On the effects of viscoelasticity in stirred tanks

Özcan-Taskin, N. Gül (1993). On the effects of viscoelasticity in stirred tanks. University of Birmingham. Ph.D.

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Abstract

Mixing viscoelastic fluids is common to many chemical and biochemical process industries where the rheological properties of the bulk change considerably over the time course. The objectives of this study were to investigate the effects of viscoelasticity in mechanically agitated vessels (on: i- the power consumption and flow patterns in single phase and gassed systems, ii- mixing time under unaerated conditions and iii- cavities in the presence of gas) and to study the performance of InterMIGs in comparison to the classical six bladed disc turbines. Model viscoelastic fluids prepared exhibited only slight shear thinning properties (Boger fluid type), hence the effects of viscoelasticity could be studied in the absence of other rheological properties. Results obtained with these fluids were compared to those with viscous Newtonian glycerol covering the transitional flow regime (50< Re< 1000). Additionally, some work was also conducted in water for a preliminary characterisation of InterMIGs.

In the relatively low range of Elasticity numbers (El < 3.5 x 10\(^-\)\(^3\)) covered, secondary flow patterns were not reversed. The power drawn under unaerated conditions was higher in viscoelastic fluids (at a given Reynolds number) for both impeller types that had to compete with mutually opposing viscoelastic forces.

An experimental set-up to measure mixing times in viscous fluids (using the fluorescent dye-fibre optic technique) was installed. Reduced secondary circulations in viscoelastic fluids resulted in longer mixing times.

Power consumption under aeration was also higher in viscoelastic fluids than that in Newtonian glycerol. Different from the findings under unaerated conditions, this enhancement was independent of the level of viscoelasticity. Cavities, hence the power drawn under aeration, were in general stable with respect to the variations in the gas flow rate in viscous fluids. This stability was found to be accentuated by viscoelasticity.

InterMIGs underwent viscoelastic effects more severely on account of the complicated interaction of the viscoelastically driven flows with the flows associated with the inner and outer blades of these impellers. They presented a better choice in low and high viscosity Newtonian fluids and their performance was comparable to that of a single Rushton turbine in viscoelastic fluids.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

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