An experimental study of the mixing characteristics of viscoplastic fluids in dual-impeller agitation systems

We study experimentally the agitation of viscoplastic Carbopol™ 980 (C980) fluids in a 2.5-L vertical unbaffled cylindrical vessel using central dual-impeller stirring systems, examining the mixing characteristics of different combinations of standard six-blade Rushton turbines (RTs) and downward-pumping 45°-pitched four-blade turbines (PBTs) of identical diameter D = 41 mm. We consider the effects of rotational speed (as expressed by the modified power-law Reynolds number, Re m ), impeller separation ( G / D = 0.73 and 1.22) and a variety of arrangements of the rotors on the mixing characteristics of these systems. Phenomena of cavern segregation and internal flow compartmentalisation are revealed and explained in terms of expected flow patterns. Dual-RT stirrers are found to produce highly symmetrical flow patterns, influenced by the impeller separation, along with strong time-dependent compartmentalisation between the internal flows of both caverns. This flow compartmentalisation can be, however, avoided in dual-PBT systems due to the downward-pumping nature of this system, which also allows for the achievement of a state of full tank homogeneity through cavern engulfment. Comparing the total equivalent volumes of the caverns, V c tot , for the examined dual-impeller systems, the mixing effectiveness, as characterised by the growth of V c tot with Re m , is maximised for the RT-PBT and PBT-RT arrangements with a separation of G / D = 1.22, for which V c tot is 70–100 % larger at Re m = 18.5 compared to the dual-RT and dual-PBT arrangements. Although the RT-PBT arrangement performs well, it does not achieve a homogeneous mixing state of the viscoplastic fluid throughout the entire vessel within a reasonable time. Thus, the most efficient mixing system is that featuring a PBT rotor overlying a RT. The findings demonstrate the significant influence of the selection and geometrical arrangement of dual-impeller systems on the mixing of viscoplastic fluids. • Effects of rotational speed, impeller separation and rotor arrangements considered. • Dual-RT stirrer produces strong compartmentalisation between flows in two caverns. • Dual-PBT system allows full tank homogeneity through cavern engulfment. • RT-PBT impeller does not provide complete homogeneous mixing within a reasonable time. • Most efficient mixing system is that featuring a PBT rotor overlying a RT impeller.