Mixer helical flow

One of the benefits of static mixers like those mentioned above are the secondary flow patterns set up by the swirling helical flow. 

Helical flows are not only found for 2-D mixer designs. In analogy with findings for macro-scale alternating helical coils, microstructured 3-D designs were proposed [49, 153], For reasons of limitations of today s micro fabrication, not real helices were made, but easier to fabricate structures such as complex micro channels based of L-shaped elements arranged in a 3-D fashion. 

M 72] [M 73] [P 65] A linear dependence of the pressure drop on the volume flow rate was observed both for the intersecting and the helical microstructured mixers (see Figure 1.154) [2], The pressure drop of the helical device is lower than that of the intersecting mixer. A flow rate of about 11 h 1 was found for the helical mixer at a pressure of about 150 mbar, whereas the intersecting mixer gives 0.75 1 h 1 at about 250 mbar. 

M 80] [P 70] Cross-sectional images of the flow patterns, gained by confocal microscopy, demonstrate the interfacial stretching and hence are indicative of mixing in the barrier-embedded mixer [58], At the non-barrier entrance structure clearly the helical flow induced by the slanted grooves is evident. In the barrier... 

Top entering mixers are heavy duty equipment. They are usually fixed to a rigid structure or tank mounting. Either radial flow or axial flow turbines may be used. Speeds vary from 50 to 100 rpm and usually require a double set of helical gearing or a single set of worm gears to achieve these low speeds. Therefore, they are more expensive than single reduction mixers. 

The Kenics STATIC MIXER unit is a series of fixed, helical elements enclosed within a tubular housing. The fixed geometric design of the unit produces the following unique patterns of flow division and radial mixing simultaneously. 

The Kenics mixer, Figure 10.14(a), for example, consists of a succession of helical elements twisted alternately in opposite directions. In laminar flow for instance, the flow is split in two at each element so that after n elements the number of striations becomes 2". The effect of this geometrical progression is illustrated in Figure 10.14(b) and points out how effective the mixing becomes after only a few elements. The Reynolds number in a corresponding empty pipe is the major discriminant for the size of mixer, one manufacturer s recommendations being... 

Figure 10.14. Some kinds of in-line mixers and blenders, (a) Mixing and blending with a recirculating pump, (b) Injector mixer with a helical baffle, (c) Several perforated plates (orifices) supported on a rod. (d) Several perforated plates flanged in. (e) Hellical mixing elements with alternating directions (Kenics Corp.). (f) Showing progressive striations of the flow channels with Kenics mixing elements.

M 81] [P 70] A microscopy-image analysis of the color formation due to a reactive approach reveals that the micro mixer with helical elements and barriers gives a better performance than the micro mixer with helical elements but without barriers and a reference pipe structure without either helical elements or barriers (see Figure 1.167) [3], The pipe gives the expected profile with two colorless fluid compartments on top and at bottom, separated by a colored interface. This is indicative of the absence of any swirling, secondary flow. 

Based on the above relation, for most practical operations, this reactor behaves much hke the plug-flow reactor, where ReF = pdtL/pOT, ReM = pdiNjp. Here, 0T is the average residence time. The relationship between power consumption and mixing time reveals the similarity of this vessel to the double helical-ribbon mixer. The fraction of dead space in this apparatus appears to be small. The relationships described above are valid for the fluids in the viscosity range of 50-5000 poises. 

An in-line measurement is performed in a process line an on-line measurement is performed in a bypass loop from the main process line and the food may be returned to the main process line after measurement is performed. A near-line measurement is performed on a sample taken from a process line which is often discarded after measurement. Because foods are complex materials (e.g., suspensions, emulsions, gels), stmctural changes may take place during sampling (e.g., flow through a valve) for on-line and near-line measurements (Roberts, 2003). Nevertheless, in principle, the previously described capillary flow, and rotational concentric cylinder, plate-cone, and mixer viscometers may be used for in-line, on-line, and near-line measurements. In this respect, Tamura et al. (1989) proposed a helical screw rheometer as an on-line viscometer. The empirical measurement methods described previously are used primarily in near-line measurements. 

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