There are several flaws in the two-fluid model, which is mostly applicable to the hit flow pattern and is used to anticipate transitory flows. A unified model known as the "two fluid/two-flow-pattern model" is proposed in this work. It begins with the existence of two fundamental flow patterns: separated flow (stratified or annular) and dispersed flow (with bubbles or droplets). It is commonly recognized that slug flow is nothing more than a random pattern composed of time- and space-sequential sequences of separated flow (Taylor bubbles) and bubbly flow (liquid slugs). A continuous function of time and location, which represents the probability that the separated flow will occur, is intended to characterize the flow pattern. The mass and momentum conservation equations, which are defined for each phase and every fundamental flow pattern and averaged throughout the cross section, constitute the basis of the model. The fundamental equations are distributed by the probability of the related basic pattern to produce the generic two-phase flow equations for mass and momentum. Our focus has been limited to physical scenarios short of interfacial mass transfer. The order of magnitude of this term has been calculated for slug flow: the results show that it cannot be ignored, especially in transient flows.

Two-fluid model, liquid slugs, Phase fractions, drift-flux models.