Cavern Sizes in Agitated Fluids With a Yield Stress

Abstract

Highly viscous, non-Newtonian Xanthan gum solutions and two transparent model fluids with similar rheological properties have been studied under aerated (up to I vvm) and unaerated conditions in a 0.29m diameter agitated vessel. Rushton disc turbines of size 113 and 112 of the tank diameter have been used alone and also in conjunction with 6-bladed, 45°-pitch axial Row turbines of the same size at speeds up to 24 rev/s. enabling specific power inputs of up to 15 W/kg to be imparted.Flow patterns were studied by flow visualisation and hot film anemometry. When the fluids have a yield stress, the fluid divides into a turbulent well-mixed cavern which increases in size with increasing speed with the remainder stagnant. A model for the size of the cavern fits the experimental data well lor both aerated and unaerated mixing. Large diameter combinations produce good mixing at about I to 2 W/kg which is about 113 to 114 of that required with small diameter combinations. Single disc turbine impellers are unsatisfactory.

Biomass processing into ethanol: pretreatment, enzymatic hydrolysis, fermentation, rheology, and mixing

“…Solomon et al [137] proposed a mathematical model for mixing cavern, assuming a spherical shape centered on the impeller. Hot film anemometry and Xanthan gum were used in the experiments.…”

Mixing in the Fermentation and Cell Culture Industries

“…Solomon et al (1981a) proposed a physical model to estimate cavern sizes based on a torque balance to predict its diameter, D c . They assumed that the cavern was spherical, that the predominant motion at the cavern boundary was tangential in nature (applicable to radial flow impellers), and that the stress imparted by the impeller at the cavern boundary was equal to the fluid yield stress.…”

Onion-Like Structure of Viscoelastic Surfactant Solution Flow Induced by 4-Blade Paddle Impeller in a Vessel

“…e onion-like structure observed in Figure 3(b) essentially indicates the same phenomenon as that in the case of a uid lump spiral. e characteristic uid ow near the impeller was similar to "cavern" that is frequently seen when a yield stress uid is agitated (Solomon et al, 1981). However, the uid outside the onion-like structure owed moderately because the solution had a low and constant viscosity, while the uid outside the cavern was completely stagnant owing to the yield stress.…”