Stirrer power characteristics, viscoelastic

Figure 11 Power characteristics of a Rushton turbine stirrer under given geometric conditions, measured in two differently scaled vessels (scale 1 2) and fitting the flow behavior of the viscoelastic fluid [polyacrylamide (PAA) solution] by changing its viscosity. Source From Ref 13.

Some fermentation broths are non-Newtonian due to the presence of microbial mycelia or fermentation products, such as polysaccharides. In some cases, a small amount of water-soluble polymer may be added to the broth to reduce stirrer power requirements, or to protect the microbes against excessive shear forces. These additives may develop non-Newtonian viscosity or even viscoelasticity of the broth, which in turn will affect the aeration characteristics of the fermentor. Viscoelastic liquids exhibit elasticity superimposed on viscosity. The elastic constant, an index of elasticity, is defined as the ratio of stress (Pa) to strain (—), while viscosity is shear stress divided by shear rate (Equation 2.4). The relaxation time (s) is viscosity (Pa s) divided by the elastic constant (Pa). 

The power characteristic for viscoelastic fluids (Reher, 1969 Schiimmer, 1970) can also be described accurately by a relationship of the type Ne (Re ). Additional parameters to account for viscoelastic properties are needed only when the ratio of tangential to normal stresses is less than two (Reher, 1969). Power characteristics for the turbine stirrer in aerated non-Newtonian fluids (carboxymethylcellulose [CMC] and polyacrylamide [PAA] solutions) have been presented by Hocker and Langer (1962). 

The mixing power of a Rushton turbine with 6 flat blades and with 6 blades concavely curved in the direction of rotation was measured in pure and in gassed liquid of Newtonian and non-Newtonian (pseudoplastic and viscoelastic) behaviour [660]. The results were plotted as P/Pq = /(Q) and proved that the concave turbine used only 1/5 to 1/3 of the power at the same Q. In [709] power characteristics of turbine stirrers wirh 6 different shapes of concave blades are presented in the same form P/Pq = f Q). 

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