Power Demand and Mixing Performance of Coaxial Mixers in Non-Newtonian Fluids

Experiments have been carried out in a transparent dished bottom stirred tank with a diameter T of 0.48m and liquid level of 0.6T. The power consumption and mixing performance of a coaxial mixer consisting of a wall-scraping anchor and different inner dispersion impellers (Rushton turbine, 45 degrees pitched blade turbine and CBY) operating in inner impeller-only, co- and counter-rotating modes have been experimentally characterized in viscous Non-Newtonian fluids (2% and 3% w/w CMC solutions) with different rheology behaviors. The results show that, for the co-rotating modes, the power consumption of the anchor could decrease up to 5% of that for the anchor rotating-only mode, whereas for the counter-rotating dispersion modes, it could increase to two times of that for the anchor rotating-only. However, the power consumption of the inner impellers is almost independent of the anchor rotation. We propose new correlations to give better fitted power curves between the generalized Reynolds number and the power number by considering not only the impeller geometry and the characteristic speed, but also the speed ratio. For each coaxial mixer, one reasonable power curve can be generated for different experimental speed ratios and different rotation modes. Impellers in co-rotating mode are more efficient than the inner impeller-only and the counter-rotating modes in the mixing of non-Newtonian fluids. Among the above three dispersion impellers, the power consumption of the CBY-anchor combination is the lowest compared with those for the other two combinations giving the similar mixing performance.