Particle velocity characteristics of dilute to moderately dense suspension flows in stirred reactors

Measurements of particle mean and r.m.s. velocity were obtained by laser-Doppler velocimetry in solid-liquid turbulent flows in fully baffled stirred reactors driven by Rushton-type impellers of different sizes at rotational speeds of 150, 300 and 313 rpm. The effects of particle size, density and volumetric concentration were investigated. The maximum particle concentration at which the solid-phase velocity measurements could be made was improved from 0.02 to 2.5% when the refractive index of the continuous-phase was matched to that of the dispersed particles. The results showed a steep particle concentration gradient in the vertical direction below the impeller and a mild one above the impeller, that the particles lagged or led the bulk fluid when the flow direction was upwards and downwards, respectively, and that the particle turbulence levels were in general lower than those of the single-phase flow levels, especially in the impeller stream and wall jet regions. Particle velocities decreased with an increase in particle concentration, while the particle turbulence levels remained the same. The apparent relative velocity of glass particles was higher than that of Diakon by up to 2.5 times and the effect of the particle size, at least for the sizes used in the experiment, was negligible.