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Modeling of Two-Phase Flow and Slug Flow Characteristics in

Horizontal/Inclined Pipelines using CFD,

Conference Paper · October 2007

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 Modeling of Two-Phase Flow, and Slug Flow Characteristics in
Horizontal/Inclined Pipelines using CFD
W. Brandstaetter*, A. Ragab*, and S. Shalaby**
*Dept. of Petroleum Engineering and Mineral Resources, University of Leoben, Austria
** Department of Petroleum Engineering, Suez Canal Uni., Egypt
Abstract
The use of two-phase flow systems is common in the oil and gas industry. Two-fluid flow is encountered e.g. in the flow
lines, gathering systems or transportation. Even in gas pipelines where the gas enters the pipeline as a single phase fluid,
condensation of liquids can occur due to pressure and temperature drop along the line to form multiphase flow system.
The flow regimes can be grouped into: dispersed, separated, and intermittent flows or combinations of these. Intermittent
flow regimes are characterized by being non-continuous in the axial and radial direction, and therefore exhibit locally
unsteady behaviour, such as slug, plug, and elongated bubble flow, continuity of both phases is interrupted and a series on
inertial and kinetic force changes occurs throughout the line section. Therefore, the overall performance of the segment is
the summation of a series of intermittent and large and fluctuating rates of gas and liquid that dramatically can decrease the
production rates and in the worst case may cause damage and/or shut down of separation facilities.
In this study, the fluid dynamics of gas liquid slug flows in horizontal/inclined pipes are investigated using 2D and 3D
Computational Fluid Dynamics (CFD) tools to gain deeper insight into complex flow phenomena. The two-phase Volume
of Fluid model (VOF) has been used to predict the transition of segregated gas-liquid flow into slug flow patterns and a
number of cases has been studied using different pipeline geometries.
A set of simulation runs was performed to compute flow patterns in horizontal and inclined gas-liquid pipelines. The first
set of runs was done using a horizontal pipe with a 2-inch diameter and the results were verified against experimental
work. The study covers a wide range of fluid flow rates (The superficial velocities of the gas and liquid phases range from
0.05 m /s to 200 m /s and 0.05 m/s to 10 m/s respectively). The slug flow characteristics have been calculated, and new
relations between the superficial liquid velocity and liquid hold up have been derived.
The second set of runs was conducted for a pipeline with inclination angels +5, 0, and -5 degrees. The effect of pipe
inclination on the flow regime was studied and the flow patterns, liquid holdup and pressure drop have been computed.
Introduction
If gas and liquid flow simultaneously in a pipe, different flow regimes may result. One of the most common of these
patterns is slug flow in which plugs of aerated liquid are separated by regions of gas riding on a thin liquid film. Plug flow
“or elongated bubble” flow is considered a limiting case of slug flow. Pressure drop during slugging may be an order of
magnitude higher than would be the case if the flow were homogeneous or stratified[1,2]. Therefore, slug flow can pose
serious problems to the designer and operator of two-phase flow systems. The slugging problems include severe
production reduction, damaging production equipments and facilities, such as separator vessels and pipelines. Therefore, it
is important to understand the mechanism and the behaviour of the slugging operation. Generally several operations in an
oil field can cause such kind of slugging: transient effects, start-up and blow-down and pigging operations.
Two-phase flow studies in this context have a long history [3-9], and much recent effort has been directed towards the
development of general mechanistic models for steady-state two-phase flow. Most lack generality and contain predictive
inadequacies. In contrast to this CFD which is based on the numerical solution of the fundamental equations governing the
conservation of mass, momentum and energy, offers an attractive alternative, it prevails the construction of flow pattern
diagram and comparison with experimental work. The flow pattern diagram has been constructed and compared with an
experimental work of the previously publish maps.
Slug Tracking Models
Generally speaking there are two ways to locate the interface between the phases, [1]Eulerian Grid Methods (Marker-And-
Cell, MAC and Volume of Fluid, VOF[10, 11]), and [2]Lagrangian grid methods[12, 13].
Pipeline geometries and Grid generation
The geometries of gas-liquid two-phase flows in horizontal pipelines have been chosen based on experimental
investigation of published maps. The pipeline has 12.7 m long and 0.0508 m (ID=2-in) diameter. Then after verifying the
data with the experimental works, our flow patterns have been developed and sketched. The computational mesh
consisting of approximately 123000 grid cells was made by using the pre-processor Gambit 2.2.30 of the commercial CFD
package.
CFD-Calculations and the Results
For each set of test runs, the gas and liquid inlet flow was varied while the outlet temperature and pressure were
maintained constant. In order to cover a wide range of flow rates, about 62 simulated runs for each set, have been made
using various boundary conditions. Superficial velocity ranges from 0.05 to 100 m/sec and from 0.05 to 10 for gas and
liquid respectively. Over the range of flow rates and pipe inclination angles considered here, stratified and stratified wavy
flow, slug flow, bubble flow and annular flow patterns were observed.
150 Years of Petroleum Industry: Tradition & Challenges, 14-17 October, Bucharest, Romania
SPE Romanian Section Conference
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Flow Pattern Determination

The volume of fraction distribution of the gaseous and liquid phase phases in the computational domain was initialized
with a mean gas and liquid fraction of 50%. The 3-dimentional CFD Volume of Fluid model (VOF) simulations have
clearly shown the feasibility of all flow patterns simulation with the available multiphase flow model.
Based on visual observations, and the extensive data sets that were collected on ground for two-phase gas-liquid flows in
horizontal pipe, each flow regime has been identified. In Figure 1, the results of a typical slug flow simulation are
depicted. Then, every set of runs has been categorized to represent a certain region in the flow pattern diagram and we end
up with all of these categorized sets were re-plotted on a combined Mandhane[8] and Weisman[14] maps as shown in Figure
2. A good agreement between the two flow maps was observed except minor regions.
Slug Transitional Velocity
In multiphase flow, the complex relation between the phases sometimes leads to appearance new parameters, in case of
slug flow, a slug unit forms, and own its special characteristics, such as velocity, length, hold up and frequency. All of
these characteristics were computed by the CFD – VOF model. In this study, the slug transitional velocity was determined
by dividing the time required for a slug to travel between two points into the distance between the two positions. It was
observed that all of them have significant relations with the superficial velocities of the phases and plotted and verified
against the published experimental works. The best fit equation appears from this relation to relate the mixture velocity
and mean slug body velocity with a good fitting quality (R2 = 0.9517) is as follows:
Vslug = 1.9732Vm − 0.1369
Analysis of the CFD calculation shows that the slug transitional velocity mainly depends on the mixture velocity of the
phases (Vm =Vsg + Vsl) and has a considerable agreement with the published correlations [15, 16].
Average Slug length
When a pipeline is operated in slug flow, the estimation of the slug length that can be expected at the end of the pipeline is
very important in the design of separation facilities. The average slug length is a complex function of many variables: pipe
diameter and the length of the pipe, the topography of the line, the gas and liquid velocities, the physical properties, and
gas density. Basically, the slug length computed to find out the volume of the large slugs and hence design a slug catcher
or separators that are able to handle this volume. Duckler-Hubbard[17], studied slugging phenomena for 38 mm pipe
diameter and found that the slug length was 12-30D while Nicholson et al. [17] and Barnea –Brauner[18] for 25 and 51 mm
pipe diameter found approximately the same results (about 30-32D) but Gregory et al. [19] finding was 30D-375D which is
very large range. Manolis[20] studied air-water system and concluded that the slug length within a range from 10D to 25D
for 53 and 90 mm pipe diameter. Brill and Scott [21] analyzed tests in large diameter pipelines at Prudhoe Bay and found
the mean slug length is about 300-350D.
Figure 3 shows the relation between slug length and liquid velocity for a 2-inch pipeline diameter. This graph shows that
as the velocity of the liquid increases as the slug length increases. In the same time, shows also that at constant liquid
velocity, the slug length decreases by increases superficial gas velocity. Therefore, the slug length mainly increases due to
increase the superficial liquid phase velocity.
This result comes in a considerable agreement with the most famous correlations in oil field, Hill-Wood [22] for calculating
slug length for horizontal and near horizontal pipelines.

Figure 1: Time series of slug propagation

 3

Figure 2: CFD-Flow maps versus Mandhane and Weismann et al. maps.

Figure 3: Represents the slug length versus superficial liquid velocity.

Conclusions
The following main conclusions can be drawn for this modeling study:
1. The flow regime in horizontal and near horizontal pipelines heavily depends on the flow rate of the inlet fluids.
2. Volume of Fluid (VOF) solver has a large potential to construct the flow pattern diagram for horizontal and inclined
pipeline, two-phase flow.
3. Slug flow characteristics have been studied and new correlation has been developed for mean slug velocity.
4. The simulation results showed that the slug body length is not constant and increase along the pipeline while flowing
to certain extent.
5. The analysis of the results showed that the flow structure deeply modifies along the pipe.

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 4

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