Numerical Study Of The Churn-slug Transition Dynamics In Vertical Upward Air-water Flows

Abstract

In gas-liquid two-phase flows through vertical ducts, the spatial distribution of phases may assume several configurations, known as flow patterns. Slug and churn patterns are characterized by an unsteady behaviour and their boundary is a subject of interest for many researchers. Di?erent descriptions of mechanisms occurring at churn/slug transition may be found in the literature. One transition process is described by Taitel et al. (Modelling flow pattern transitions for steady upward gas-liquid flow in vertical tubes. AIChE Journal, 26, pp. 345–354, 1980), in which churn flow is an entrance condition observed upstream of a developed slug flow if there is su?cient pipe length for this to happen. In the present work, vertical upward air-water flows through a 2-inch i.d. tube are simulated with the commercial finite-volume-based code FLUENT, aiming to study this churn/slug transition process through spatial and temporal phase distribution visualizations and statistical analyses of void fraction histories at points along the domain. The incompressible Volume of Fluid (VOF) formulation allied to a Piecewise Linear Interface Construction (PLIC) technique is applied to capture the distribution of phases throughout three-dimensional domains. Turbulence e?ects are accounted through the Realizable k-epsilon model. Three di?erent tube lengths and two sets of superficial velocities are studied in order to understand their influence in liquid slug collapse and bubble coalescence processes. Void fraction histories at points distributed along the duct centerline are stored during a simulation time long enough to provide statistical data with reasonable errors. Fast Fourier Transform analyses are used to verify the transition point and flow characteristic frequencies. Probability distributions of void fraction at cross sections along the tube are also evaluated. The VOF method was able to qualitatively capture the mechanisms of churn/slug transition processes. Results for entrance length and frequencies are