Flow patterns overview

Detailed descriptions of the chemical reactor flow patterns are given in chaps 8, 10, 7 and 11. Meanwhile, a preliminary overview of the pertinent reactor flow characteristics is given to determine which modeling concepts are available describing the behavior of the relevant flows. 

FIGURE 5.2 Flow pattern inherent to segmented flow analysis. 1 = tube wall of part of a coiled reactor 2 = thin liquid layer adhering to it 3 = vortices inside the liquid phase 4 = air bubble outer large arrow = overall displacement of the segmented flow. Adapted from P.J. Worsfold, A. Townshend, C.F. Poole (Eds), Encyclopedia of Analytical Science, 2nd Edn, Oxford, 2005, v.3, p.24, E.A.G. Zagatto, PJ. Worsfold, Flow Analysis Overview, with permission from Elsevier (Ref. [4]). 

Passive micromixers rely on the mass transport phenomena provided by molecular diffusion and chaotic advection. These devices are designed with a channel geometry that increases the surface area between the different fluids and decreases the diffusion path. By contrast, the enhancement of chaotic advection can be realized by modifying the design to allow the manipulation of the laminar flow inside the channels. The modified flow pattern is characterized by a shorter diffusion path that improves the mixing velocity. In this section, an overview of the different types of passive micromixers is provided. Mixed phase passive micromixers can be categorized as ... 

Theoretical solution of the Navier-Stokes equation for prediction of the collision efficiency, E(Dp,dp), for the general raindrop-aerosol interaction case is a difficult undertaking. Complications arise because the aerosol size varies over orders of magnitude, and also because the large raindrop size results in complicated flow patterns (drop oscillations, wake creation, eddy shedding, etc.) Pruppacher and Klett (1997) present a critical overview of the theoretical attempts for the solution of the problem. A detailed discussion of these efforts is outside our scope. However, it is important to understand at least qualitatively the various processes involved. 

While the benefits of a properly designed system are many, so are the pitfalls that must be avoided by the system designer. An ill-conceived material flow pattern or poor choice of system elements can result in operational limitations or maintenance headaches which reduce the expected benefits. In this section, we will examine each element going into a system, such as silos, conveying systems, dryers, blenders, and then take an overview of how they fit together. 

Recently, Cheng et al. [10] presented a broad review on two-phase flow patterns that is suggested here to provide a comprehensive overview not only of the historical aspects of flow patterns characterization but also of the actual status quo of research on this topic. As a separate chapter on two-phase flows in microchannels is presented elsewhere in this handbook, here flow patterns will be discussed only briefly. 

Fig. 5.91 A reflected light micrograph (A) shows the layered structures in a molded bar aligned parallel to the flow direction (arrow). Variation in density and color reflect variation in orientation from layer to layer. Lateral, curved flow patterns are seen in a polarized light micrograph overview of a thin section (B) (color section). The flow layers are nearly normal to the flow direction (arrow) in the center of the bar (C) (color section).

Fig. 5.91 Lateral, curved flow patterns are seen in a polarized light micrograph overview of a thin section (B). The flow layers are nearly normal to the flow direction (arrow) in the center of the bar (C).

After introducing the concepts of macro- and micro-mixing, the importance of flow / mixing patterns in reactors is discussed. Next, an overview of the methods for describing complex flow patterns is given and the relation between the different methods is explained. Focusing on statistically stationary flow, each method is then presented and discussed in more detail in the remaining sections of this chapter. 

In Chapter 3 we considered chemical reactors with ideal macro flow patterns where the reactor behaviour was independent of scale. In Chapters 4, 5 and 6 an overview was given of various physical phenomena on the intermediate scale, some of which interact with chemical reactions. Several of these phenomena are scale dependent. To arrive at integral reactor models, we have to consider macro-flow effects, i.e. the effects of transport phenomena on the scale of the reactor dimensions. These are as a rule strongly scale dependent. 

Because the flow pattern inside gasification reactors is of prime importance to understand gasifier behavior, this section is dedicated to give an engineering-oriented and pragmatic overview how to approach the complex field of CFD utilization for coal gasification. On the example of internal circulating fluidized bed (INCI) concept development, explained in detail in Chapter 9, test calculations should be accomplished to understand the basic relationships of the system. 

In addition to the effect the insert has on the flow pattern, the loads on the insert is a very important parameter that has to be known before it can be installed in a silo. An overview of the last experimental results from the full scale silo tests will be given here. 

Figure C.6. Overview of the fabrication process. Two coverslips are placed at the top and bottom of a transparency (a) and prepolymer solution is dropped in the center (b). A thick glass slide is positioned on top of the prepolymer solution, allowing it to rest on the coverslips, and causing the prepolymer solution to flow and fill the gap (c). A photomask is aligned on top of the glass slide (d), a second glass slide is placed on top of the photomask to keep it in place, and the exposed photoresist is polymerized using UV light. After rinsing, an insoluble, patterned polymer results (e).

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