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Mixing operations are extremely common in many industries. It has been this way for a long time, which has led to a huge variety of mixing techniques. These mixing techniques vary significantly in the principles that they employ and the applications they are designed for. This makes it easier to select a technique that will perform a desired mixing operation. Even if none of the existing mixing techniques will perform the desired mixing operation to a certain standard required, the techniques can be modified to reach that standard. Some existing techniques and their principles can even be combined and modified to create new mixing techniques. The existing mixing techniques can be grouped into the following sections:

• Impellers – large variety including propeller, turbine, paddle, anchor, helical, gate, ribbon/screw, high shear teeth, rotor-stator

• Kneaders/Blade mixers (Planetary, Banbury, Sigma and Plough)

• Roll mills/mixers

• Screw Extruders

• In-line Static mixers

• In-line Dynamic mixers

• Jet mixers

Impellers

Due to the highly viscous nature of the materials that food printing requires and the small characteristic dimensions involved with mixing one voxel, it is not feasible to work with turbulent flow [56](p. 383). For this reason, a laminar flow regime is assumed for all mixing operations, which means that little can be drawn from methods that require turbulent flow, for example turbine impellers. However, some impeller designs such as the ribbon/screw and helical impellers can introduce slow but sure movement of the fluid throughout the whole mixing chamber. Figure 3-1 shows a selection of novel impeller designs. The Paravisc and Coaxial impellers are similar to anchor and helical ribbon impellers and could be valuable to food printing if applied correctly. This would particularly involve minimising surface area to allow contrast to be achieved between voxels, but this has not been explored further due to time constraints.

FIGURE 3-1-NOVEL IM PELL ER DESI GNS [73]

The anchor impeller uses close clearance with the wall to increase shear stress. These two mixing types can be combined by employing two helices to form a mixer such as the double helicone impeller (Figure 3-2).

High shear impellers would result in shear thinning in the fluids we are dealing with, so are not useful by themselves, but ensuring whatever techniques are used don’t result in shear thinning is a factor that needs to be considered.

Kneaders/Blade mixers

The planetary motion used by some mixers could potentially be very useful for minimising blade size. As the blade moves around the mixing chamber it agitates a larger volume than the same blade would with simple rotation. This means the blade can be a fraction of the width of the chamber diameter rather than almost the full width. A major benefit of reduced surface area of the blade is less residue on the blade. A different method to improve mixing of extremely viscous materials is employed by the Banbury mixer. This mixer employs a ram to ensure the mixing chamber is completely full and potentially pressurised. Even though the complex shape of the Banbury mixer means that it’s not feasible for small batches, the concept of using a ram could be used to dynamically reduce the effective size of any mixing chamber.

FIGURE 3-3-INDUSTRIAL BANBURY MIXER [74]

Screw extruders both mix and pump at the same time. This has challenged the idea of separating these two processes and has encouraged research into what ways these steps can be combined to reduce waste and/or residue, minimise time between stages and avoid extra equipment.

Non blade mixers

Rolling mills and mixers show how it can be useful to use the surface of an object to transport material. It also demonstrates another way of forcing fluid through a small clearing to increase localised shear and increase interfacial surface area, hence promoting mixing. The challenge then becomes to also create axial mixing along the length of the mixer. Using gravity to feed the material along a slanted mixer or having a mixer with a varying profile could help to achieve this. More research would be needed to investigate the feasibility of this technique.

FIGURE 3-4-ROLLING MILL/MIXER

Static Mixers

With static mixers the concepts can be directly applied by purchasing standard static mixers. There are many different types of static mixers [75], but all static mixers work by dividing the flow into different paths, stretching and recombining the flow (Figure 3-5). Static mixers are fundamentally continuous devices which employ the kinetic and pressure energy of the flowing fluid to mix its constituents.

FIGURE 3-5-HELICAL STATI C MIXER OPERATING PRINCIPL E

In the literature there is a lot of work done to study the flow in static mixers [76], [77]. Particularly large amounts of work have gone into researching the Kenics helical element

static mixers while a smaller amount of research has been done with other types of static mixers [78], [79]. This is partly to do with how long the Kenics mixers have been around and partly to do with the complexity of newer kinds of mixers e.g. Sulzer MX [80]. Even with this reasonable amount of ground work being done into static mixing technology, little or no research has been done in terms of dynamic mixing ratios. A dynamic mixing ratio is necessary for use in food printing because the ratio of the colours being mixed will constantly be changing. The only related research that was found was in the area of Functionally Graded Materials (FGM) and this did not focus on the capability of the static mixer being used [81]. There has been research that takes into account the effect that the ratio of mixing has on the resulting mixing coefficient, but it does not consider a dynamic mixing ratio [82]. The Residence Time Distribution (RTD) is an important measure to consider in relation to dynamic mixing ratios. Jin et al suggest that the RTD is affected by the diameter of the static mixers, the velocity of the flow and the arrangement of the mixing elements [83]. A dynamic mixing ratio is a mixing ratio between one or more components that changes as the process continues. This would mean that most of the time the system is not in steady state running conditions. This may be part of the reason that there is a lack of literature in this area of mixing. Another reason for the lack of research may be that there are few applications that require a dynamic mixing ratio. Most applications of static mixers have static mixing ratios for a consistent and uniform process output. The degree to which the components in the static mixer have been mixed is the main topic that has been researched. This is definitely applicable to food printing, but is limited in its usefulness due to the dynamic nature of our system. For most applications, it comes down to increasing efficiency of the mixing system, either by reducing the pressure drop and therefore reducing energy required to mix, or reducing the size to reduce the cost of the mixers. For any previous research to be useful, the flow regime had to correspond to the flow regime likely to be used in food printing. Due to the highly viscous nature of the food mixture and relatively low flow rates, the flow regime has been accepted to be laminar. This makes much of the existing research of little value as turbulent mixing in static mixers is very different to laminar mixing.

Because useful literature on the topic of dynamic mixing ratios was unable to be found, two options were then considered. The first option was to use Computational Fluid Dynamics (CFD) to examine the flow patterns that might occur while mixing with a dynamic mixing ratio. This option was not undertaken for reasons explained in Section 9.1. It was perceived that even though the geometry of the mixers is reasonably simple, the dynamic nature of the

The second option was to build a test rig to physically test this kind of mixing. This was the option selected for this research.

Oscillatory mixers

Research into oscillatory mixing has been around for many years, with the Karr column being the earliest documented reciprocating plate column (RCP) [84]. There are two common plate profiles. These two plate profiles have a ring on the outside of the wall of the column in which they operate, but differ in their actuation attachment location. Some RCPs use a central rod for the actuation, so have attachments between this central rod and the outer ring while others have rods directly on the outer ring, between each consecutive ring (Figure 3-6).

FIGURE 3-6-PLATE PROFILE S

The latter configuration means there is free space in the middle of the mixer and produces different flow patterns. Neither of these configurations would allow the mixing then ejecting action required for our application.

Although not very common, oscillatory mixers can provide more effective mixing than rotary type mixers in certain circumstances [85]. The author of [86] shows the two types of oscillatory mixing, either directly oscillating the mixing elements, or causing the fluid to oscillate past fixed baffles or mixing element. The authors of [87] suggest that the majority of the mixing that occurs in RCPs is that of vortices in turbulent flow, which occurs at high Reynolds numbers. Due to the high viscosity of the fluids our application requires, these Reynolds numbers cannot be achieved and therefore flow will be laminar. This indicates that this mixing technique is not very efficient in the laminar regime. However, modifying the technique to something more like a standard hand-held paint mixer has potential to achieve great benefits.