Now, the difficult part of understanding mixing: a strong vortex isnt necessarily a good thing. If the shaft of the mixer, when viewed from above, points straight across the tank, strong rotational flow will occur. With this installation, a vortex will form around the rotating shaft. Mixers typically rotate clockwise when looking from behind the motor, so that rotation is transferred to the fluid. The result is clockwise rotational flow in the tank. What is wrong with rotational flow? If the objective of a process step is to add a powdered ingredient to the liquid, strong surface motion with a vortex may help. If the object is to achieve uniform mixing from the top to the bottom of the tank, rotational flow isnt the best solution.
Figure 2. Particles are poorly distributed on one side of the tank with the shaft aimed straight across the tank.
Figure 2 shows the drawbacks of rotational motion. While it looks as if the surface of the fluid is moving well, particles in the top-view photograph swirl around the outside of the tank. The side-view photo shows that most of the settling particles are on one side of the tank. The side-view photo also shows a small vortex on the surface above the impeller location. Unfortunately, any still picture of agitation shows only a part of the story, because agitation is an active process. These still photos dont adequately show the rotational swirl and random patterns of the actual mixing.
Greater rotational speed or a larger impeller will draw a deeper vortex from the liquid surface. When the vortex reaches the impeller, air will be drawn into the fluid. The entrained air may not only cause foaming or bubbles in the product, but the air disrupts the liquid flow through the impeller and reduces the effectiveness of the mixing. A strong vortex reaching the impeller is usually a sign of poor mixing.
Because the impeller creates axial flow, aiming the mixer shaft far to the right will create a rotational flow in the opposite direction of the inherent swirl (Figure 3).
Figure 3. Induced motion reverses the direction of rotation compared with inherent motion.
The top-view photo shows that particles are driven from the side where the impeller is located primarily because the dished bottom redirects the axial discharge flow. The side view shows that the settling particles are driven toward the surface, but only on the side away from the mixer. While creating some vertical motion with this location, the uniformity of mixing is unsatisfactory.
Achieving a compromise between clockwise and counterclockwise motion seems like a good objective for both side-to-side and top-to-bottom motion. Angling the shaft to the right just enough to counter rotational flow produces an effective bulk mixing pattern (Figure 4).
Figure 4. Moderate orientation towards the right, improves vertical mixing.
In the graphic pattern, the circles with dots in them represent the heads of arrows for flow coming toward the surface with arrows going away from them representing the flow moving across the surface. The arrows pointing toward circles with crosses in them represent the flow across the surface and the feathers on arrows of flow going down into the tank. The top-view photo shows how particles are well distributed across the tank, while the side-view photo shows settling particles well suspended into a large portion of the batch.
Of course, pointing the mixer to the left will further enhance the inherent rotational flow shown in the first set of figures. A strongly rotational flow sweeps nearly all of the particles to the opposite side of the tank and leaves them on the bottom (Figure 5).
Figure 5. Tilting the shaft to left may improve agitation of viscous liquids or the draw of solids from the surface of the liquid at the expense of uniform mixing.
The side view photo shows a strong vortex leading from the surface almost down to the impeller. Heres a bit of irony: the stronger the vortex becomes, the poorer the mixing. Sometimes the vortex helps draw powders from the surface, but it wont mix them well. This enhanced-motion position also may provide mixing for more viscous fluids.
Perhaps the most difficult concept to grasp about the mixer location is that the best overall mixing motion has a surface that shows the least swirling motion. The surface motion with the best mixing shows a slight upward boil where the flow is up along the side of the tank and the motion across the surface is from side to side or even random.
Remember that violent surface motion may be deceptive. When trying to develop viscosity or create an emulsion, the vertical motion with the portable mixer will reduce the chances of vortex formation and air incorporation. To handle a more viscous fluid, a larger mixer may be needed. If the rotational flow creates a vortex, small air bubbles will be dispersed in the fluid. Small air bubbles are difficult to remove from a viscous fluid. Emulsion formation requires high impeller tip speeds. Those high speeds may also cause problems with air incorporation and emulsion formation, especially if the mixer is not well positioned.