Effect of rheological properties on power consumption with helical ribbon agitators

Abstract

The influence of shear thinning and viscoelasticity on the power required for the mixing of viscous liquids using six different helical ribbon agitators has been investigated. Four Newtonian and 12 non-Newtonian fluids prepared using several polymers dissolved in varying concentrations in different solvents cover a wide range of rheologicalproperties. By a careful choice of test media, thespecific and combined effects of shear thinning and viscoelasticity on the power requirement have been examined. Simple models are proposed to predict the effective shear rate in the tank from the knowledge of the torque or power number. The effective shear rate predictions compared with the effectiveshear rate estimated using thescheme of Metzner and Otto (1957) show that they slightly depend on the shear thinning properties. Fluid's elasticity increases appreciably the power requirement, and departures from the generalized Newtonian power curve in the laminar regime are observed at smaller Reynolds numbers for viscoelastic fluids. Bottom wall resistance of the mixing vessel makes a negligible contribution to the power consumption.

Calculation of Metzner Constant for Double Helical Ribbon Impeller by Computational Fluid Dynamic Method

“…Flow patterns enhanced by DHR have been studies by many investigators, such as Bourne and Butler [27], Carreau et al [6]. However, Delaplace et al [4] did a survey on existing literature and found that, in spite of the different DHR impeller geometries, the primary circulation patterns are approximately the same, namely the liquid between the blades and the wall flows downwards, inwards along the bottom, upwards in the core near the shaft and radically outwards near the liquid surface of the tank.…”

“…Of all close clearance impellers, the helical ribbon impeller, due to its high mixing efficiency [3] and its unique feature of producing axial flow in a stirred tank [4], is widely chosen in mixing highly viscous Newtonian fluids and non-Newtonian fluids. In industrial applications, power consumption is one of the most important factors concerning designers [5][6][7][8]. For Newtonian fluids, theoretical and experimental studies [9,10] have shown that the power number N p is inversely proportional to the Reynolds number Re in the laminar flow:…”

Development of a mixer-viscometer for studying rheological behavior of settling and non-settling slurries

“…This is because either the narrow gap or geometry would not accommodate the particle size (Roos et al 2006), inability to maintain the particles suspended or inability to achieve the desired homogeneity. Vane or helical ribbon mixer geometry types have been proposed as potential candidates for slurry characterization (Roos et al 2006;Metzner and Otto 1957;Patterson et al 1979;Carreau et al 1993). …”

Flow characteristics of Newtonian and non-Newtonian fluids in a vessel stirred by a 60° pitched blade impeller

“…7% larger than the Newtonian glycerin mixture. It should be mentioned that another viscosity model which is more appropriate for CMC solutions is the simplified Carreau model [17] which was also used by [19] and may result in a better estimate of viscosity than that of the power law model. However, it was shown by [20] that the agreement between the measured and estimated (using the power law) values for the range of shear rate that used in this experiment was very good.…”

“…This flow transition changed the overall flow pattern in the mixing vessel from a double vortical structure to a single large vortex with a sudden reduction in power consumption by about 12% [10]; similar sudden drop in power was reported by the measurements of [16] who used a different power measuring technique to that used in the present study. Non-Newtonian fluid effects, such as shear-thinning and elasticity, on power consumption in mixing vessels stirred with other types of impellers than pitched-blade were investigated by [17] and [18] who used a helical ribbon and double planetary mixers, respectively. The former study showed that in the laminar flow there is no effect of elasticity on the power consumption at low Reynolds number, Re, while at the higher value of Re the fluid elasticity caused an increase in power consumption.…”