Numerical Study of Gas–Liquid Two-Phase Flow Regimes for Upward Flow in a Helical Pipe

This study presents numerical investigations of air–water two-phase flows for upward flow in a vertical helical pipe. The numerical simulations were done in accordance with the experimental work of Zhu et al. 2017, which aimed at identifying the different flow regimes and the transition maps of the flow [Chem. Eng. J. 2017, 308, 606–618]. Accordingly, the same geometrical parameters and flow conditions were retained in the simulations. The main objectives are (1) to assess the ability of the volume of fluid (VOF) model to accurately predict the two-phase flow regimes and (2) to provide further details about the two-phase interaction processes. This includes the distribution of the two phases, the velocity profiles, and the secondary flow structures under two-phase flow conditions. Mesh and time step independence tests were first performed to minimize discretization errors. The numerical results were then validated against experimental velocity profiles from the literature, showing very good agreement. Flow conditions corresponding to all the possible two-phase flow regimes were considered in the simulations, discussing the accuracy of the numerical modeling and some characteristics of each distinct flow regime. In most cases, the numerical flow regimes are very comparable to the experiments. However, very fine bubbles could not be resolved with the employed mesh resolution (7.64 million cells); they would require an even finer, computationally too expensive mesh. Finally, the internal flow details have been compared between single- and two-phase flows, revealing very complex streamlines and velocity profiles under two-phase flow conditions compared to the Dean vortices observed for single-phase flows.