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Axial hydrodynamics

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  1. Introduction
  2. The characteristics of fluidization regimes
  3. The behavior of a high velocity circulating fluidized bed
  4. Extending entrainment models developed originally for bubbling fluid beds
  5. Findidngs
  6. Comparing hydrodynamic characteristics among different flow regimes of gas fluidized beds
  7. Conclusion

Many studies have appeared in the literature dealing axial solids distribution along CFB risers (e.g. Yerushalmi et al, 1976: Kwauk et al, 1986: Arena et al 1991, Bai et al 1992). It is most often represented as either a plot of the radially averaged solids suspension density or voidage versus height. This profile is experimentally determined from the pressure distribution along the riser, recognizing that the suspended solids constitute the major contributor to the pressure drop.

The experimental work of Yerushalmi et al (1976) is generally considered as the pioneering academic study of the axial solids distribution in circulating fluidized bed riser. They discussed the densification at the riser base and proposed several advantages of operating in the fast fluidization regime.

[...] Axial profiles of solids volume fraction ) are directly measured by means of quick closing valves and compound to those estimated from pressure drops. Bed blow out experiments are carried out by suddenly turning off solids control valve to find out instantaneous solid mass flux. They have also provided a model by taking into account of solids concentration profiles ) as the result of diffusion/segregation mechanism with uniform particle cluster size through out the columns to determine the derivatives of S-shaped axial voidage profiles at the theory of elutriation, explicitly takes into account the effect of the size of the column, the flux of circulating solids and the solids particle densities. [...]

[...] Values for the limiting voidage and the axial location of the interface were determined from previously published correlations; Fujima et al (1991) modeled the axial voidage profiles as due to ?chain growth? or cluster formation. Choi et al (1991) developed empirical correlations to predict the average axial voidage profiles. Rhodes et al (1991) studies particle motion at the wall of a riser in a cold model CFB using both high-speed video and optic fiber probes. They observed that the particles at the wall were descending in the form of sworms having voidages in the range of 0.8 to The sworms resulted from the transport of solids from the core of the riser. [...]

[...] One of the authors M.Kwauk, presented a semi-empirical formulation for the axial voidage distribution on the basis of the cluster diffusion and segregation model. Avidan A.A and Yerushalmi.J (1985) proposed a fluidization map depicting the slip velocity versus the solid volumetric concentration at the bottom section of the bed. They concluded that the slip velocity provides more relevant measures of the interaction between gas and solid and the formation of clusters in FFB is responsible for higher slip velocities. They have not examined that the possibility of annular flow formation by which the observed slip velocity might have been explained without curing the concept of clusters. [...]

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