Fluidization represents an important particulate and multiphase operation, featuring dynamic interactions between a continuum fluid and a discrete phase. It is typically realized in a vertical column or pipe. Various fluidization regimes occur, depending on the property of the fluidizing particles, flow rate, and external field force applied. This chapter describes gas–solid fluidization represented by dense-phase fluidized beds and circulating fluidized beds. Fluidization under the gas–liquid–solid flow conditions is also illustrated with the inclusion of its limiting condition of two-phase flows. Basic topics of fluidization include the fluidization regime classification and characteristics, phase-interaction mechanisms in the dense and dilute phase fluidization as well as nanoparticle fluidization, fluidized bed systems, and multiscaled transport phenomena, such as clustering, agglomeration, breakup, and coalescence of dispersed particles or bubbles. For the numerical modeling of fluidization systems, the Eulerian–Eulerian modeling is extensively used and often coupled with the DEM models or kinetic theory models for collision-induced transport in the dispersed phase.

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.