Numerical Study Of The Churn-slug Transition
Dynamics In Vertical Upward Air-water Flows
Author(s)
F. H. J. Imada, F. Saltara & J. L. Bali˜no
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 sucient 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
Keywords
multiphase flow, slug flow, churn/slug transition, computational fluid dynamics, volume of fluid
Related Book
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