Much work has been done to properly characterize the two-phase flow of liquid and gas in pipes. The importance of these flows extends over several branches of Engineering, notably nuclear and petroleum engineering. During their transportation, these two phases are subjected to several factors - gravity, viscosity and density gaps, different relative velocities between the phases and turbulence, to name a few relevant ones - that act in the spatial distribution of the liquid-gas interfaces bringing forth those that are known as flow regimes or, most commonly, flow patterns. The type of flow known as slug flow stands out as one of the most frequent, or the most frequent thereof, among these patterns. It main characteristics are intermittency and the continuous succession of two well-defined structures: an elongated bubble, which is a bulky accumulation of gas wrapped in a thin liquid film and the liquid slug, a significant amount of liquid containing mostly liquid with gas bubbles of variable size in its interior. Those two-structures, together, constitute that what is known a slug unit or, from a more computational standpoint, a unit cell. Due to their complexity, modelling slug flows has been a challenge over the last four decades. In the present day, steady-state one-dimensional models based on the unit cell concept and more accurate physical representations based on two-fluid models or slug tracking models which embed transient flow capabilities are available. Nevertheless, and notwithstanding the efforts that have been carried out, those models still require closure relationships that could bring three-dimensional flow features into the one-dimensional model, and those relationships must be backed by trustworthy experimental data. With this scenario in mind, the present work brings forward a brief review on two-phase, liquid-gas slug flows in circular pipes, be they horizontal, vertical or inclined, and a comprehensive state-of-the-art on the closure relationships aimed at predicting flow parameters such as the front bubble velocity, slug frequency and liquid slug holdup. An analysis of the performance of these correlations with experimental data for horizontal slug flows from the Universidade Estadual de Campinas' 2-Phase Flow Group (2PFG/DE/UNICAMP) was carried out and its results are presented.

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