Flow patterns in rheologically evolving model fluids produced by hybrid dual mixing systems

The flow patterns produced by two dual mixing systems composed of independe ntly driven impellers were studied. The dual impellers included a turbine r otating at high speed (Rushton or Smith) and a slowly rotating helical ribb on agitator (HR). Visualizations and power input were used to evaluate mixi ng performance. The influence of the rotational speed ratio on the flow pat terns was evaluated. For high shear-thinning fluids, N-T/N-HR modifies the flow patterns considerably. Three typical behaviors were found with shear t hinning fluids: segregation of two principal flow patterns (N-T/N-HR < 10), turbine dominance (N-T/N-HR > 10), and a well-distributed flow pattern thr oughout the tank (N-T/N-HR = 10). For low-viscosity fluids, the motionless HR reduced the vortex length and the T-HR systems eliminated vortex when th e impellers rotated in opposite directions at N-T/N-HR = 10. Finally, a rel ationship between the dimensionless vortex length and the Froude number is proposed for individual turbines as well as for the turbine-motionless HR s ystems.