S Martin & J.R. Williams 
Multiphase Flow Research [PDF ebook] 

In fluid mechanics, multiphase flow is a generalisation of the modelling used in two-phase flow to cases where the two phases are not chemically related or where more than two phases are present. Each of the phases is considered to have a separately defined volume fraction , and velocity field. Conservation equations for the flow of each species, 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 field, 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. This new book presents the latest research in the field.