Turbulent Multiphase Flow

When the flow is highly turbulent multiphase, there are only two practical design options  

When the flow is laminar, either single or multiphase, there is only one design class option static or motionless mixers. Other pipeline mixing devices described for turbulent flow are not usable for even the simplest mixing applications in the laminar regime. All rely on turbulence and cannot function at low Reynolds numbers. The only alternative technology is in-line dynamic mixers, which include extruders, rotor-stator mixers, and a variety of rotating screw devices. None of these has the benefits of simplicity and the little or no maintenance characteristic of static mixers. In-line mechanical mixers are discussed briefly later in the chapter. 

3A Liquid-Liquid and Gas-Liquid Dispersion. When the flow is laminar and multiphase, elongational flow static mixers are required to mix and disperse additives into viscous bulk streams. The dispersion of a low viscosity immiscible additive into a viscous mainstream is a common and very difficult 

It is important to note that in most cases the value static mixing brings to heat transfer does not translate to turbulent flow, where cost and pressure drop cannot be justified by the heat transfer enhancement. Turbulent flow heat transfer processes are best handled by empty tubes and tubes with spiral wrapped cores or tubes containing twisted tapes. These other pipeline devices, though not discussed here, are nevertheless important in industry. The reader is encouraged to seek other literature if interested (see Burmeister, 1983a,b). 

6 Tubular Plug Flow Reaction. When the application is a laminar flow tubular reactor, static mixing internals can provide great benefits in terms of 

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The fluxes of mass, momentum, and energy of phase k transported in a laminar or turbulent multiphase flow can be expressed in terms of the local gradients and the transport coefficients. In a gas-solid multiphase flow, the transport coefficients of the gas phase may be reasonably represented by those in a single-phase flow although certain modifications... 

Turbulence is the most complicated kind of fluid motion. There have been several different attempts to understand turbulence and different approaches taken to develop predictive models for turbulent flows. In this chapter, a brief description of some of the concepts relevant to understand turbulence, and a brief overview of different modeling approaches to simulating turbulent flow processes is given. Turbulence models based on time-averaged Navier-Stokes equations, which are the most relevant for chemical reactor engineers, at least for the foreseeable future, are then discussed in detail. The scope of discussion is restricted to single-phase turbulent flows (of Newtonian fluids) without chemical reactions. Modeling of turbulent multiphase flows and turbulent reactive flows are discussed in Chapters 4 and 5 respectively. 

For multiphase flow processes, turbulent effects will be much larger. Even operability will be controlled by the generated turbulence in some cases. For dispersed fluid-fluid flows (as in gas-liquid or liquid-liquid reactors), the local sizes of dispersed phase particles and local transport rates will be controlled by the turbulence energy dissipation rates and turbulence kinetic energy. The modeling of turbulent multiphase flows is discussed in the next chapter. 

Most attempts at modeling complex, turbulent multiphase flows rely on the practices followed for single-phase flows, with some ad hoc modifications to account... 

The recent progress in experimental techniques and applications of DNS and LES for turbulent multiphase flows may lead to new insights necessary to develop better computational models to simulate dispersed multiphase flows with wide particle size distribution in turbulent regimes. Until then, the simulations of such complex turbulent multiphase flow processes have to be accompanied by careful validation (to assess errors due to modeling) and error estimation (due to numerical issues) exercise. Applications of these models to simulate multiphase stirred reactors, bubble column reactors and fluidized bed reactors, are discussed in Part IV of this book. 

Sha WT, Slattery JC (1980) Local Volume-Time Averaged Equations of Motion for Dispersed, Turbulent, Multiphase Flows. NUREG/CR-1491, ANL-80-51... 

Balachandar, S. 2009 A scaling analysis for point-particle approaches to turbulent multiphase flows. International Journal of Multiphase Flow 35, 801-810. 

Turbulent Multiphase Flows with Heat and Mass Transfer... 

Many industrial processes are stiU designed on the basis of the assumptions of plug flow and steady-state uniform two-phase flow. For this chapter, much evidence has been collected with respect to the abundant occurrence of transient mesoscale coherent structures, strands, or clusters in various turbulent multiphase flows, at least at scales and under conditions relevant to industrial processes. This evidence is from a variety of sources experimental observations in academic laboratories, results from hydrodynamic stability analyses, and computational simulation studies (both of the LES and the DNS type). Unfortunately, this evidence comprises many indecisive and even contradictory reports about the drivers behind these structures, clusters, and strands, and about their dependence on density ratio, particle size, volume fractions, operating conditions, and so on. 

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