CFD modeling of two-phase annular flow toward the onset of liquid film reversal in a vertical pipe

This paper presents a CFD simulation to film reversal phenomena in an annular flow in a vertical pipe of diameter 76.2 mm by performing a 2D axisymmetric numerical simulations using ANSYS Fluent^® version 16.1 commercial software. In a vertical annular flow, the gas phase is driven by the pressure gradient along the pipe while the liquid is, mainly, driven by the shear force acting at the liquid-gas interface. When the gas velocity decreases, the liquid film will start falling down causing a problem known as “liquid loading” or film reversal phenomenon. The aim is to predict the onset of film reversal so as to allow necessary measures to be taken to mitigate the problem of liquid loading. Simulations were made using air and water as the main working fluids in a 3 m long pipe. The model results agreed well when compared with experimental results, from literature, regarding pressure gradient measurements and critical velocity predictions. Due to the presence of the viscous sublayer, the shear stress fluctuates with high amplitude in the vicinity of the pipe wall where the liquid phase dominates and gradually reduces towards the pipe center. The mass transfer at the interface (entrainment and deposition of liquid droplets) may also affect the shear stress and its space and time variations.