Multiphase flow is a flow in which more than one fluid is present. In general, the fluids consist of different chemical species, such as air-water. In some applications, they may represent different thermodynamic phases of the same species, such as steam-water.

It is important to distinguish between multicomponent and multiphase flow. A multicomponent fluid is assumed to consist of a mixture of chemical species that are mixed at the molecular level. In this case, single mean velocity and temperature fields, and so on, are solved for the fluid. Examples are gaseous mixtures, and solutes in liquids.

The fluids in a multiphase flow are assumed to be mixed at macroscopic length scales, much larger than molecular. Examples are gas bubbles in a liquid, liquid droplets in a gas or in another immiscible liquid, and so on. In this case, it is necessary to solve for different characteristics such as velocity and temperature fields for each fluid. These may interact with each other by means of interfacial forces and heat and mass transfer across the phase interfaces.

For example, if cold wet particles are injected into a fast flowing stream of hot air, the particles will be accelerated by drag, they will be heated up by heat transfer across the phase boundary, and they will be dried by evaporation of water into water vapor at the phase boundary.