Power consumption of a dual turbine–helical ribbon impeller mixer in ungassed conditions

Abstract

Ungassed power measurements in a dual coaxial mixer composed of a helical ribbon and a Rushton turbine were carried out in laminar mixing conditions for Newtonian and non-Newtonian shear thinning fluids. For the Newtonian case, the power draw constant K p for the hybrid geometry was not the sum of the individual impellers. This was explained by considering the radial discharge flow in the turbine region as well as the top-to-bottom circulation pattern of the helical ribbon impeller. For the non-Newtonian fluids, the results showed that, at a given Reynolds number, power consumption decreases as the shear thinning behaviour increases. A dimensionless and unique representation of the power draw data was obtained by shifting the non-Newtonian power draw results to the Newtonian curve. This was carried out with a K s function defined from the K p( n) data. The predictions for K s were found to be in good agreement with those obtained using the classical method of A.B. Metzner and R.E. Otto (AIChE J., 3 (1957) 3–10). It was observed that pseudoplasticity tends to shift the upper limit of the laminar region toward Reynolds numbers higher than 10.