Multiphase flow

Multiphase flow is a generalisation of the modelling used in two phase flow to cases were the two phases are not chemically related (eg dusty gases) or where more than two phases are present (eg in modelling of propagating steam explosions)

Each of the phases is considered to have a separately defined volume fraction (the sum of which is unity), and velocity field. Conservation equations for the flow of each species (perhaps with terms for interchange between the phases), can then be written down straightforwardly.

The momentum equation for each phase is less straightforward. It can be shown that a common pressure field can be defined, and that each phase is subject to the gradient of this, weighted by its volume fraction. Transfer of momentum between the phases is sometimes less straightforward to determine, and in addition, a very light phase in bubble form has a 'virtual mass' associated with its acceleration. (The virtual mass of a single bubble is about half its displaced mass).

These terms, often called 'constitutative relations', are often strongly dependent on flow regime.