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Local shear and skin friction on particles in three-phase fluidized beds

Identifieur interne : 001483 ( Main/Exploration ); précédent : 001482; suivant : 001484

Local shear and skin friction on particles in three-phase fluidized beds

Auteurs : A. H. Essadki [Maroc] ; I. Nikov [France] ; H. Delmas [France]

Source :

RBID : Pascal:05-0359493

Descripteurs français

English descriptors

Abstract

An extension of the electrochemical shear-rate measurement technique is carried out in this work to evaluate the friction force and the shear stress on a particle in two and three phase fluidized beds. Using this technique, the skin friction on a sphere has first been validated for single phase flow. In two- and three-phase fluidized bed, the significance and the direction of the velocity gradient at the wall are discussed. In the case of three phase fluidization, glass spheres (2mm in diameter, ρs = 2532 kg m-3) and plastic spheres (5 mm in diameter, ρs = 1388 kg m-3) were used. This choice provides very different bubbly flows due to different balances of coalescence and break-up of bubbles. The contribution of the frictional force is more important in "coalescent" fluidized beds than in "break-up" fluidized beds. The effect of gas injection is depending on the fluidized particle effect on bubble coalescence and break-up. Correlations have been developed linking frictional force to gas hold-up. The correlations recommended for frictional force in fluidized beds for both systems, (i.e., coalescence and break-up) are as follows: ● Glass spheres (2 mm diameter, coalescence regime): F = 2.43 Re0.052ε0.4g, standard deviation = 6%. ● Plastic spheres (5 mm diameter, break-up regime): F = 0.123 Re0.3ε0.1g, standard deviation = 4%. F is a dimensionless force defined by F = Ff/Pa, where Pa is the effective weight of the sphere. In the case of inverse fluidization, where the solid phase consisted of expanded polystyrene particles 5 mm in diameter (ρs = (350-550) kg m-3), the average frictional force remains almost unaffected by gas injection due to opposite effects on terminal particle velocity and on turbulence.


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Le document en format XML

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<term>Fluidization</term>
<term>Fluidized bed</term>
<term>Friction</term>
<term>Gas holdup</term>
<term>Gas injection</term>
<term>Glass</term>
<term>Hydrodynamics</term>
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<term>Contrainte cisaillement</term>
<term>Verre</term>
<term>Matière plastique</term>
<term>Ecoulement bulle</term>
<term>Coalescence</term>
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<term>Injection gaz</term>
<term>Corrélation</term>
<term>Analyse corrélation</term>
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<front>
<div type="abstract" xml:lang="en">An extension of the electrochemical shear-rate measurement technique is carried out in this work to evaluate the friction force and the shear stress on a particle in two and three phase fluidized beds. Using this technique, the skin friction on a sphere has first been validated for single phase flow. In two- and three-phase fluidized bed, the significance and the direction of the velocity gradient at the wall are discussed. In the case of three phase fluidization, glass spheres (2mm in diameter, ρ
<sub>s</sub>
= 2532 kg m
<sup>-3</sup>
) and plastic spheres (5 mm in diameter, ρ
<sub>s</sub>
= 1388 kg m
<sup>-3</sup>
) were used. This choice provides very different bubbly flows due to different balances of coalescence and break-up of bubbles. The contribution of the frictional force is more important in "coalescent" fluidized beds than in "break-up" fluidized beds. The effect of gas injection is depending on the fluidized particle effect on bubble coalescence and break-up. Correlations have been developed linking frictional force to gas hold-up. The correlations recommended for frictional force in fluidized beds for both systems, (i.e., coalescence and break-up) are as follows: ● Glass spheres (2 mm diameter, coalescence regime): F = 2.43 Re
<sup>0.052</sup>
ε
<sup>0.4</sup>
<sub>g</sub>
, standard deviation = 6%. ● Plastic spheres (5 mm diameter, break-up regime): F = 0.123 Re
<sup>0.3</sup>
ε
<sup>0.1</sup>
<sub>g</sub>
, standard deviation = 4%. F is a dimensionless force defined by F = F
<sub>f</sub>
/P
<sub>a</sub>
, where Pa is the effective weight of the sphere. In the case of inverse fluidization, where the solid phase consisted of expanded polystyrene particles 5 mm in diameter (ρ
<sub>s</sub>
= (350-550) kg m
<sup>-3</sup>
), the average frictional force remains almost unaffected by gas injection due to opposite effects on terminal particle velocity and on turbulence.</div>
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