Numerical investigation of horizontal two-phase plug/bubble flows: Gravitational effect on hydrodynamic characteristics

Abstract

Abstract This paper investigates hydrodynamic characteristics of horizontal gas–liquid plug/bubble flows under various gravity levels. A three-dimensional computational fluid dynamics (CFD) model based on the Volume of Fluid (VOF) approach, was developed and validated with experimental at normal gravity observations and previously published correlations in literature. The gravity levels used for the simulations include 2 g o (hyper), g o ( = 9 . 81 m/s earth), 0 . 38 g o (Mars), 0 . 17 g o (Moon), and 1 0 − 4 g o (microgravity). The simulations were conducted for two pairs of working fluids (air–water and air–oil) each for two different superficial velocities. Results indicate that the maximum time-averaged void fraction at selected tube cross-sections is shifted toward the center line at reduced gravity due to a decreased buoyancy force. At microgravity, axisymmetric bubbles exist at the center of the tube creating thin liquid films at the tube wall, while at hyper-gravity, a bubble is pushed to the top region of the tube, leaving a thick liquid film at the bottom. The time-averaged void fraction is slightly increased by reducing the gravity which indicates the volume of the gas phase slightly expands at reduced gravities. A simplified theoretical model predicts the onset of the gravity condition well in which a bubble detaches from the tube wall. Bubble detachment causes a slight increase in the translational velocity of gas bubbles. Pressure drop of gas–liquid plug/bubble flows shows a complex behavior as gravity is reduced from hyper ( 2 g o ) to micro ( 1 0 − 4 g o ) . However, for all of the two-phase flow conditions used in this study, the minimum pressure drop occurs near the normal-gravity.