Modelling Of Stratified Two Phase Flows Using An
Interfacial Area Density Model
Author(s)
T. Höhne & C. Vallée
Abstract
Stratified two-phase flow regimes can occur in chemical plants, nuclear reactors
and oil pipelines. A relevant problem is the development of wavy stratified flows
which can lead to slug generation. The slug flow regime is characterized by an
acceleration of the gaseous phase and by the transition of fast liquid slugs, which
carry a significant amount of liquid with high kinetic energy. It is potentially
hazardous to the structure of the system due to the strong oscillating pressure
levels formed behind the liquid slugs as well as the mechanical momentum of the
slugs. Because these flow conditions cannot be predicted with the required
accuracy and spatial resolution by the one-dimensional system codes, the
stratified flows are increasingly modelled with computational fluid dynamics
(CFD) codes. In CFD, closure models are required that must be validated. The
recent improvements of the multiphase flow modelling in the ANSYS CFX code
make it now possible to simulate these mechanisms in detail. In order to validate
existing multiphase flow models and to further develop these, measurement data
with a high-resolution in time and also in space are needed. For the experimental
investigation of co-current air/water flows, the HAWAC (Horizontal Air/Water
Channel) was built. The channel allows in particular the study of air/water slug
flow under atmospheric pressure. Parallel to the experiments, CFD calculations
were carried out. The two-fluid model was applied with a special turbulence
damping procedure at the free surface. An Algebraic Interfacial Area Density
(AIAD) model on the basis of the implemented mixture model was introduced,
which allows the detection of the morphological form of the two phase flow and
the corresponding switching via a blending function of each correlation from one
object pair to another. As a result this model can distinguish between bubbles,
droplets and the free surface. The behaviour of slug generation and propagation
at the experimental setup was qualitatively reproduced by the simulation, while
local deviations require a continuation of the work. The creation of small
instabilities due to pressure surge or an increase of interfacial momentum should
be analysed in the future. Furthermore, experiments with pressure and velocity
measurements are planned and will allow quantitative comparisons, also at other
superficial velocities.
Keywords: CFD, stratified flow, slug flow, HAWAC.
Keywords
CFD, stratified flow, slug flow, HAWAC
Related Book
Other papers in this volume
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