cfd simulations of low liquid loading mpf in horiz pipes fedsm2014-21856

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CFD Simulations of Low Liquid LoadingMultiphase Flow in Horizontal Pipelines

CONFERENCE PAPER AUGUST 2014

DOI: 10.1115/FEDSM2014-21856

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5 AUTHORS, INCLUDING:

Carlos F Torres

University of the Andes (Venezuela)

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Mazdak Parsi

DNV GL, Katy, TX, USA


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Eduardo Pereyra

University of Tulsa


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Cem Sarica

University of Tulsa


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1 Copyright 2014 by ASME

CFD SIMULATIONS OF LOW LIQUID LOADING MULTIPHASE FLOW IN

HORIZONTAL

PIPELINES

Hamidreza KaramiThe University of Tulsa

Tulsa, OK, 74104

Carlos F. TorresThe University of Tulsa

Tulsa, OK, 74104

Mazdak ParsiThe University of Tulsa

Tulsa, OK, 74104

Eduardo PereyraThe University of Tulsa

Tulsa, OK, 74104

Cem SaricaThe University of Tulsa

Tulsa, OK, 74104

ABSTRACTLow Liquid Loading is a very common occurrence in wet gas

pipelines where very small amounts of liquid flow along with

the gas, mainly due to condensation of hydrocarbon gases and

water vapor. The effects of low liquid loading on different flow

characteristics, and flow assurance issues such as pipe corrosion

prove the necessity of analyzing the flow behavior in moredepth. In this study, CFD simulations are conducted for a

horizontal pipe where liquid and gas are supplied at separate

constant rates at the inlet. The liquid is introduced at the

bottom to help shorten the developing section. The simulations

are conducted with Ansys Fluent v14.5 using Volume Of Fluid

(VOF) as the multiphase model. The analysis targets, mainly,

the shape of the interface, velocity fields in both liquid and gas

phases, liquid holdup, and shear stress profile. On the other

hand, experiments are conducted in a 6-inch ID low liquid

loading facility with similar testing condition. Experiments are

conducted with water or oil as the liquid phase for a liquid

volume fraction range of 0.0005 - 0.0020 of the inlet stream.

For all cases, several flow parameters are measured includingliquid holdup and interface wave characteristics. A comparison

is conducted between CFD simulation results, model

predictions, and experimental results, and a discussion of the

sources of discrepancy is presented. Overall, the results help

understand the low liquid loading flow phenomenon.

INTRODUCTIONLow liquid loading flow is the flow condition wherein the

liquid flow rate is very small as compared to the gas flow rate

It is widely encountered in gas condensate pipelines. Meng e

al. (2001) defined it as the flow conditions when liquid flow

rate is less than 1100 m3 per MMsm3 gas flow rate. Even

though the pipeline is fed with single phase gas, condensation o

the heavier components of gas phase along with traces of water

results in three phase flow. The presence of these liquids in thepipeline, although in very small amounts, can influence

different flow characteristics, such as pressure distribution

Many issues like hydrate formation, pipe corrosion, pigging

frequency, and downstream facility design associated with

pressure and holdup are also affected. Therefore, understanding

of the flow characteristics of low liquid loading gas-oil-water

flow is of great importance in transportation of wet gas.

Limited amount of studies have been conducted on low

liquid loading flows. Most of the existing experimental studie

utilized small diameter pipes. The predictions of the existing

models for pressure gradient, liquid holdups, wetted-wal

perimeter, liquid entrainment, and etc. are not satisfactory fo

low liquid loading flow (Fan,2005; Dong, 2007; Gawas 2013).Computational fluid dynamics (CFD) tools are very helpfu

means to solve governing equations for fluid flow under

different conditions and complement experimental work

Several investigators have tried to use CFD to simulate

multiphase flow in pipeline, and in particular, low liquid

loading stratified wavy flow. In one of the earliest attempts

Shoham and Taitel (1984) combined a 2-D momentum equation

and an eddy viscosity turbulence model, and presented the

Proceedings of the ASME 2014 4th Joint US-European Fluids Engineering Division Summer Meeting

FEDSM2014

August 3-7, 2014, Chicago, Illinois, USA

FEDSM2014-21856


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computations for the liquid phase. However, in the gas phase,

only bulk flow calculations were made to estimate gas and

interfacial shear stress values. Later, Newton and Behnia

(2000) tried to predict pressure gradient and liquid holdup using

another two-dimensional simulation and with the assumption of

smooth interface. They also presented some wall and interfacial

shear stress values. Mouza et al. (2001) used a flat interfaceand separate conduits for the two phases, coupled at the

interface to simulate the stratified flow. They tried to simulate

the waves at the interface with a static equivalent roughness,

which led to some uncertainty. In their method, the film

thickness is an input to obtain velocity and shear stress profiles.

They conducted the simulations with CFX using the

homogeneous model.

Bartosiewicz et al. (2008) also used VOF approach to

obtain transition from stratified smooth to stratified wavy flow,

wave celerity, and critical wave number. They observed a good

match between simulation results, experimental data, and linear

inviscid theory. They also compared VOF and multi-field

approaches and found some discrepancies in wave amplitudeand wave growth. This study uses the capability of Volume Of

Fluid (VOF) model to capture the gas-liquid interface in

transient flow.In this study, a horizontal pipe is considered. The

liquid and gas are introduced separately with constant rate inlet

boundaries. The liquid inlet is placed at the bottom to help

shorten the flow developing section. The analysis targets are

mainly the shape of the interface and the liquid holdup.

EXPERIMENTAL DESIGNIn this study, the experimental data used are from the tests

conducted in a 6 in. ID flow loop. Figure 1 shows the

schematic of the facility.

Figure 1. Low Liquid Loading 6-in. ID Facility Schematic

The flow loop consists of two parallel sections, with 6-in.

(0.15 m) ID pipes. Each section is 56.4 m long. Acrylic

visualization sections about 8 m long are provided at the end of

each section. The inclination angle can change from 0,

horizontal case, to 2 in inclined case. Two back pressure

valves installed at the outlet of the separator control the

pressure in the flow loop. The facility has been previously

utilized by several researchers to investigate two or three phase

low liquid loading flow.

The test fluids are air and water. Air is provided using two

different compressors and water phase is provided, using tap

water from the Tulsa city water supply. Three differentia

pressure transmitters, with uncertainty of 0.2 in. of water areused to get the pressure drop values.

Five quick-closing valves (QCV) are used to bypass the

flow and at the same time trap the liquid in the test sections

The liquid trapped in the QCV is pigged out with a specially

designed pigging system and is drained into graduated cylinders

to measure the oil and water volumes. The uncertainty of liquid

holdup measurements was analyzed by some pigging efficiency

calibration tests. After two pigging operations, at least 98% o

the liquid is drained out of the section. An approximated value

of 100 ml. was added to the measured liquid volume in all the

tests to take into account the residual liquid in the section.

A set of conductivity probes was used to conduct wave

characteristics analysis, mainly for two-phase flow of water andair. Two probes were positioned 6 inches apart in the facility

The cross correlation between the wave signals from the two

probes gives the wave celerity of the interface. Both static and

dynamic calibration tests were conducted for the probes, and

the results were repeatable, and very close for both probes. The

resulting calibration curve was used for further wave

characteristics analysis and estimation of liquid film thickness a

the bottom of the pipe.

The liquid superficial velocity is kept either at 1 or 2 cm/s

and the gas superficial velocity is varied between 10 m/s and

22.5 m/s. These superficial velocity values ensure the low

liquid loading stratified flow.

The experimental results obtained for different parametersare used to benchmark CFD simulation results and documen

discrepancies. In particular, liquid holdup and liquid film

thickness are considered and analyzed in this work.

CFD SIMULATION DESIGNThe purpose of the CFD simulations was to obtain liquid

holdup for different flowing conditions, and compare the

outputs with experimental data. For this objective, symmetry in

tangential direction condition is assumed. Half a pipe i

considered with an inner diameter of 6 in. and a length of 15 ft

In order to shorten the developing section, inlet plate was

divided into two zones. Liquid was entered from the bottomzone, and the gas was entered from the other inlet zone. Figure

2 shows the schematic of the section geometry and meshing.


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Figure 2. Test Section Geometry Schematic

The entrance velocities are corrected by the area of the

entrance for each phase. The equations used to obtain liquid

and gas velocity values at their inlet zones are:

1,

,2 zon e

pipeSL

inletLA

Av

v . (1)

2,

,2 zon e

pipeSG

inletGA

Avv . (2)

The outlet boundary is a pressure boundary with given

pressure value of 9 psig in order to simulate the experimental

conditions. The liquid phase is water with given default fluid

properties, and the gas phase is air. The multiphase flow model

used was Volume Of Fluid model (VOF) with two Eulerian

phases, in an explicit time integration fashion. As a viscous

model, standard k- (2 eqns.) is used with default model

constants and standard wall functions.

Gravitational acceleration is activated in negative y-direction with given default value of 9.81 m/s2. Transient runs

are obtained with a total run time of between 15 to 20 seconds

for all cases. The time step value is chosen to vary using

Courant-Friedrich criterion, which is one of the most common

ways to check the stability of an explicit scheme, with one as

the global Courant number.

As the initial condition, water volume fraction is chosen to

be 0.002, which is close to the no-slip condition. This is

expected to shorten the stabilization period. Standard

initialization with Pressure Implicit with Splitting of Operator

(PISO) scheme for pressure-velocity coupling was chosen.

The pipe geometry and meshing is designed using Gambit,

a commercial software for grid generation. In order to analyzethe effects of meshing quality, three different meshing types are

implemented, namely, coarse, medium, and fine meshing. The

total number of grid blocks in the coarse meshing is 96000;

while for the medium mesh, this number increases to 224250

grid blocks. The fine meshing design includes the highest

number of grid blocks with 864000 grids.

Two different experimental conditions are tried with each

mesh quality. In order to decide if a meshing quality is working

well for an experimental condition, the resulting liquid holdup

values at the outlet were compared. Figures 3, 4, and 5 are

showing the acquired results for the two experimental cases

using coarse, medium and fine mesh, respectively.

Figure 3. Coarse Mesh Results for Two Cases

Figure 4. Medium Mesh Results for Two Cases

Figure 5. Fine Mesh Results for Two Cases

Using the coarse mesh, the lower gas flow rate case with

vSG value of 10 m/s is simulated smoothly. This means that a

continuous liquid film is simulated at the bottom of the pipe

along with some minor fluctuations in liquid volume fraction


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These oscillations could be due to the interfacial wave structure.

However, for the case of higher gas flow rate, as shown in Fig.

3, a continuous film cannot be simulated, and very large

oscillations in liquid film are an indicator that liquid is flowing

in a separated surging manner. Overall, the coarse mesh was

not able to capture the whole fluid behavior, especially for the

cases of higher gas flow rate.The cases with medium and fine mesh are showing

smoother curves for liquid volume fraction. This shows that in

both cases a continuous liquid film can be formed in the

simulation. Fine mesh simulations take significantly longer

times. Therefore, medium mesh is used as the main simulating

tool in further analysis. However, results with both mesh

qualities are presented in the next section.

Different discretization methods can produce different

results in a computational work. In this study, the least squares

cell based method is used for gradient spatial discretization.

The momentum is discretized in a second order upwind fashion,

and first order upwind method is used for turbulent kinetic

energy and dissipation rate discretization.Volume fraction spatial discretization method is an

important parameter in a multiphase VOF CFD simulation.

Two methods are tried for this option. Geo-reconstruct method

and compressive method are the tested options. Three

experimental cases are simulated using both methods for

volume fraction discretization. Figure 6 shows the liquid

volume fraction results using each method along with the

experimental results for the three cases.

Figure 6. Simulation Results with Two Different Volume

Fraction Discretization Methods

The geo-reconstruct method seems to over-predict and the

compressive method seems to under-predict the value of liquid

volume fraction. On the other hand, if the fluctuations in liquid

fraction are interpreted as the result of the interfacial wave

structure, geo-reconstruct method does a better job predicting

this phenomenon. Especially for the higher gas flow rate case

with vSGvalue of 15 m/s, the wavy structure is clearer. Overall,

geo-reconstruct method seems to be a better option for a

volume fraction spatial discretization method. This method has

been used for the simulations conducted in this study.

RESULTS AND ANALYSISSeveral different parameters can be acquired as the outpu

of a CFD simulation. For a multiphase low liquid loading flow

some of these parameters are liquid holdup, liquid film

thickness at the bottom of the pipe, velocity profiles at liquid

and gas phases, pressure drop, wave structure, and liquiddroplet entrainment. The entrainment phenomenon is one of the

most important ones since it significantly affects othe

parameters. It has been studied by several researchers

especially in recent years. However, simulating the entrainmen

process with CFD requires a very fine mesh design that comes

with an excessive run time. The mesh design applied in thi

study is not able to capture the entrainment phenomenon. This

is considered as one of the main sources of uncertainty of the

results, especially for higher gas flow rate cases where

entrainment becomes more significant.

The results obtained for different parameters are analyzed

separately, and the simulation predictions are compared to

experimental results, where applicable. These parametersinclude liquid holdup, liquid film thickness, wave

characteristics, velocity profile, and wall shear stress.

Liquid Holdup

The liquid holdup is the areal fraction of the liquid phase in

the test section. It is measured as an area weighted average o

the liquid phase volume fraction in the outlet plate. Due to the

waves formed at the interface, the volume fraction value

changes with time. Therefore, the average value of the volume

fraction over a period of time was considered to be the liquid

holdup for the given case.

Figure 7 shows the contours of volume fraction for two

cases, one with very low vSGvalue of 10 m/s at the right, andone with the highest vSG value of 22.5 m/s at the left. Both

cases have vSL value of 2 cm/s. However, the increase in ga

flow rate has increased the drag force significantly, dropping the

liquid level in the section and decreasing the liquid holdup.

Figure 7. Liquid Fraction Contours for Lowest and Highest

Gas Flow Rate Cases

vSG= 22.5

m/s

vSG= 10

m/s


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One of the most common models used in an ordinary

stratified wavy flow in pipelines is the model developed by

Taitel & Dukler (1976). This model combines the momentum

balance equations for gas and liquid phases and formulates all

of the variables as functions of the liquid film thickness.

Wettability effects are ignored, and the interface is assumed to

be flat. The liquid holdup is calculated from the film thicknesswith a simple geometrical relationship. The results of this

model are used for comparison purposes.

The CFD simulation results are shown for five cases, using

medium mesh design, along with three cases using fine mesh

design. Figure 8 shows the liquid holdup results from the

experiments, compared with the CFD simulations and Taitel &

Dukler model.

The medium and fine meshes are typically used for low and

high gas flow rate cases, respectively. The experimental results

have been predicted fairly well using Taitel & Dukler model.

The CFD simulations results are in an acceptable range. A

small over-prediction is observed for medium mesh simulation

results, especially for higher gas flow rate cases. Theentrainment phenomenon cannot be simulated accurately, unless

the grids are small enough to capture very small droplets ripped

off the liquid surface. This can be the reason for the

discrepancy.

Figure 8. Liquid Holdup Results from Experiments,

Simulation, and Modeling

Liquid Film Thickness

In the experimental work, the liquid film thickness is

obtained by time-averaging the voltage signals from a set of

conductivity probes. These probes are placed at the bottom ofthe pipeline and their signals are indicators of the thickness of

the water layer. The results of the tests conducted with different

vSGvalues, and vSLof 2 cm/s are used for the following analysis.

The interface is assumed to be flat and the entrainment

phenomenon is neglected in Taitel & Dukler model. This

means that the liquid holdup is related with a simple

geometrical relationship to the film thickness. The multiphase

stratified wavy flow can be modeled with flat interface

geometry, especially for a large diameter pipeline. However

when the vSGvalue increases and the flow pattern gets closer to

annular flow, the turbulence and surface tension effects become

dominant and the interface curvature increases. For the

experimental range of this study, the flat interface seems to be a

good option for the geometry design.

In order to obtain film thickness values from CFDsimulations, a line is created at the center of the pipeline

connecting the bottom to the top of the pipe. The liquid phase

volume fraction on this line is averaged over the run time

Since the entrainment cannot be detected in the simulation, it is

assumed that the whole liquid fraction is flowing in the film

Multiplying this fraction with pipe diameter gives the liquid

film thickness. Figure 9 shows the comparison between the

experimental results, model prediction, and CFD simulation

results with the two given mesh size. Taitel & Dukler mode

tends to under-predict the film thickness and CFD simulations

tend to over-predict it. The method used for the estimation o

film thickness, which includes all of the liquid on the centerline

as part of the film, can be the reason for over-predictionOverall, considering different sources of uncertainty involved

with the simulation, the results are within an acceptable range.

Figure 9. Liquid Film Thickness Results from Experiments

Simulation, and Modeling

Interfacial Wave Characteristics

The waves in the liquid-gas interface are of significan

importance in a stratified wavy flow. By increasing the ga

flow rate, these waves can change from 2-D waves to 3-D

waves, and then to roll waves. One of the most interesting

observations in the CFD simulations is the structure of thewaves. Figure 10 shows a snapshot of these waves in the

simulation.

Different wave characteristics, such as the wave celerity

frequency, amplitude, and length can be used to identify the

interfacial wave. In order to increase the accuracy of prediction

for these parameters, a very high quality mesh is needed. A

comparison between the simulation and experimental results is

not completed at this stage. The difference in definition of the


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wave can affect the results for the wave amplitude and wave

frequency. However, the wave celerity experimental results

have been compared to the interfacial velocity from CFD

simulations in the next section.

Figure 10. Wave Structure along the Flow in a CFD

Simulation

Velocity Profile

The velocity profile in a single-phase fluid flow in the

pipeline is symmetric with a parabolic shape. But for the

multiphase flow, and especially in horizontal pipelines, the

gravitational effects are dominant and the symmetry vanishes.

The velocity magnitude in the liquid phase is much smaller, and

the point with maximum velocity is deviated from the center of

the pipeline. Unfortunately, there are no experimental data

available for comparison purposes at this stage. However, the

simulation results will be discussed here.

Figure 11 shows the velocity contours for two cases, one

with very low vSGvalue of 10 m/s at the right, and one with the

highest vSG value of 22.5 m/s at the left. Both cases have vSL

value of 2 cm/s.

Figure 11. Velocity Profile Contours for Lowest and Highest

Gas Flow Rate Cases

The scales are different at the two plots. However, they

both show a similar behavior. The position of the point with

maximum velocity has shifted towards the top of the pipeline.

And also, the velocity magnitude is much smaller in the liquid

phase, compared to the gas phase.

Figure 12 shows the velocity profiles obtained with respect

to the vertical position on the centerline. These plots are

presented for different values of superficial gas velocity

Finding the exact position of maximum velocity requires a very

fine mesh. However, for all of the cases, the point with

maximum velocity has a relative vertical position of about 0.75.

Figure 12. Velocity Profile vs. Relative Vertical Position for

Different vSGValues

In all these curves, at a point around the interface position

the rate of velocity change increases sharply. This point can be

considered as the interface between liquid and gas phases, and

the velocity at this point can be identified as the velocity of

liquid particles at the interface. Figure 13 shows a comparison

between this value and the value of wave celerity, acquired

experimentally. The wave celerity, which is the speed of wave

propagation, is naturally higher than the velocity of the liquid a

the interface. However, the trends are very similar for both

cases.

Figure 13. Simulation Interfacial Velocity and Experimenta

Wave Celerity

Wall Shear Stress

Similar to velocity profile, the wall shear stress value is

uniform in a single phase fluid flow in pipeline. Bu

introducing the second phase results in deviations from the

uniform behavior. Having the liquid film at the bottom and gas

phase on top Taitel & Dukler (1976) proposed three differen

average shear parameters into the simplified momentum balance


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equationsThese are liquid-wall shear stress (WL), gas-wall shear

stress (WG), and liquid-gas interfacial shear stress (I).

In this section, local values of WL and WG are obtained

from CFD simulations, and the results are compared with

outputs of Taitel & Dukler model. There are no experimental

data to compare with these predictions at this stage. Figure 14

shows the shear stress profiles obtained from CFD with respectto the vertical position on the centerline.

Figure 14. Shear Stress vs. Relative Vertical Position for

Different vSGValues

For all cases, the wall shear stress predicted by CFD is

almost constant in the gas phase, with a slight decreasing rate,

as the vertical position decreases. Then, it increases sharply to

a maximum value in the interface, and has a slight decrease in

the liquid film area. This trend can be easily justified,

considering that shear stress is a function of the fluid density

and velocity. The decreasing rate in both phases is due to the

velocity decrease, and the jump in the interface is due to the

jump in density from gas to liquid phase.In Taitel & Dukler, the WLand WG values are assumed to

be constant along the pipeline. Figure 15 shows WL and WGvalues predicted by Taitel & Dukler compared with the CFD

simulation outputs. For the CFD results, the shear stress at the

very bottom of the pipe is used as represent WLand the value at

the top of the pipeline is used to represent WG.

Figure 15. Comparison of Shear Stress Values between CFD

Simulation and Taitel & Dukler Model

The values of WGare matching very well from the mode

to the CFD simulation. However, the values of WLare under

predicted by the model, compared to the CFD simulation. The

uncertainties in prediction of shear stress in the liquid are very

high, considering the very thin liquid film in stratified low

liquid loading flow. The rate of shear stress change inside theliquid film is very high, and in order to increase the accuracy of

the simulation, meshing should be very fine. Having an

accurate estimation of the shear stress profile, especially close

to the interface, is a very helpful in any effort to model stratified

wavy flow.

CONCLUSIONSA set of CFD simulations are conducted for a piece o

pipeline where liquid and gas are supplied at separate constan

rates at the inlet. The results obtained for different parameter

are compared to some experimental data from a 6-inch ID

facility. In addition, Taitel & Dukler (1976) is used for thepurpose of model comparison.

Three different mesh sizes are investigated. The medium

and fine mesh simulation outputs are used in further analysis

Geo-reconstruct method is selected as the discretization method

for the volume fraction.

The results for liquid holdup and film thickness are in an

acceptable agreement with the experimental results. The

interfacial wave structure, velocity profiles and deviation from

symmetric conditions are also observed. The values of wal

shear stress in the liquid and gas phases are compared with the

predictions from Taitel & Dukler. The WG values are very

close, but the WL values show some discrepancy. This

discrepancy can also be related to unsteadiness of the liquidfilm due to the complex wave structure.

NOMENCLATUREWL Liquid-Wall Shear Stress (Pa)

WG Gas Phase Wall Shear Stress (Pa)

I Liquid-Gas Interfacial Shear Stress (Pa)

Apipe Pipe Cross-Sectional Area (m2)

Azone,1 Zone-1 (Liquid Inlet) Area (m2)

Azone,2 Zone-2 (Gas Inlet) Area (m2)

vSL Liquid Superficial Velocity (m/s)

vSG Gas Phase Superficial Velocity (m/s)

vL Liquid Actual In-Situ Velocity (m/s)

vG Gas Actual In-Situ Velocity (m/s)

ACKNOWLEDGMENTSThe authors wish to thank the Tulsa University Fluid Flow

Projects (TUFFP) members for their support of this research.

REFERENCESBartosiewicz, Y., Lavieville, J., and Seynhaeve, J.: A firs

assessment of the NEPTUNE_CFD code: Instabilities in a


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stratified flow comparison between the VOF method and a two-

field approach, International Journal of Heat and Fluid Flow

29 (2008) 460478.

Dong, H.-K.: "Low liquid loading gas-oil-water flow in

horizontal pipes", U. of Tulsa, Tulsa, 2007.

Fan, Y.: "An investigation of low liquid loading gas-liquid

stratified flow in near-horizontal pipes", U. of Tulsa, Tulsa,2005.

Gawas, K.: Low liquid loading in gas-oil-water pipe

flow, PhD Dissertation, The University of Tulsa, 2013.

Meng, W., Chen, X.T., Kuoba, G.E., Sarica, C., and Brill,

J.P.: "Experimental study of low-liquid-loading gas-liquid flow

in near-horizontal pipes". SPE Production and Facilities, 16,

240-249, 2001.

Mouza, A. A., Paras, S. V., and Karabelas, A. J.: CFD

code application to wavy stratified gas-liquid flow, Trans

IChemE, Vol 79, Part A, July 2001.

Newton, C.H. and Behnia, M.: Numerical calculation of

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Flow, 26:327337, 2000.Shoham, O. and Taitel, Y.: Stratified turbulent-turbulent

gas-liquid flow in horizontal and inclined pipes, AIChE J,

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Taitel, Y. and Dukler, A.E.: "A model for predicting flow

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cfd simulations of low liquid loading mpf in horiz pipes fedsm2014-21856

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