Review of droplet entrainment in annular flow: Interfacial waves and onset of entrainment

•Review of the most recent literature on annular two-phase flow.•Emphasize all variables and processes occurring in the liquid–gas interface.•Show the existing scattering found when expressions are compared to each other.•Highlight the gaps of knowledge still existing.•Propose new correlations: liquid film thickness and wave celerity and frequency.

Annular two-phase flow has been vastly investigated because of its large and deep involvement in industrial processes, particularly in nuclear engineering. This paper reviews most of the recent literature on the matter, with emphasis in all those variables and processes occurring in the liquid–gas interface that cause droplet entrainment. Further than presenting correlations, the paper shows the existing scattering found when expressions are compared to each other and it highlights the gaps of knowledge still existing. Additionally, based on some of the open data, alternate equations are derived for key variables in the annular flow descriptions, like liquid film thickness and wave celerity and frequency.