Mixing of viscoelastic fluids with helical-ribbon agitators. I — Mixing time and flow patterns

Batch mixing of viscous fluids with helical-ribbon agitators in 2.4 liter and 13 liter vessels has been studied for agitator speeds up to 200 RPM. Seven different agitators of different dimensions were employed in this work. Mixing times were measured using a decoloration technique and circulation times were determined by the tracer bead method. In addition, velocity profiles were obtained from streak photographs using selective illumination of the vessel and PVC powder as tracer particles. It was found that the mixing times of Newtonian fluids, which agreed with previously published data, were considerably (3 to 7 times) shorter than those of the viscoelastic fluids. The mixing time was strongly affected by the fluids' elasticity; increasing as the fluid elasticity increased. The velocity profiles were qualitatively similar for all the fluids but showed decreased axial circulation and increased circumferential flow as fluid elasticity increased. However, mixing is not only a function of the axial circulation (impeller pumping rate) but also is a function of the perturbations superimposed on the main flow. A simple, first approximation model based on the impeller geometry and flow patterns is proposed to correlate the circulation capacity and mixing time data for the various geometries studied. On a etudie, en employant des vitesses d'agitation atteignant 200 tours a la minute, le melange discontinu de fluides visqueux assure par des agitateurs a ruban helicoidal dans des vaisseaux de 2.4 litres et 13 litres; on a utilise a cette fin sept agitateurs differents de dimensions variees. On a mesure les temps de melange par une methode de decoloration et determine les temps de circulation au moyen d'une methode a base de perles marquees. On a, en outre, obtenu des profils de vitesse a partir de photographies avec stries, en utilisant un eclairage selectif du vaisseau et de la poudre de chlorure de polyvinyle qui servait pour les particules marquees. On a trouve que les temps de melange des fluides newtoniens, qui correspondaient a ceux qui ont deja ete publies, etaient bien plus faibles (3 a 7 fois) que les temps de melange des fluides visco-elastiques. L'elasticite des fluides a considerablement affecte le temps de melange, qui augmentait avec celleci. Les profils de vitesse se sont averes qualitativement semblables pour tous les fluides, mais ils ont indique une circulation axiale moindre et un ecoulement circonferentiel accru, a mesure que l'elasticite des fluides augmentait. Toutefois, le melange est une fonction non seulement de la circulation axiale, (taux de pompage du rotor), mais aussi des perturbations sur-imposees sur l'ecoulement principal. On propose un modele simple et fournissant une premiere approximation, lequel est base sur la geometrie du rotor et les types d'ecoulement, en vue d'etablir des correlations entre les resultats relatifs a la circulation et aux temps de melange, dans le cas des diverses formes geometriques etudiees.