Powder Technol.

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 minimum fluidization to pneumatic transport. Through measuring the lateral distributions of solid concentration in the range epsilon(mf) similar to 1.0 of the cross-sectionally averaged voidage, it was found that the first variational stage, from dense uniform (minimum fluidization) to nonuniform, is completed via the rapid decrease of the particle concentration in the bed core, and the succeeding second stage, from nonuniform to dilute uniform (pneumatic transport), is implemented via the quick decrease of the particle concentration 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 bed center, which can be used for the entire spectrum of gas-solid fluidization regimes prevailing in CFB risers. Further analyses demonstrated that such two variational regions are related also to the regime transition and the lateral distribution features of the transient solid concentration fluctuations. (C) 1999 Elsevier Science S.A. All rights reserved.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 minimum fluidization to pneumatic transport. Through measuring the lateral distributions of solid concentration in the range epsilon(mf) similar to 1.0 of the cross-sectionally averaged voidage, it was found that the first variational stage, from dense uniform (minimum fluidization) to nonuniform, is completed via the rapid decrease of the particle concentration in the bed core, and the succeeding second stage, from nonuniform to dilute uniform (pneumatic transport), is implemented via the quick decrease of the particle concentration 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 bed center, which can be used for the entire spectrum of gas-solid fluidization regimes prevailing in CFB risers. Further analyses demonstrated that such two variational regions are related also to the regime transition and the lateral distribution features of the transient solid concentration fluctuations. (C) 1999 Elsevier Science S.A. All rights reserved