9.123. For preliminary design purposes, the pressure drop problem can be made more manageable by applying one of two simplifications. In homogeneous models, the two phases are assumed to flow as a single phase mixture possessing mean flow properties and a suitable single-phase friction factor is developed to represent the two-phase flow. Separated-flow models consider the phases to be segregated into two separate streams, with each moving at a different velocity. Empirical correlations are then used to determine a so-called friction multiplier (described in the next paragraph) which is a function of flow parameters. The different models vary in so­phistication and may be specific to given flow regimes. Furthermore, models must be used with care since the behavior being modeled is inherently complex [15] with numerous simplifications necessary for a practical description.