Multiphase flows, mesoscale structures

Figure 1 Schematic examples of multiphase systems in which a discrete dispersed phase is moving through, or moved by, a continuous fluid phase. The discrete phase can be a solid (left), a gas (center), or a liquid (right). In many cases, inhomogeneous mesoscale structures appear in the spatial distribution of the discrete phase, caused by interplay of hydrodynamic flow and local energy dissipation. More complicated cases with three or more phases are also possible, such as encountered in slurry reactors (where solid particles are also present in the continuous liquid phase) or trickle bed reactors (where the droplets are sprayed on a packed bed of particles). To focus on the essentials, the topical sections will focus mostly on the two-phase examples depicted here.

Zhang DZ, VanderHeyden WB The effects of mesoscale structures on the macroscopic momentum equations for two-phase flows, Int J Multiphase Plow 28 805—822, 2002. 

The origin of mesoscale structures in dispersed multiphase flows may be investigated in three ways ... 

These three approaches will now first be discussed in some detail this leads to an intermediate conclusion with respect to the dominating role of fluid-particle interaction in the development of mesoscale structures. On this basis of this conviction, the main body of this chapter will be devoted to fluid-particle interaction in dispersed multiphase flows and a proper description of the forces involved. 

It is the author s conviction that in many (turbulent) dispersed multiphase flows—except probably in very dense multiphase flow systems—the origin of mesoscale structures is in the fluid—particle interaction, with a secondary role for particle-particle interaction (coUisions, coalescence, breakup). Clustering of particles is believed to be intimately connected with the chaotic dynamics of fluid accelerations, as particles converge toward each other where and when the divergence of the acceleration field is positive (Goto... 

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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