions are related also to the regime transition and th e lateral distribution features of the transient solid concentration fluctu ations. (C) 1999 Elsevier Science S.A. All rights reserved.