A mixed flow occurs when the individual phases form a homogeneous mixture. If the mixed phases move without slip, which means without speed difference, they can be described as a homogeneous fluid. In this approach, the individual phases are thus combined into a single fluid. The substitute fluid thus obtained represents the properties of the phase mixture. An exchange of impulse, energy or also a mass transport between the phases takes place via a macroscopic model. The mass transport corresponds to evaporation or condensation. The associated latent heat is accordingly considered in the energy equation, either as an equilibrium model or as an imbalance model for rapid state changes. The latter is required, for example, to model supcooled nucleate boiling.
This homogeneous-fluid method can be used if the volumetric ratio of the individual phases varies only insignificantly. This applies, among other things, to steam and dust flows, to emulsions or to the description of cavitation. However, the necessary condition, no slip, for this method is not always met. By introducing a slip ratio into the formulation, even mixed flows that do not sufficiently meet this basic condition can be considered.