Core-annular flow

Figure 10.16. Illustration of the definition of intermittency index for three limiting cases (after Brereton and Grace, 1993a) (a) Ideal cluster flow (b) Core-annular flow (c) Uniform dispersed flow.

Models Based on the Core-Annular Flow Structure... 

Dc Diameter of the central core region in the core-annular flow Lz Length of the vertical section of the L-valve... 

In the core-annular flow model, the mass balance of the solid phase can be expressed as... 

Aul and Olbricht [4] reported the results of an experimental study of low-Reynolds number, pressure-driven core-annular flow in a straight capillary tube. The annular film was thin compared to the radius of the tube, and the viscosity of the film fluid was much larger than the viscosity of the core fluid. The photographs showed that the film was... 

Bannwart, A. C. (2001). Modeling aspects of oil—wattn core—annular flows. Journal of Petroleum... 

Blyth MG, Luo H, Pozrikidis C (2006) Stability of axisymmetric core-annular flow in the presence of an insoluble surfactant. J Fluid Mech 548 207-235... 

HYDRODYNAMIC MODELING, 278 Radial Gas Velocity Profiles, 278 Gas Mixing, 280 Radial and Axial Dispersion, 281 Core-Annular Flow, 284 Contact Efficiency, 285... 

Advances in Engineering Fiuid Mechanics Core-Annular Flow... 

Flow nature Bubbles and emulsion phase Dispersed dilute phase and dense phase Core-annular flow, particle accumulation near wall and particle clustering Dispersed dilute flow with a thin particle layer on the wall... 

Various core-annular models have been developed to describe the gas-solid flow (Horio et al., 1988 Bai et al., 1995 Bolton and Davidson, 1998). The main difference among these models lie in the degree of complexity and in the assumptions associated with simplifications. Core-annular flow structures become dominant in the upper dilute region. Thus, when the dilute flow is predominantly present in the riser, models based on core-annular structures can be applied to reactor models. Kunii and Levenspiel (1990) extended the conventional fluidized bed model (a dense lower region coupled with a freeboard upper region) to cir-... 

Most approaches assume a sigmoidal voidage profile and/or perfect core annular flow as noted above, these are oversimplified. 

A phenomenon encountered with non Newtonian mixtures is a tendency for the low-viscosity constituent to migrate to regions of high shear and to lubricate the flow. One example is the core annular flow of crude oil in water, where the more viscous material is lubricated by the less viscous material. In the case of emulsions and certain non-Newtonian slurries, lubrication occurs by a slip layer of water on the wall. 

Figure 9. Idealised clustering and core-annular flow patterns and the corresponding plots of intermittency index versus radius (Brereton and Grace, 1992b).

Figure 11 allows us to draw some important conclusions about the true nature of the CFB fluid mechanics. From it we can infer that the overall flow structures are as shown in Figure 13. A core-annular flow structure dominates, with particles carried up in the central core and travelling down at the column walls. Along the height of the unit there is a net particle transfer from core to annulus which creates the decrease in overall bulk density with height. Superimposed upon the internal structure is a net flux through the unit which, depending upon the particles, gas velocity, solids flux, and exit employed, may be large or small compared to the net internal circulation. Typically, it is desirable that it be small to assure temperature uniformity. However, in reactions where plug flow of solids is desirable, this may not be the case.

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