Slug-to-churn vertical two-phase flow regime transition study using an interface tracking approach

•Direct numerical simulation (DNS), implementing the level set method, of vertical two-phase flow regime transition from slug flow to churn-turbulent flow in a pipe using the PHASTA code.•Behavior of the two-phase flow simulation is shown to be strongly dependent on the pipe length and mesh resolution.•The DNS is shown to be in good agreement with experiments and theory.•The simulations show the transition from slug to churn flow is related to when the wake of the bubble reaches the nose of the following bubble. Direct numerical simulation (DNS) coupled with an interface tracking method (ITM) is used to demonstrate the applicability of DNS to the study of vertical two-phase flow regime transition. The study focuses on the slug flow to churn-turbulent regime transition. PHASTA, a finite element based flow solver which utilizes the level set method for interface tracking, has been used to perform the presented simulations. A domain size study has been conducted in order to find pipe dimensions that match the natural wavelength of the periodic slug flow. A mesh resolution study has been completed. The results show that using DNS to simulate the slug-to-churn transition is within the capabilities of the code as the simulations agree well with experimental data and empirical knowledge. The simulations were analyzed to find flow features such as the velocity profile in the wake of a Taylor bubble and the bubble interface shape evolution during breakdown. An understanding of such features could help identify the driving physics behind the transition phenomenon.