Multiphase modeling employs the notion of interpenetrating continua. Although phases are mixed at length scales much larger than molecular, they are also assumed to be mixed at length scales smaller than you want to resolve.

Thus, each phase is assumed to be present in principle in each control volume, and assigned a volume fraction equal to the fraction of the control volume occupied by that phase. This variable appears as <fluid>.Volume Fraction in the results file. The variable <fluid>.Conservative Volume Fraction is also available but should not usually be used for postprocessing. The conservative volume fraction is used by the CFX-Solver; it may exceed 1 and may not sum to unity over all fluids during the solution. However, these criteria are enforced in a converged solution.