Power Demand and Mixing Performance of Helical Ribbon Coaxial Mixers with Newtonian Fluids

Abstract

The power consumption and mixing performance of a coaxial mixer with different rotating speeds consisting of a helical ribbon (HR) agitator and four dispersion impellers (45° pitched blade impeller, Rushton turbine, 2-blade hydrofoil, and Pfaudler) in syrup solutions of different concentrations were evaluated in an elliptical dished-base cylindrical stirred vessel with a diameter of T=0.48 m and a filled aspect ratio H/T=1. The results show that stagnant zone is effectively eliminated and mixing time is significantly shortened with a coaxial mixer. The power consumption of HR is not affected by the inner dispersion impeller as greatly as the Anchor is, but the effect of the dispersion impeller increases with the increase of its diameter. On the other hand, the inner dispersion impeller is less affected by HR if the speed ratio RN between the high- and low-speed impellers increases. The power curves with different speed ratios fit well with each other when the modified definitions of generalized Reynolds number Re* and power number Np* are used considering the influence of the speed ratio. On the basis of the mixing time at a given power consumption, a Pfaudler-HR presents a better mixing performance than an Anchor as the outer agitator for a coaxial mixer.