Mixing multiphase flow

Phase average models apply to well mixed multiphase flows, i.e. when the exact shape of the interfaces is not known, or not relevant e.g. bubbly flows. The principle could be applied under the two-fluid, six-equation model, where separate eonservation equations are required for each phase with appropriate exehange forces, or the homogeneous. Algebraic Slip model. Under sithermal, incompressible flow conditions, the equations of laminar motion for phase A are expressed in the two-fluid formalism as follows ... 

These methods hardly take spatial distributions of velocity field and chemical species or transient phenomena into account, although most chemical reactors are operated in the turbulent regime and/or a multiphase flow mode. As a result, yield and selectivity of commercial chemical reactors often deviate from the values at their laboratory or pilot-scale prototypes. Scale-up of many chemical reactors, in particular the multiphase types, is still surrounded by a fame of mystery indeed. Another problem relates to the occurrence of thermal runaways due to hot spots as a result of poor local mixing effects. 

For multiphase systems a rough distinction can be made between systems with separated flows and those with dispersed flows. This classification is not only important from a physical point of view but also from a computational perspective since for each class different computational approaches are required. For multiphase systems involving multiphase flow both Eulerian, mixed Eulerian-Lagrangian, and two-material free surface methods can be used. An excellent review on models and numerical methods for multiphase flow has been presented by Stewart and Wendroff (1984). A similar review with emphasis on dilute gas-particle flows has been presented by Crowe (1982). 

Multiphase flows involving dispersed phases (particles, droplets or bubbles) using mixed Eulerian-Lagrangian approaches both with one-way and two-way coupling... 

For multiphase reactive systems of types (a) and (b), at least one of the reactants has to reach the reaction zone from a different phase. In such systems, generally mass transfer between these two different phases (and its interaction with chemical reactions) is of primary importance and turbulent mixing is often of secondary importance. For such systems, modeling multiphase flows as discussed in Chapter 4 is directly applicable. The only additional complexity is the possibility of interaction between mass transfer and chemical reactions. The typical interphase mass transfer source for component k between phases p and q can be written (for the complete species conservation equation, refer to Chapter 4) ... 

Hirt and Nichols [12] demonstrated the volume of fluid (VOF) method and started a new trend in multiphase flow simulation. It relies on the definition of an indicator function y. This function allows us to know whether one fluid or another occupies the cell, or a mix of both. In the conventional volume of fluid method [12], the transport equation for an indicator function y, representing the volume fraction of one phase, is solved simultaneously with the continuity and momentum equations as follows ... 

The statistical description of multiphase flow is developed based on the Boltzmann theory of gases [37, 121, 93, 11, 94, 58, 61]. The fundamental variable is the particle distribution function with an appropriate choice of internal coordinates relevant for the particular problem in question. Most of the multiphase flow modeling work performed so far has focused on isothermal, non-reactive mono-disperse mixtures. However, in chemical reactor engineering the industrial interest lies in multiphase systems that include multiple particle t3q)es and reactive flow mixtures, with their associated effects of mixing, segregation and heat transfer. 

---