Tangential flow mixers

The forces applied by an impeller to the material contained in a vessel produce characteristic flow patterns that depend on the Impeller geometry, properties of the fluid, and the relative sizes and proportions of the tank, baffles and impeller. There are three principal types of flow patterns tangential, radial and axial. Tangential flow is observed when the liquid flows parallel to the path described by the mixer as illustrated in Figure 7. 

Figure 7. Tangential flow pattern generated by a paddle mixer.

Figure 1.115 Top photographs of vortex-flow pattern formation at various flow rates when contacting water and dyed water (acid blue) solutions in the tangential-flow cyclone micro mixer. Bottom comparison of the experimentally derived image of one vortex pattern with the predicted image by CFD simulation [131] (source IMM).

Turbine designs are intermediate between paddles and propellers. Turbines are effective mixers over a wide viscosity range and provide a very versatile mixing tool. The ratio of radial to tangential flow, the predominating parameters with this impeller, increases as the operating speed increases. Pitched-blade turbines are sometimes used to increase axial flow. Baffles must be used to limit swirling unless the turbine is shrouded. 

When the flow pattern in a mixed tank is primarily tangential, the fluid discharge from the impeller to the surroundings and its entrainment into the impeller are small. Also, fluid transfer in the vertical direction is at a minimum. The mixing effect is lowest when the rotational velocity of the liquid approaches that of the mixer. 

The material flow in an internal mixer is oriented vertically. On top of the internal mixer a more or less sophisticated feeding system can be found. Underneath the mixer a downstream device, such as a sheeting extruder with one or two tangential screws, or two roll mills are placed. 

An analysis of the flow behavior which is more or less confirmed by numerical simulations is presented by Leblanc. " The total flow in an internal mixer (tangential type) can be divided into a laminar, circulating flow which is pushed by the rotor tip, an axial directed flow, as an effect of the helix angle between axis and rotor wing and a leakage flow between rotor tip and mixing chamber waU (refer to Figure 35.6), or... 

Over the years various types of mixers have been designed. The most ffeqnently used today are the T-mixers (stopped-flow), four (or more)-jet tangential mixers (ffeeze-qnench), the Berger-ball mixer (stopped-flow) and the coaxial... 

Figure 2 Drawings of commonly used mixers in continuous-flow and stopped-flow devices. Mixing in the four-jet tangential mixers occurs in the orifice of the Pt inlay (MHQ) or in a Teflon gasket (RFQ). Ri and R2 denote reactant 1 and reactant 2, respectively...

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