(Smith, 1968) assumes that kinetic energy of the liquid is equivalent to that of the two-phase mixture and a constant fraction k of liquid phase is entrained with the gas phase. The value к = 0.4 was chosen to correspond with the best agreement to experimental data for flow in a vertical tube. Using the Smith correlation, the slip is defined as follows.

where x is the mass quality, pg is the density of the gas phase and p1 is the density of the liquid phase.

3.3.2 Drift-flux model

The main formulation of drift-flux model was developed by (Zuber and Findlay, 1965). This model takes into account both the two-phase flow non-uniformity and local differences of velocity between the two phases. The slip is defined as follows.

where U gJ is averaged gas phase drift velocity.

Where m is the mass flux

The remaining two unknowns are empirical and (Lellouche et al., 1982) is used to estimate these.

3.3.3 Schrage correlation

The correlation by (Schrage, 1988) is based on empirical data from an experimental test section, which measures void fraction directly. This test section has two valves capable of isolating a part of the flow almost instantaneously.

The correlation is based on physical considerations and assumes two different hypotheses:

The Schrage correlation is as follows:

Eg / Egh = 1+0.123 Fr-°-191lnx

with

This correlation was established with an air-water mixture, but it remains valid for any other phase flow.