Paper No 7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30June 4, 2010
The Effect of Gas and Liquid Velocities on Frictional Pressure Drop in Two Phase Flow for
Large Diameter Vertical Pipe
M.Zangana*, GP.van der Meulen and B.J.Azzopardi
Process and Environmental Engineering Research Division, Faculty of Engineering, University of Nottingham
University Park, Nottingham, NG7 2RD, United Kingdom
*Corresponding author enxmhsz@nottingham.ac.uk
Keywords: gasliquid flows, frictional pressure drop, wall shear stress, large diameter pipes
Abstract
The results of 600 runs carried out on a 127 mm diameter, 11 m tall vertical pipe are reported. The work has concentrated on
pressure drop and wall shear stress. The overall pressure drop over a length of the test section measured using a differential
pressure cell and the data on liquid holdup obtained using the conductance probe rings in three different locations on the test
section. The wall shear stress also measured employing a hot film probe with Dantec 55R47 electronics.
Frictional pressure drop data was extracted from the measured total pressure drop and the liquid hold up, three major areas can
be identified as a result of liquid superficial velocity changes. The result shows that the frictional pressure drop in third area
namely at 5.1m/s is much higher than the lower liquid velocities (i.e. in first and second area). The time varying data of
frictional pressure drop fluctuated between positive and negative values for the highest gas velocity studied, 16.2 m/s. This is
unexpected, that type of behavior has been reported for lower gas velocities. An obvious effect of liquid superficial velocity on
wall shear stress has been observed, both measured and calculated timeaveraged wall shear shown a good agreement, a similar
trend on the effect of gas velocity changes on the wall shear stress reported.
Introduction
Oil and gas developments in offshore industry facing a
special challenge due to the pressure drop during
transportation of the gas and the liquid from the sea bed to
the platforms, especially after moving into deeper water.
Therefore the large diameter risers has been employed in
order to reduce the production costs.
Only Few works are known in such size of the pipes, and a
different behaviors of the flow are reported from what
have been observed in smaller pipes: e.g., Cheng et
al.(1998),and Ohnuki et a! 2111 11 ), shown that conventional
slug flow does not occur clearly in vertical two phase flow
for large diameter pipes. There appears to be a direct
transition from bubble flow to chum flow. Ombere (2006),
also did not observe the traditional Taylor bubble of slug
flow within the range of his work. These were not the only
differences in the behavior of gas and liquid flow in large
diameter pipes, but also more different characteristics have
been reported, e.g., Azzopardi et al.(1983), who studied the
disturbance waves in annular two phase flow in a vertical
large pipe diameter (125mm) showing that they are
circumferentially localized, in contrast to what Hewitt and
Lovegrove (1969) found in small pipe diameter (32mm)
that the waves are coherent around the pipe circumference.
In spite of importance of large diameter pipes and effect of
such pipes on the flow behavior as mentioned, there is still a
dearth of experimental data over a wide range of conditions
particularly in churnannular and annular region, specially
on some parameters like a frictional pressure drop which is
known as a very important parameter for the design of
pipelines.
Work related to frictional pressure drop for two phase flow
in pipes known to be few and the majority of these works
focused on small diameter pipes, e.g., Sawai et al (2001),
Shannak 2'" 1') and Fukano et al (1997). Therefore more
effort are needed and specially to concentrate on larger
diameter pipes as such pipes now a day getting larger
demand by the industry.
The present work investigates the effect of gas and liquid
superficial velocity on frictional pressure drop in two phase
flow for a large diameter pipe over a wide range of
conditions. Data on wall shear stress in vertical pipe are
presented, both measured and extracted data are
compared ,and the effect of film thickness on wall shear
stress are discussed.
Nomenclature
(dp/dx)t
(dp/dx)f, F,
(dp/dx)g
d, D,
De
g
Ku
Ug
U
g7*
Total pressure drop(Pa/m)
Frictional pressure drop(pa/m)
Gravitational pressure drop(pa/m)
Inner diameter of pipe(m)
The distance between ring probes(m)
Gravitational acceleration mi! 
Kutateladze number
The thickness of the conductance ring
probes(m)
Gas superficial velocity(m/s)
Dimensionless gas velocity
Paper No
Greek
letters
/ Liquid holdup
SP Gas phase density (Kg/m3)
PG Pg
pL, Liquid phase density (Kg/m3)
Zw Wall shear stress(NI m')
CT Surface tension(N/m)
Subsripts
f Frictional
G,g Gas
L,l Liquid
t Total
w Wall
e Electrode
Experimental Facility
The experiments have been carried out using a closed loop
facility of 127mm id and 11 m tall riser (Figure 1). Liquid is
stored in the main separator and pumped into the riser base.
The gas phase is compressed by two liquid ring vacuum
pumps operated in parallel and delivered to the riser base.
The phases come together in the mixer and from this point
the flow develops along the tall riser. The flow is then
directed horizontally into the downcomer and back into the
separator. Here the gas is separated from the liquid and the
fluids are fed to the compressors and pump. The flow of
both fluids can be regulated by valves and the flow rates
monitored by flow meters. System pressure can be up to 5
barg. Gas and liquid superficial velocities of up to 17 m/s
and 1.5 m/s, respectively, can be achieved. Temperatures
and pressures are measured at various points throughout the
experimental facility. The vertical pipe is equipped with a
transparent acrylic resin section (Figure 2) wherein following
measurements are located:
Pressure drop: The total, time averaged, pressure drop is
being measured by an electronic differential pressure
detector/transmitter (Rosemount 1151 smart model), with a
range of 0 37.4 kPa and an output voltage from 1 to 5 V, i.e.
a resolution of 9.35 kPa per volt. Two pressure tappings,
separated by an axial distance of 12.9 pipe diameters, across
the transparent section, are connected to the differential
pressure device via stainless steel tubes. The tubes were
filled with water to keep the density constant. This was
assured by an efficient purging procedure which eliminated
the risk of gas fractions in the pressure lines. The latter
procedure was repeated at the start of each set of the
experiments.
Wall shear stress: The hot film probe driven by Dantec
55R47 electornics is employed to measure wall shear stress,
and was carefully flush mounted on the pipe wall (Figure 2),
this to avoid any disturbance to the flow. It was positioned
at 63.1 pipe diameter from the riser base. The probe
calibrated by measuring the frictional pressure drop in a
single phase flow using an electronic differential pressure
transmitter (Rosemount 3051 smart model), with a range of
0 6.23 kPa and an output voltage from 1 to 5 V. As the wall
shear stress in two phase flow is relatively higher than that in
a single phase flow therefore the calibration performed in a
high liquid flow rate (the liquid superficial velocity ranged
from 0.52 to 1.32 m/s), with locating the probe in the same
7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30June 4, 2010
position as it has been used during the experimental work due
to its sensitivity to its position, Millar (1980). The hot film
probe is the heat transfer based technique, so the changes in
temperature in the system has a significant effect on the result.
Most of possible problems with the operation of such probes
are described by Martin(1983) and Bruun (1996). Therefore
a high level of care has been taken during this work to keep
the temperature changes within the satisfactory range by:
First: Temperature profile data for both single and two phase
flow on this facility has been collected experimentally
(Figure 3) and the ratio of temperature changes over the time
found. Second: All the presented data obtained at 1%
overheat ratio at a certain temperature(25.270C) and the
measurement started before and ended after reached the
mentioned temperature. During the experiments the
temperature in the riser was monitored and recorded
continuously. The only data accented were those for which
the rig temperature was within 0.1C of the reference
temperature (i.e.25.270C).
Liquid holdup: The conductance probe rings, used for phase
distribution measurements in the present study, are located at
62.7, 63.5 and 65.5 pipe diameters from the riser base,
respectively. The stainless steel rings are flush mounted with
the pipe wall. They were located using cylindrical dowels
placed at either side of wall sections
The thickness, s, of the rings is 3mm and the distance, De,
between the probes is 25 mm, insulated by nonconducting
acrylic resin. This is a electrode separation distance to pipe
diameter ratio, DJDt, of 0.20. Data acquisition was
performed through a PC equipped with a National Instrument
(NI) DAQ card. An existing data acquisition programme in
Labview was adapted to convert the voltage output of the
probes into a cross section averaged void fraction. The data
acquisition rate was 1 kHz.
Figure 1: The large scale closed loop facility.
Figure 1: The large scale closed loop facility.
Figure 2: showing the test section, the location of the
measurement techniques and the hot film probe as mounted
flush inside the pipe.
Paper No
30 [Single Phase(Uls=0 54m/s)
Two Phase Flow(Uls=0 04 m/s
S and Ugs=11 09m/s)
;28
S26
E 24
22   .
20
0 50 100 150 200 250
Time(min)
Figure 3: showing the temperature profile in the closed
loop facility for single and two phase flow.
Results and Discussion
A total of 600 experimental runs were carried out measuring
total pressure drop and liquid holdup for a range of liquid
superficial velocities from 0.Olm/s to 0.7m/s and the gas
superficial velocities from 3m/s to 16.25m/s. The wall shear
stress was measured selectively at liquid superficial velocity
of 0.008, 0.01, 0.02, 0.04, 0.08 and 0.13 m/s over a number of
gas superficial velocities. The system pressure during
experiments was set at 2 barg. The frictional pressure drop
data was obtained from the measured total pressure drop and
the liquid hold up employing the following steady
momentum equation for vertical upward annular flow, Sawai
et al (2 I4):
dp
=F.L +((1P)pG +Pp,)g
dx
7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30June 4, 2010
minimum pressure drop. The frictional pressure drop has a
similar movement over the mentioned range of liquid
superficial velocities.
The second area can be identified by two particular
conditions, namely 0.053 and 0.06 m/s for the liquid
superficial velocity. In this area the inclination of the graph
for frictional and total pressure drop is not behaving exactly
in the same way as for the low liquid velocity graphs, the
general movement has a smoother nature rather than
exponential.
The trend becomes more complicated with increasing the
liquid superficial velocity, namely in the ranges of 0.07 to
0.7m/s, i.e., the third area, where the effect of gas velocity
changes become less sensible compared with the previous
two areas specially for the frictional pressure drop.
However both frictional and total pressure drop getting
higher as the liquid superficial velocity increase. In overall
both total and frictional pressure drop are showing similar
trends in Figures 4 and 5.
4000
P3000
2000
'a
1000
0
A 0 02n/s
S0 1251/s
* 0 163m1/s
* 0 33ms
x 0 46nms
Llqmd Superficial Velocity
E 0 03m/s A 0 073m/s X 0 09m/s
= 0 06m/s 0 145m/s 0 Im/s
*0 085m/s X 0 185m/s 0 296m/s
0 265ms 0 368m/s A 0 51ms
0061m/s 041nm/s A 0 55m/s
X 0 05m/s
0 0 21m/s
S0 237m/s
0 7m/s
A 0 01 m/s
ot .
0A
Eh~"Y~Xy~~:;~t
AA AA AZ~W~~Xt~
0 5 10 15 20
Gas Superficial Velocity(m/s)
Figure 4: Timeaveraged pressure drop as function of
superficial gas velocity.
Where FwL is the frictional pressure drop and is given by
4
FwL =Tw
d
Where d is the inner diameter of pipe, and rw is the wall
shear stress.
The second term in the right hand side in Equation (1) is the
gravitational pressure gradient, where / is the liquid holdup,
g is the gravitational acceleration, PG and PL are the
densities of gas and liquid phase respectively.
The timeaveraged total pressure drop and frictional pressure
drop are plotted in Figures 4 and 5 respectively.
Three major areas can be identified from both these figures,
as a result of gas and liquid superficial velocity changes.
The first area being at low liquid velocities (0.014, 0.02 and
0.03m/s), where the flow behaves on the similar way as it has
been reported by Van der Meulen et al (2i 1' where the total
pressure, drops rapidly as the superficial gas velocity
increases, then the trend changes to gradual decrease of
pressure drop as the superficial gas velocity increasing
(described as an annular flow region), after moving through a
possible chumannular transitional region with no clear
4000
3000
> 2000
 1000
0
Liquid superficial velocity
(mr/s)
.002 003 0007 009 X005 "0125 0163 0085 6021
S01 006 A0145 0 265 0368 .0237 A033 0185 00296
041 A0558 046 0061 6051 S07 A001
0 5 10 15 20
Gas Superficial Velocity (m/s)
Figure 5: Frictional pressure drop as function of superficial
gas velocity.
It is remarkable that the liquid superficial velocity plays such
a significant role upon the fictional pressure drop. Such
effect can be seen very clearly in Figure 6; the time varying
frictional pressure drop at liquid superficial velocities of 0.02,
0.06 and 0.51m/s and gas superficial velocity of 12.7m/s. It
is apparent that frictional pressure drop is much smaller for
the low liquid superficial velocities, i.e., 0.02 and 0.06m/s,
than the higher liquid velocity, i.e., 0.51m/s, where big peaks
can be seen in the frictional pressure drop.
7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30June 4, 2010
9000
900 Liquid Superficial Velocity (mis)
q 7000 051 006 002
5000
3000
" 1000
1000 0H 20 25 3U
Time(s)
Figure 6: Time varying frictional pressure at superficial gas
velocity of 12.7m/s.
What is also noticeable in Figure 6 is that the frictional
pressure drop is fluctuating between negative and positive
values, this was an unexpected behavior specially for the
high gas superficial velocity, for instance 16.2m/s, Figure 7.
At such condition, i.e., 0.02 m/s for the liquid and 16.2 m/s
for the gas superficial velocity, the liquid film expected to
become unidirectional and flow upward on the wall of the
pipe according to the typical definition of annular flow in
vertical pipe by researchers, e.g., Azzopardi (2006), and
Hewitt and Taylor (1970 ).
Time(s)
Figure 7: Time varying frictional pressure at liquid
superficial velocity of( ,112m and gas superficial velocity of
16.2m/s.
The Kutateladze number Ku which is given by equation
(3) [Wolf (1995)], for the present conditions, i.e., 0.02 m/s
liquid and 16.2 m/s gas superficial velocity, is about 4.39.
This is the usually quoted transition value of 3.1 and
accordingly the annular flow pattern was expected.
However the behavior of frictional pressure drop mentioned
above and illustrated in Figure 7, leads to the an alternative
explanation that the direction of the flow still changing.
That is, the flow pattern is affected by the diameter of the
pipe. Therefore Kutateladze number might not be suitable
to indicate the churannular transition in such diameter of
the pipe, as it does not contain pipe diameter. In contrast the
dimensionless gas velocity for reversal flow (U ) which is
given by equation 4 as such effect has been taken in account.
2 g 25
Kug =Ug pg 0>3.2
cgcr p pg
Ug Ug o , >1
gdo (pi pg)
The timeaveraged wall shear stress at liquid superficial
velocity of 0.02 and 0.13m/s, which are from the low and
high liquid velocity areas respectively, i.e., the first and
second area, are plotted in Figure 8.
14
A 0 02 m/s
12 0 13m
10
S6
4
2
6 8 10 12 14 16
Gas Superficial velocity(m/s)
Figure 8: time.averaged wall shear stress as a function of
gas superficial velocity at liquid superficial velocities of 0.02
and 0.13m/s.
The effect of liquid superficial velocity in Figure 8 is very
obvious, where much higher wall shear stress can be seen as
the liquid velocity increases. This might be explained as the
effect of liquid film flowing on the pipe wall when it
becomes thicker with increasing the liquid velocity, which
lead to higher shear stress on the wall of the pipe as a clear
relation between film thickness and wall shear stress has
been observed, Figure 9. Again the graph can be divided
into two areas of low and high liquid velocity. For the low
liquid velocity a gradual decrease of wall shear stress can be
seen as the gas velocity increases after a sharp slope in the
wall shear stress values in contrast for the high liquid
velocity area as the gas superficial velocity increase the wall
shear stress increase as well before starting decrease.
Time(s)
Figure 9: Showing the relation between time varying wall
shear stress and the film thickness at 0.13m/s liquid
superficial velocity and 14.89m/s gas superficial velocity.
Timeaveraged wall shear stress are calculated from the
extracted frictional pressure drop data and equation (2), both
measured and calculated wall shear stress are plotted in
Paper No
2 4 8
Film Thinckess(mm)
wall shem sthess(Pa)
7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30June 4, 2010
Figure 10 as a function of gas superficial velocity, which
shows in general a very good agreement between them, with
similar movement of the graph, except few conditions
specially at low gas superficial velocity.
14 
10
2 12 8
^ 10 0
8 A
S0 13 measured Wss
S6 A 02 measured Wss
4.1O 00 13 calculatedWss
S 4 A 0 02 calculated Wss
2
6 8 10 12 14 16
Gas Superficial velocity(m/s)
Figure 10: Measured and calculated wall shear stress as a
function of gas superficial velocity.
The variation of frictional and gravitational pressure gradient
at liquid superficial velocities of 0.02 and 0.13m/s with
various gas velocities are plotted in Figures 11 and 12
respectively. The frictional pressure gradient which is
extracted from the measured wall shear stress using equation
(2) (termed the measured values) are presented in this graphs,
this is as well as the extracted frictional pressure gradient
from total pressure drop and liquid holdup as mentioned
previously (termed the calculated values). The gravitational
pressure gradient are obtained from the liquid holdup data.
From Figures 11 and 12 it can be seen that the frictional
pressure drop for high liquid velocity is much bigger than the
low liquid velocity, i.e., 0.02m/s, the movement of the graphs
due to the gas velocity changes is the gradual decrease in
timeaveraged pressure drop ,the frictional and gravitational
pressure drop. For the low liquid velocity a sudden decrease
can be seen with increasing gas velocity before start
decreasing gradually. the negative slope of the frictional
pressure drop against gas velocity for liquid velocity below
0.lm/s, described by Sawai and Kaji (2001) as the
characteristic feature of chum flow.
1100 1
Y
?i
Cn
Ef
a^
Cn
a
100 4 6 8 1 12 14 16
Gas Superficial velocity(m/s)
Figure 11: Variation of frictional and gravitational pressure
gradient against gas velocity at 0.02 liquid superficial
velocity.
1000
S800
S 600
A 400
: o(dp/dx)t
o (dp/dx)g
S 200 (dp/dx)fmeasured
(dp/dx)f calculated
4 6 8 10 12 14
Gas Superficial velocity(m/s)
Figure 12: Variation of frictional and gravitational pressure
gradient against gas velocity at 0.13m/s liquid superficial
velocity.
Conclusions
The results of the study presented here consist of
experimental data on total time averaged and frictional
pressure drop over a wide range of conditions and selectively
on wall shear stress in large diameter vertical pipe. Based
on discussion presented above the following conclusions can
be mad :
(1) The effect of liquid velocity changes on total and
frictional pressure can be identified in thee major areas,
namely low liquid velocity, 0.01,0.02 and 0.03m/s, the
intermediate area, 0.05 and 0.06m/s, and the high liquid
velocity area, from 0.07 to 0.7m/s), where accordingly
the trends due to changes in the gas velocity. A sudden
decrease in total and frictional pressure drop can be seen
as the gas velocity increase in the low liquid velocity
area before the change become gradual. After a
smoother trend in the second area, the effect of gas
velocity on the trend become more complicated in the
third area.
(2) The changes in liquid superficial velocity has a
significant effect on frictional pressure. This effect can
be observed from the varying time frictional pressure
drop where a big peaks of frictional pressure drop can
be seen at high liquid velocity, i.e., 0.51m/s.
(3) Negative and positive values in the fluctuating frictional
pressure drop fluctuation have observed for a high gas
velocity, i.e., 16.2 m/s. This was unexpected.
(4) The liquid superficial velocity has a clear effect on time
averaged wall shear stress. Measured and calculated
time averaged wall shear stresses show a very good
agreement. A similar trend with gas velocity has been
observed for wall shear and for frictional pressure drop.
(5) An obvious effect of liquid film thickness on wall shear
stress observed from their time varying data.
Paper No
Paper No
Acknowledgements
This work has been undertaken within the Joint Project on
Transient Multiphase Flows and Flow Assurance. The
Authors wish to acknowledge the contributions made to this
project by the UK Engineering and Physical Sciences
Research Council (EPSRC) and the following: Advantica;
BP Exploration; CDadapco; Chevron; ConocoPhillips; ENI;
ExxonMobil; FEESA; IFP; Institutt for Energiteknikk; Norsk
Hydro; PDVSA (INTERVEP); Petrobras; PETRONAS;
Scandpower PT; Shell; SINTEF; Statoil and TOTAL. The
Authors wish to express their sincere gratitude for this
support. The Ministry of Higher Education in the Kurdistan
Regional Government and Koya University are highly
acknowledged for their contribution to PhD candidate M.
Zangana.
7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30June 4, 2010
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Van der Meulen, G. P., Zangana, M., Zhao, D. and Azzopardi,
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Conference on Experimental Heat Transfer, Fluid Mechanics
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(21" r ).
Wolf ,A. Film structure of vertical annular flow. PhD thesis,
Imperial College London (1995).
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