Two-phase flow is a flow in which two different aggregate states of a substance or of two different substances are simultaneously present. The possible combinations include gaseous/liquid (see Gas content of fluid handled), gaseous/solid and liquid/solid (see Solids transport).
Examples of two-phase flow are transport processes where one medium is transported in another (e. g. hydraulic or pneumatic transport), or the frequently undesirable entrainment of gas or vapour bubbles (see Cavitation) which have come out of solution in fluid flows.
In two-phase flow, further factors are involved in addition to flow parameters such as the Reynolds number. The concentration, which can be expressed in the form of volume ratio or mass ratio of the two phases, represents an important factor.
Various flow or phase distribution patterns can occur in a two-phase flow. These are influenced by the concentration ratio of the phases, the slip ratio (difference between the flow velocities of the two phases) and the orientation of the piping (horizontal, vertical). Flow patterns of gaseous/liquid two-phase flow include bubble flow, plug flow and film flow.
Various, primarily empirical, calculation methods are applied to establish the head loss in a two-phase flow in straight pipes. Some of them take the influence of the flow pattern and the orientation of the piping (horizontal, vertical) into account.
In the case of gaseous/liquid two-phase flow, a distinction must be made between flows whose gaseous phase represents the vapour state of the liquid also present (e. g. water/steam) or flows in which the gaseous phase is a different substance (e. g. water/air). In the first of these two cases, phase change phenomena such as vaporisation and condensation may play an important role.
Different densities of the two phases mean that phase separation in the form of the settling of solid substances or the rising of bubbles may occur in two-phase flows at low flow velocities under the influence of gravity. This kind of separation will be even more pronounced under the influence of centrifugal forces (e. g. in elbows, impellers) and at higher flow velocities, with the constituents of lower density being subjected to the action of forces which drive them towards the centre of the bend or centre of rotation.
This effect can result in the cut-off or stalling of the flow (see Operating behaviour) in a centrifugal pump handling a gaseous/liquid two-phase flow. In the case of liquid/solid two-phase flow containing abrasive solid particles (see Abrasion), the possibility of wear on components exposed to the flow must be taken into account. This is particularly likely to take place in the region of curved stream lines (see Solids transport, Pulp pumping).
The lift effect based on buoyancy of a two-phase fluid involving a mixture of water and air is exploited in vertical underwater piping through the use of air lift pumps, while three-phase flow is utilised for hydrotransport (e. g. air lift) (see Type of pump).