Concept of Swirling and Vortex Formation in Agitated Vessel

Concept of Swirling and Vortex (Unbaffled Tanks)

• If a low viscosity liquid is stirred in an unbaffled tank by a centrally mounted agitator, there is a tendency for a swirling flow pattern to develop, for the lighter fluid (usually air) to be drawn in to form a vortex at the surface of the liquid. This reduces the degree of agitation and mixing.

• The above said phenomenon takes place in unbaffled tanks regardless of the type of impeller. A typical flow pattern in an unbaffled tank for either axial or radial flow impeller is shown in Fig.

• In vortexing, the surface of the liquid takes roughly U-shape and efficient mixing no longer takes place. A vortex is produced owing to the centrifugal force acting on the rotating liquid.

• Thus, there is a limit to the rotational speed that may be used, since once the vortex reaches the impeller, severe air entrainment may occur (air may be sucked in). In addition to this effect, the swirling mass of liquid generates an oscillating surge in the vessel which when coupled with the deep vortex may create a large fluctuating force acting on the agitator shaft.

Fig. a : Vortex formation and circulation

 pattern in an unbaffled agitated vessel

Fig. b : Vortex at very high

 impeller speed

Prevention of Swirling and Vortex Formation 

• There are three methods for the prevention of swirling and vortex formation : 

 (i) Off-centre mounting of the impeller. 

 (ii) Use of baffles. 

 (iii) Use of diffuser ring with turbines. 

Fig. c : Propeller‚ off-centre

and angular (unbaffled)

Fig. d : Side entering propeller

• In small vessels, the impeller can be mounted off-centre as shown in Fig. In larger vessels, the agitator may be mounted in the side of the tank with a shaft in horizontal plane but at an angle with radius, as shown in Fig.

• In large vessels with vertical agitators, the most common method of reducing swirling is to install baffles along the side of the vessel, which hinder rotational flow  without disturbing radial or longitudinal flow.  

• In an unbaffled vessel, there are strong tangential flow currents and vortex formations at moderate speeds but in the presence of baffles, the vertical flow currents are  increased and there is more rapid mixing of the liquid.  

• With side entering, inclined and off-centre propellers, baffles are not needed.

Fig. e : Flow pattern with

turbine impeller in baffled tank

Fig. f : Flow pattern in a baffled tank

with centrally mounted propeller agitator

• In case of turbines, the principal currents are radial and tangential. The tangential components induce (lead to) swirling and vortexing that must be stopped by the  baffles or by the use of diffuser ring for impeller to be most effective.