Chapter 1 provides an overview of the concepts and exemplified applications of multiphase flows. It illustrates the distinctly different transport patterns or phenomena of individual phase in a multiphase flow, which have either naturally caused or intentionally designed consequences.

The chapter conveys the basic definitions of a multiphase flow, the phase interactions, and the associated modeling approaches, which include the difference between a multiphase flow and a multicomponent single-phase flow, the difference between a dilute-phase multiphase flow and a dense-phase multiphase flow, the difference between a continuum phase and a discrete phase in describing the flow regimes, and the difference in Eulerian–Lagrangian modeling and Eulerian–Eulerian modeling. Some interesting and unique phenomena of multiphase flows are discussed by case studies.

Chapter 5 delineates the Eulerian–Lagrangian modeling commonly used for dilute-phase multiphase flows. The most essential part is the formulation of a Lagrangian trajectory model to describe the transport of individual particles, which requires proper formulation of particle–fluid and particle–particle interactions involved. The basic modeling considerations include the type of Lagrangian trajectory models such as deterministic trajectory model, stochastic trajectory model, particle-cloud tracking model, or discrete element method; the degree of phase coupling between Lagrangian models of discrete phases and Eulerian model of continuum phase such as one-way or two-way particle–fluid coupling or four-way particle–fluid and particle–particle coupling flow; the field coupling with particle transport such as the electrostatic field induced by the suspended charged particles or thermal radiation among particles; and the turbulence modulation between particles and eddy transport. The Lagrangian model of particle motion, typically in a simplified form of additivity of individual modes of fluid–particle interactions, is employed in the Lagrangian modeling of discrete phase transport.