Group Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Title: P1.4 - Flow structures and transition between stratified and non-stratified horizontal oil-water flow
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Permanent Link: http://ufdc.ufl.edu/UF00102023/00437
 Material Information
Title: P1.4 - Flow structures and transition between stratified and non-stratified horizontal oil-water flow Interfacial Flows
Series Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Physical Description: Conference Papers
Creator: Yusef, N.
Al-Wahaibi, T.
Al-Wahaibi, Y.
Al-Ajmi, A.
Publisher: International Conference on Multiphase Flow (ICMF)
Publication Date: June 4, 2010
 Subjects
Subject: oil-water flow
flow pattern transition
stratified
non-stratified
 Notes
Abstract: Flow characteristics during horizontal oil-water flow and the transition from stratified to non-stratified patterns were studied experimentally. The experimental investigations were carried out in a horizontal acrylic test section with 25.4 mm ID with water and oil (density 875 kg/m3 and viscosity 11.7 mPas) as test fluids. A highspeed video camera was used to study the flow structures and the transition. Six flow patterns were identified for the range of conditions investigated. These are stratified (stratified smooth, SS, and stratified wavy, SW), bubbly (Bb), dual continuous (DC), annular, (AN), dispersed oil in water (Do/w) and dispersed water in oil (Dw/o). Each pattern was compared with those reported in the literature. The effect of oil and water velocities on flow patterns transition and in particular, to the transition between stratified and non-stratified flow was carefully investigated. For example, at superficial oil velocity less than 0.1 m/s, the flow was initially stratified (at low superficial water velocity). As the water velocity increased, the flow changed to bubbly. On the other hand, at superficial oil velocity greater than 0.1 m/s, the flow changed from stratified to dual continuous as the water velocity increased. In the stratified region, the high speed pictures revealed that the amplitudes of the waves increased as the superficial velocities of both phases increased. It was also found that no drops were found when waves were absent and waves have to grow in amplitude before the transition to bubbly or dual continuous flows occurred. This agreed with the results reported by Al-Wahaibi and Angeli (2007). The effect of oil viscosity on flow patterns and transition between stratified and non-stratified flow was found by comparing the current results with those reported by Angeli and Hewitt (2000) and Raj et al. (2005).
General Note: The International Conference on Multiphase Flow (ICMF) first was held in Tsukuba, Japan in 1991 and the second ICMF took place in Kyoto, Japan in 1995. During this conference, it was decided to establish an International Governing Board which oversees the major aspects of the conference and makes decisions about future conference locations. Due to the great importance of the field, it was furthermore decided to hold the conference every three years successively in Asia including Australia, Europe including Africa, Russia and the Near East and America. Hence, ICMF 1998 was held in Lyon, France, ICMF 2001 in New Orleans, USA, ICMF 2004 in Yokohama, Japan, and ICMF 2007 in Leipzig, Germany. ICMF-2010 is devoted to all aspects of Multiphase Flow. Researchers from all over the world gathered in order to introduce their recent advances in the field and thereby promote the exchange of new ideas, results and techniques. The conference is a key event in Multiphase Flow and supports the advancement of science in this very important field. The major research topics relevant for the conference are as follows: Bio-Fluid Dynamics; Boiling; Bubbly Flows; Cavitation; Colloidal and Suspension Dynamics; Collision, Agglomeration and Breakup; Computational Techniques for Multiphase Flows; Droplet Flows; Environmental and Geophysical Flows; Experimental Methods for Multiphase Flows; Fluidized and Circulating Fluidized Beds; Fluid Structure Interactions; Granular Media; Industrial Applications; Instabilities; Interfacial Flows; Micro and Nano-Scale Multiphase Flows; Microgravity in Two-Phase Flow; Multiphase Flows with Heat and Mass Transfer; Non-Newtonian Multiphase Flows; Particle-Laden Flows; Particle, Bubble and Drop Dynamics; Reactive Multiphase Flows
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Bibliographic ID: UF00102023
Volume ID: VID00437
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: P14-Yousef-ICMF2010.pdf

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7th International Conference on Multiphase Flow,
ICMF 2010, Tampa, FL, May 30 -June 4,2010

Flow structures and transition between stratified and non-stratified
horizontal oil-water flow

N. Yusuf, T. Al-Wahaibi*, Y. AI-Wahaibi and A. AI-Ajmi

Department of Petroleum and Chemical Engineering, Sultan Qaboos University, P.O.Box 33, Al-Khoud,
P.C. 123, Oman.
Email: alwahaib@isqu.edu.om

Keywords: Oil-water flow, flow pattern transition, stratified, non-stratified


Abstract

Flow characteristics during horizontal oil-water flow and
the transition from stratified to non-stratified patterns
were studied experimentally. The experimental
investigations were carried out in a horizontal acrylic test
section with 25.4 mm ID with water and oil (density 875
kg/m3 and viscosity 11.7 mPas) as test fluids. A high-
speed video camera was used to study the flow structures
and the transition.
Six flow patterns were identified for the range of
conditions investigated. These are stratified (stratified
smooth, SS, and stratified wavy, SW), bubbly (Bb), dual
continuous (DC), annular, (AN), dispersed oil in water
(Do/w) and dispersed water in oil (Dw/o). Each pattern
was compared with those reported in the literature. The
effect of oil and water velocities on flow patterns
transition and in particular, to the transition between
stratified and non-stratified flow was carefully
investigated. For example, at superficial oil velocity less
than 0.1 m/s, the flow was initially stratified (at low
superficial water velocity). As the water velocity
increased, the flow changed to bubbly. On the other hand,
at superficial oil velocity greater than 0.1 m/s, the flow
changed from stratified to dual continuous as the water
velocity increased. In the stratified region, the high speed
pictures revealed that the amplitudes of the waves
increased as the superficial velocities of both phases
increased. It was also found that no drops were found
when waves were absent and waves have to grow in
amplitude before the transition to bubbly or dual
continuous flows occurred. This agreed with the results
reported by Al-Wahaibi and Angeli (2007). The effect of
oil viscosity on flow patterns and transition between
stratified and non-stratified flow was found by comparing
the current results with those reported by Angeli and
Hewitt (2000) and Raj et al. (2005).

Introduction

The simultaneous flow of two immiscible fluids (e.g. oil
and water) in pipes is a common phenomenon in chemical


and petrochemical industries. It is known that the
fundamental difference between single phase flow and
two-phase flow is the existence of flow patterns or flow
regimes in two-phase flow. When a mixture of two fluids
flows simultaneously in a channel, the two phases can
distribute themselves in several configurations which are
largely dependent on the physical properties of the fluids,
like density and viscosities, the operational variables such
as flowrate and volume fraction of each phase, and the
geometry of the channel (pipe material, diameter and
inclination, etc). The flows are usually stratified at low
velocities, but as the flow rate increases, transition from
stratified to non-stratified flow patterns occurs. In
stratified flows the two fluids flow in segregation with the
layer of the less dense (usually oil) flowing at the top
while the denser (usually water) flows at the bottom.
When the flow is stratified, separation of the two phases
can be easily achieved, and energy is save as pressure
drop in stratified flow is minimal in relative to other flow
patterns.

The need to understand the different types of flow pattern
transitions that may occur in liquid-liquid flow is of
immense importance as design variables such as pressure
drop, mass and heat transfer coefficients, hold-up, rate of
chemical reactions etc are strongly dependent on flow
pattern structure. Yet determination or prediction of flow
pattern is a central problem in two-phase flow analysis.
Different researchers have worked on oil-water flow
using oil of various physical properties and pipes of
different diameters. They all observed transition from
stratified to non-stratified patterns in their studies, except
for those who used oil with same density with water (see
Charles et al. 1961). One of the first investigators to
classify flow patterns for the co-current flow of liquid-
liquid two-phase flow was Russell et al. (1959). They
studied the flow of water and white mineral oil (po=
834kg/m3, to= 18cP) for a range of flow rates and
reported transition from stratified to non-stratified flow.
Charles et al. (1961) were the first to draw a flow pattern
map based on superficial oil and water velocities. Later
on several investigators have classified oil-water flow
patterns based on their own investigations: Guzhov










7th International Conference on Multiphase Flow,
ICMF 2010, Tampa, FL, May 30 -June 4,2010


(1973), Cox (1985), Scott (1985), Arirachakaran et al.
(1989), Nidler and Mewes (1995), Trallero (1996),
Fairuzov et al. (2000), Angeli and Hewitt (2000), Limin
Yang (2003), Lovic and Angeli (2005), Rodrigeuz et al
(2006) and Raj et al. (2006). Theoretically, different
models were developed to predict the transitions between
stratified to non-stratified flows (see for example, Trallero
et. al., 1997 and Al-Wahaibi and Angeli, 2007)

In this study, experimental investigations were carried out
in a horizontal acrylic test section with 25.4 mm with
water and oil (density 875 kg/m3 and viscosity 12 cP) as
test fluids. The effect of oil viscosity on flow patterns and
transition between stratified and non-stratified flow were
investigated by comparing the current results with those
reported by Angeli and Hewitt (2000) and Raj et al.
(2005) since they used pipe with similar geometry with
this work.


Experimental Set-up

An experimental liquid-liquid flow facility has been
constructed for this study at the Sultan Qaboos University
Engineering Research Laboratory. The schematic diagram
is shown in Figure 1. The test fluids are oil and water with
average properties as shown in Table 1. Each fluid is
transferred from their storage tank with a pump to the test
section made up of 25.4mm acrylic pipe that consists of
two eight-meter long parts connected via U-bend. The
two fluids enter the test section from two pipes via a Y
like-junction. The water phase is allowed to enter from
the bottom while the oil joined from the top to reduce the
effect of mixing. Two flow meters one with a maximum
capacity of 20,000 1/hr and the other with a maximum
capacity of 30 1/min is attached to each of the flow lines
(water and oil) which are regulated through pin valves to
control the flow rate of the fluid. The flow meters have
accuracy of 0.5% full scale. The mixture returns via a
PVC pipe to a separator tank which allows the two phases
to separate and hence return to their respective storage
tanks.
High-speed camera and visual observation were used to
identify the different flow patterns and the transition from
one pattern to another. The camera was located 6.5m from
the first eight meter part of the test section. At this point it
is believed that the flow is fully developed as preliminary
investigation shows. The camera is a Troubleshooter
system that can record up to 1000 fps. In this work,
500fps was selected and the images are then transferred
and analyzed using MiDAS 4.0 express software.


Viewing
area


'res e 0
Tank
9


Pressure switch
Non return valve
Ball valve
Pressure gauge


Figure 1: Schematic diagram of the flow-loop

Table 1: Physical properties of fluids @ 30C


Parameters Mineral oil Water
Density (g/cm3) 0.875 0.998
Viscosity (cP) 12 0.001
Interfacial Tension (mN/m) 11.7

Experimental Results

The flow patterns identified in this work for the range of
superficial oil and water velocities investigated are
presented in Figure 2 in terms of superficial water and oil
velocities. The flows are classified into six patterns. These
are stratified (stratified smooth, SS, and stratified wavy,
SW), bubbly (Bb), dual continuous (DC), annular, (AN),
dispersed oil in water (Do/w) and dispersed water in oil
(Dw/o).

Stratified (ST): (stratified smooth, SS, and stratified
wavy, SW): where the two fluids flow in separate layers
at the top and bottom of the pipe according to their
densities.

Dual continuous (DC): where both oil and water form
continuous layers at the top and bottom of the pipe
respectively but drops of one phase appear into the
continuum of the other phase.

Annular (AN): where water forms an annular film at the
wall and oil flows in the pipe core.

Bubbly (Bb): where the oil appears in the form of
elongated drops (slightly longer or shorter than the pipe
diameter) within water continuum.










7th International Conference on Multiphase Flow,
ICMF 2010, Tampa, FL, May 30 -June 4,2010


Dispersed oil in water (Do/w): where the pipe cross
sectional area is occupied by water containing dispersed
oil droplets.

Dispersed water in oil (Dw/o): where oil is the continuous
phase and water is present as droplets across the pipe
cross sectional area.

Stratified flow

Stratified flow appears at low superficial oil and water
velocities, and it is initially characterized by a smooth
interface with no drops and waves. This is described as
stratified smooth (SS) flow. The smooth interface
becomes wavy as the superficial oil and water velocities
are increased and this is termed as stratified wavy (SW)
flow. Stratified flow appeared at low oil and water
velocities because force due to gravity dominates at low
velocities as momentum instabilities are minimal.
Stratified flow was observed within the range of
superficial oil velocity (Uso) of 0.06 0.33m/s and
superficial water velocity of 0.1 0.48m/s.


10

.* *ST DBb AAN xDC xDo/w *Dw/o













u ii velocity TUso. in
ID 25.m 12 X 0. g/cm1

amplitude of te waves is also observed to grow with
0 >0< * XXXX X


01 -- -. .. ...--
0.01 0.1 1 10
superficial woil velocity. For. ml

Figure 2 Experimental flow pattern map of this study
ID = 25.4 mm, t = 12 cP, p = 0.875 g/cm3, o =11.7 mN/m

The flow is found to change from stratified smooth to
stratify wavy as superficial water velocity increases. The
amplitude of the waves is also observed to grow with
increase in superficial water velocity. For example, at U,
= 0.06 and 0.33m/s, the wave amplitude grow as U-
increases as shown in Figure 3(a & b). The observed
amplitudes are found to be larger for higher superficial
water velocities for the same superficial oil velocity. This
is why the required superficial water velocity to initiate
drop formation decreases as superficial oil velocity
increases. At low superficial oil velocities up to 0.1m/s,
stratified flow existed up to U,, = 0.48m/s. But for Uo >


0.1 m/s, the transition from stratified to other flow patterns
is found to occur at lower U. as can be seen from the
flow pattern map in Figure 2.

Stratified flow is observed to transform into two types of
flow pattern as superficial water velocity increased. They
are transition to bubbly and dual continuous flow.
Transition to bubbly flow is observed at low superficial
oil velocity, while at relatively higher velocities, the
stratified flow changed to dual continuous flow.


U,, = 0.1 m/s U, =0.28m/s U,, = 0.40m/s


U,, = 0.1 m/s U,, =0.28m/s U, = 0.40m/s
(b)
Figure 3: Effect of increase in U,, on wave amplitude @
(a) U,, = 0.06m/s (b) U,, = 0.21m/s.

Transition from stratified to bubble flow

At Uso less than 0.1m/s, the flow is observed to change
from stratified to bubbly flow. This is largely due to the
increase in turbulence of the water phase at high water
velocity and because the layer of the oil phase is thin at
low superficial oil velocity. The probability of the waves
at the interface breaking the thin layer of oil is very high,
thereby, creating a continuous water phase with the oil
phase dispersed non uniformly as bubble (oil drops
slightly longer or smaller than the pipe diameter).


The transition from stratified flow to bubbly flow is
clearly illustrated in Figure 4. At superficial water
velocity U,, = 0.1m/s and Uo = 0.06m/s, the flow is
stratified smooth (SS). As U, increases from 0.1 to
0.35m/s, the flow becomes stratified with wavy interface
(SW), though the waviness is of small amplitude and the
oil layer is thinner. As U,, is further increased to 0.48m/s,
the cross sectional area of the pipe occupied by the oil is
smaller and the wave amplitude also increased. Further
increase in U,, to 0.54m/s changes the flow from
stratified to bubbly flow.











7th International Conference on Multiphase Flow,
ICMF 2010, Tampa, FL, May 30 -June 4,2010


U ,= 0.1m/s U.= 0.22m/s U,, = 0.35m/s


u,. -U.4tum/s


Us,, -U.4 '.Z1/


Us, U.D33/s


U, = 0.42m/s U,,= 0.48m/s Us = 0.54m/s

Figure 4: Transition from stratified to bubbly flow @ Uso
= 0.06m/s

Transition from stratified to dual continuous flow

Stratified flow is observed to transform to dual continuous
flow pattern at Uso > O.1m/s at the same or even lower
than U,. where transition to bubbly flow occurred. This is
attributed to the fact that at these flow conditions, the oil
layer is thick enough to resist the turbulence of the water
phase from breaking its continuous flow. Instead of
breaking the continuity of the layer, the relative
movement between the phases increased, which increases
the amplitude propagation eventually breaking the
interfacial tension between the oil and water, hence drop
formed at the interface.

The effect of increasing superficial water velocity at Uo >
0.1m/s is presented in Figure 5. As the water velocity
increases from 0.1 to 0.35m/s the stratified flow becomes
wavier for the same Uo = 0.21m/s. The wavy amplitude
continue to increase until it gets to the critical wave
amplitude (see Al-Wahaibi and Angeli, 2007) where
drops of oil are seen in water continuum at U, = 0.42m/s.
This is defined as the onset of drop formation.


Figure 5: Effect of increase in water velocity on Uso @
0.21m/s

At U, < 0.42m/s, the transition from stratified to DC
flow is found to occur at higher superficial oil velocity
(Uso = 0.21m/s) as compared to the transition at U,, =
0.48m/s where the flow is seen to change from stratified
flow to DC at lower superficial oil velocity (Uso =
0.14m/s). This is because at higher U,,., two forces of
destabilization are present; namely; the momentum force
due to high velocity of water and the force due to the
viscosity difference between the two fluids, while at low
U_, the contribution of the difference in velocity is less.
Figure 6 illustrates the effect of increasing superficial oil
velocity at U, = 0.22 and 0.42m/s where transition from
stratified to DC flow is observed at Uo = 0.29 and
0.14m/s respectively.


Uso = U.uom/s


Uso= U.14m/s


USO U. 1III/SU


Uso -U.










7th International Conference on Multiphase Flow,
ICMF 2010, Tampa, FL, May 30 -June 4,2010


uLso- u.Lz1i nI


uLso- u.z.yii


(b)
Figure 6: Effect of increasing superficial oil velocity on
stratified flow (a) at U,, = 0.22m/s, and (b) at U, =
0.42m/s

It is also of importance to note that unlike the effect of
increasing Uo where two transitions (stratified to bubbly
flow and stratified to DC flow) occur as shown in Figures
4 and 5, only transition from stratified to dual continuous
flow is observed as Uo is increasing. This is because the
cross sectional area of the pipe occupied by the oil phase
increases as the oil velocity increases.

Comparison with other studies

Different researchers have reported the transition from
stratified to other flow patterns in horizontal oil-water
flows. For the purpose of comparison, in this study, the
work of Nadler and Mewes (1997), Trallero et al. (1997),
Angeli and Hewitt (2000), Lovick and Angeli (2004), Raj
et al. (2005), and Al-Wahaibi et al. (2007) are compared
with that of this study.

Stratifiedflow
Similar to this study, all the researchers (Trallero et. al.,
1997; Nadler and Mewes, 1997; Angeli and Hewitt, 2000;
Lovick and Angeli, 2004; Raj et al., 2005 and Al-Wahaibi
et al., 2007) observed stratified flow pattern in their
studies. This is because the occurrence of stratified flow is
majorly a function of density difference between the two
fluids. All the researchers used fluids with density ratio
( P/A.) less than unity. The only difference is the extent
to which the flow pattern extends. Charles et al. (1961),
used fluids with ( /p.) = 1 and did not observe
stratified flow in their study.

Trallero et al. (1997) used acrylic resin pipe of 50 mm
internal diameter (ID) and oil and water with viscosity
and density ratio of /FI ) = 26.9 and 0.85
respectively, while Nadler and Mewes (1997) employed a


Perspex pipe of 59 mm ID and oil with f of 22-35
and /1a = 0.84. They observed stratified flow up to
superficial water velocity of about 0.2 and 0.22m/s
respectively and superficial oil velocities of about 0.2m/s.
While in this study, stratified flow was observed up to
superficial water and oil velocities of 0.42 and 0.33m/s
respectively. Angeli and Hewitt (2000), Raj et al. (2005)
both used acrylic pipe of similar ID (25.4m/s) and oil with
' u/ = 1.6 and 1.4 and 0A = 0.8 and 0.84
respectively. In their studies, stratified flow extends to
U,, of about 0.23 and 0.3m/s and U,, of about 0.35m/s
respectively. Lovick and Angeli (2004) and Al-Wahaibi et
al. (2007) used stainless steel pipe of 38 and acrylic pipe
of 14mm ID using similar oil with 'u/ = 5.5, and
/r. of 0.83. They observed stratified flow up to
superficial water velocity of 0.72 and 0.5m/s respectively.


Transition to bubbly flow pattern

Few investigators have reported transition from stratified
flow to bubbly flow pattern in liquid-liquid flow. Raj et
al. (2005) and Al-Wahaibi et al. (2007) observed
transition from stratified to bubbly flow pattern with
increase in superficial water velocity at low superficial oil
velocities. Raj et al. observed transition from stratified to
bubbly flow pattern at lower superficial water velocity
compared to this work. In this work the transition was
observed at Us, > 0.3 m/s while in Raj work, it was
observed at 0.54m/s. Moreover, the transition was
observed up to superficial oil velocity of 0.15m/s in Raj et
al. as compared to 0.lm/s in this study. This could be
attributed to the difference in viscosity of the oil used for
both studies (Raj et al. used oil with viscosity of 1.2cP,
while in the present study oil with viscosity of 12cP is
employed). Since the occurrence of bubbly flow pattern is
attributed to the turbulence of the water phase and the
thickened of the oil layer, transition from stratified to
bubbly flow pattern is expected to extend to higher
superficial oil velocity for oil of lower viscosity, and the
transition is expected to occur at lower superficial water
velocity.

In Al-Wahaibi et al. 2007, transition from stratified to
bubbly or slug flow patterns extended to higher
superficial oil velocity (0.16m/s) compared to this work,
and occurred at superficial water velocity above 0.4m/s.
This could also be attributed to the difference in
viscosities of the oils as well as the pipe diameter used by
Al-Wahaibi et al. Since the viscosity of the oil used by
them is lower than that of this study, it is expected that
transition from stratified to bubbly flow pattern could










7th International Conference on Multiphase Flow,
ICMF 2010, Tampa, FL, May 30 -June 4,2010


extend to higher superficial oil velocity, and occur at a
lower superficial water velocity.

Trallero et al. (1997), Nadler and Mawes (1997) and
Lovick and Angeli (2004) did not report transition from
stratified to bubbly flow in their work. This could be due
to the relatively larger pipe diameter used by them.

Transition to dual continuous flow pattern

Trallero (1996) and Nadler and Mewes (1997) reported
early formation of DC flow pattern compared to this
study. They observed stratified flow pattern up to
superficial water velocities of 0.2m/s and 0.22m/s, and
superficial oil velocities of about 0.2m/s respectively, and
above these superficial velocities, stratified flow
transformed to DC flow pattern. Since they used oils with
higher viscosities compared to this work, it is expected
that the transition from stratified to DC flow pattern
should occur earlier in their work as higher viscosity
difference between oil and water is known to cause
greater instability.

Angeli and Hewitt (2000), Lovick and Angeli (2005), Raj
et al (2005) and Al-Wahaibi et al. (2007) reported late
transition from stratified to DC flow pattern. This is
because they all used oils with lower viscosities compared
to the oil used in this work.

Effect of viscosity

Angeli and Hewitt (2000) and Raj et al. (2005) used pipe
made from acrylic material and similar geometry with the
pipe used in this study. The viscosity of their oil is also
similar (1.6 and 1.2cP respectively), while the one used in
this study is 12cP. The transition from stratified to other
flow patterns obtained in their study is found to differ
from that of this study. In this study as an example, the
transition from stratified to dual continuous flow occurred
at higher superficial water velocity compared to both
Angeli and Hewitt (2000) and Raj et al. (2005), while the
same transition (from stratified to dual continuous flow) is
observed to occur earlier in this study as superficial oil
velocity increases. This can be attributed to the difference
in viscosities of the oils, as both the pipe material and
diameters are the same.

Figure 7 is a comparison of the flow pattern maps of
Angeli and Hewitt (2000) and Raj et al. (2005) with that
of this study showing the transition from stratified flow to
other flow patterns. For example, at superficial oil
velocity Uso = 0.21m/s, transition from stratified to dual
continuous flow occurs in this study at U,, = 0.54m/s,
while in Angeli and Hewitt (2000) and Raj et al. (2005)
the transition occurred at U, = 0.20 and 0.30m/s
respectively. The transition is delayed in this study


because of the large viscosity difference between the oil
and water compared to them. As the superficial water
velocity increases, the instability due to momentum also
increases, the large viscosity difference helps to dissipate
the energy that causes instability, hence, transition from
stratified to dual continuous flow is delayed.

On the other hand, the transition from stratified to dual
continuous flow is observed to occur earlier in this study
as superficial oil velocity increases, for example, at
superficial water velocity of U,, = 0.12m/s, transition
from stratified to dual continuous flow occurs at Uo =
0.21m/s in this study and Uo = 0.33 and 1.2m/s in Angeli
and Hewitt (2000) and Raj et al. (2005) respectively. This
is because as the velocity of the oil increases, the
instability due to the different velocity profiles at the
interface of the two layers increases, thereby causing
early transition.

From the comparisons above, it can be deduced that the
dual effect of viscosity reported in literature (Yil, 1967)
affects flow as follows; when superficial oil velocity is
kept constant and superficial water velocity is increasing,
large viscosity difference between two fluids will assist in
damping the energy that causes instability, while on the
other hand, when superficial water velocity is constant
and superficial oil velocity is increasing, large viscosity
ratio will increase instability.

Although the viscosity of the oil used by Angeli and
Hewitt (2000) is more than that used by Raj et al. (2005)
yet transition from stratified to dual continuous flow
occurred earlier in Angeli and Hewitt (2000), this can be
attributed to the entry condition of the fluids. Raj et al
(2005) introduced their fluids through a mixer comprising
of two concentric pipes with the oil introduced through
the annulus and water through the tube. This method will
prevent lateral mixing of the two fluids near the entry
point. While Angeli and Hewitt (2000) used a T-junction
with a 90 elbow immediately downstream and before the
test section to introduce their fluid into the test section.
This may increase the possibility of lateral mixing of the
fluid at the entry point, hence causing early transition.

Nadler and Mewes (1997) and Trallero et al. (1997) used
oil with higher viscosity and pipe with larger internal
diameter compared with this study but observed early
transition from stratified to dual continuous flow pattern
as superficial water velocity increases. It is likely that
there is a critical viscosity at which the viscosity effect
dominates the flow. Above the critical viscosity, the effect
of instability due to momentum is overpowered by the
large viscosity difference between the fluids, hence early
transition occurs. In contrast, both Nadler and Mewes
(1996) and Trallero et al. (1997) observed early transition
as superficial oil velocity increases. This is so because










7th International Conference on Multiphase Flow,
ICMF 2010, Tampa, FL, May 30 -June 4,2010


their large viscosity difference will cause larger difference
in the velocity profiles at the interface of the two fluids,
thus early transition occurs.

ST Bb
2 x DC Rajetal.
------- Angeh &Hewitt


I i 3 X X X


st n l-o fi a H D
i f. x xDC
3 ST ,





0.02
0.02 0.2 2
superficial oil velocity, i/s

Figure 7: Comparison of the flow pattern transition from
stratified to non-stratified flow of Angeli and Hewitt
(2000), Raj et al. (2005) and this study.


Although, both Nadler and Mewes (1997) and Trallero et
al. (1997) used pipe with larger internal diameter
compared with this study, it is expected that late transition
from stratified to dual continuous flow occurs as larger
pipe ID favours stratification. But early transition was
observed as superficial water velocity increases. This also
suggests that the early transition is due to the high
viscosity difference between the fluids.


Conclusions

The effect of oil and water velocities on flow patterns
transition and in particular, to the transition between
stratified and non-stratified flow has been carefully
investigated in this study. Two types of transition are
observed as oil and water velocities increases. These are
transition from stratified to bubbly flow and transition
from stratified to dual continuous flow pattern. At very
low superficial oil velocities, stratified flow transformed
to bubbly flow as superficial water velocity increases.
This is due to the increase in turbulence of water phase as
superficial water velocity increases, hence breaking the
thin layer of the oil phase. But as superficial oil velocity
increases, increase in water superficial velocity causes
stratified flow to transform into dual continuous flow.
This is because the layer of the oil phase is thick enough
to resist the turbulence of the water phase. Hence, the


wave amplitude at the interface grows and eventually
drops of one phase are seen in the continuum of the other
phase.
From the comparison of this study with the works of
Angeli and Hewitt (2000) and Raj et al. (2005), the effect
of oil viscosity on flow patterns transition between
stratified and non-stratified flow can be summarized as
follow, when superficial oil velocity is kept constant and
superficial water velocity is increasing, large viscosity
difference between two fluids will assist in damping the
energy that causes instability, while on the other hand,
when superficial water velocity is constant and superficial
oil velocity is increasing, large viscosity ratio will
increase instability.
Finally, the experimental results of the transition from
stratified to non-stratified flow was compared with
Trallero (1995) and Al-Wahaibi and Angeli (2007)
models. None of these models were found to predict the
transition satisfactorily.

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7th International Conference on Multiphase Flow,
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