SAR mixers

For the first step, the lamellae division, a splitting plane, basically acting like a knife, can be useful [7,140], not only for precise cutting but also for avoiding friction forces to deform the lamellae. SAR mixers were described with [7,140] and without a splitting plane [43,125, 126, 141, 142],... 

This SAR mixer, based on multiple collisions of one stream, was described by industry in a patent [143]. The same company has established the first production... 

M 57] [P 52/ForaMobius-typefour-stage SAR mixer complete mixing was achieved at flow rates up to 285 pi min4 [142],... 

At fast flow rates, no color change was observed in the SAR mixer [141]. At lower flow rates, the formation of the green manganate was evident. At still lower flow rates, the brown manganese dioxide was observed. The exact flow rates were not specified. 

M 58] [P 53] The mixing process was investigated by a pH-driven color reaction [43], A complete change of the color was detected at the mixer outlet, i.e. mixing was assumed to be complete. This was found for SAR mixers with five and 20 mixing elements, hence a low number of mixing steps seems to be sufficient. 

Figure 1.129 Optical inspection of multi-lamination in the SAR mixer.

Figure 1.130 Optical inspection of mixing in the SAR mixer. The applied total volume flow rate of 0.2 I IT1 is the same as for the experiment shown in Figure 1.129. Starting from a bi-lamination of yellowish iron ion (Fe3+) and transparent rhodanide (SChT) solutions, homogeneous mixing is achieved in the eighth mixing step, indicated by the deep brown color of the iron-rhodanide complex formed [7] (by courtesy of RSC).

It must be noted that SAR mixers generally work at small Re. However, some secondary recirculation flows can be generated, as demonstrated by particle tracking simulation [90]. 

The main disadvantage of SAR mixers is the complex fabrication process required to make a 3D structure. However, an effect on the liquid stream similar to that exploited by SAR can be achieved by a planar, packed bed configuration that enhances trans-channel coupling. Melin et al. [93] fabricated and tested multiple... 

Figure 3.19 Mixing element of a SAR mixer (left) and optical micrographs showing lamellae multiplication in the first three mixing elements (right).

The actual system consisted of two parallel rows with 10 SAR elements each [141]. The channels underwent a linear change of the width and depth to turn and reshape the flow. The mixer was an element that was intended to be integrated in a system comprising a mixing unit, reaction channel and an optical detector with crossed cylindrical lenses and optical fibers. 

Figure 1.123 Schematic of the SAR process in a Mobius-type SAR micro mixer [141] (by courtesy of Kluwer Academic Publishers).

Mixer type Mobius-type SAR micro mixer Top layer thickness 250 pm... 

Mixer type Fork-element SAR Number of mixing 5,10,15,20... 

This SAR device, named a stack mixer, is composed of two plates which both contain microstructures [125,126], By face-to-face positioning of these microstructures, a micro channel network yielding the SAR path is formed. 

Mixer type Stack SAR micro Number of parallel 2... 

Mixer type Up-down curved SAR micro mixer Splitting layer material Stainless steel... 

This micro mixer extends the SAR concepts mentioned above, the sequential lamination being actively supported by the use of a separation plate for flow splitting (see Figure 1.126) [140], As a result of the splitting, two sub-channels are formed, initially at the same height level within the device. Then, one of these sub-channels undergoes a downward movement in a lower level of the device. The two flow channels are so sandwiched, the first carrying the two fluids separate from each other. At certain locations there are conduits within the sandwich to achieve flow recombination on one of the levels of the sandwich. This procedure is repeated many times. 

Mixer type Separation-plate SAR micro mixer Width of channels for inlet branches 150 pm... 

M 60] [P 54] SAR flow splitting can be performed using split channels or done inline in one channel. Concerning the quality of flow splitting in the latter case, CFD simulations were performed on the example of a so-called caterpillar micro mixer [7]. 

M 60] [P 54] The impact of having truly separated flows was shown by CFD simulations [7]. An essential part of the flow splitting, besides having split channels which recombine later, is a splitting plane which cuts the flow like a knife. These design aspects are central parts of an optimized SAR caterpillar mixer. 

Fig. 13 Micromixer combining SAR and chaotic advection approaches (a) Serpentine laminating micromixer (SLM) and (b) concentration contours along the mixers channels, (Reproduced from [131] by permission of The Royal Society of Chemistry), (c) Staggered overlapping crisscross micromixer (SOC p-mixer) and (d) corresponding cross-section view showing concentration profiles after flowing through two junctions (Adapted from [132] with permission. Copyright lOP Publishing)...

Figure 3.32 Evolution of Bo as function of Re for SAR and SCR micro-mixer. (Data taken from Ref. [20].)...

Figure 4.19 Caterpillar mixer, (a) Schematic of SAR showing structured walls, (b) Snapshot of the 600 im size caterpillar mixer. The size is defined for entrance channel. Courtesy Fraunhofer ICT-IMM, Germany.

One of the promising designs is the modified Tesla structures [46]. It uses the Coanda effect to split part of the fluid stream and direct it so that it recombines with the opposing flow of the other part of the stream. Coanda effect micromixer relies on the redirection of a flow by a special guiding structure that creates new interfaces within the flow [47]. This special passive structure provides good mixing at low flow rates. In this way the Coanda mixer can also be seen as a special realization of the SAR approach using recycle flows [32]. 

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