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

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 (2 mm in diameter, ρ s = 2532 kg m - 3 ) and plastic spheres (5 mm in diameter, ρ s = 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 Re 0.052 ɛ g 0.4 , standard deviation = 6 % . • Plastic spheres (5 mm diameter, break-up regime): F = 0.123 Re 0.3 ɛ g 0.1 , standard deviation = 4 % . F is a dimensionless force defined by F = F f / P a , where P a 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 s = ( 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.