effect of oil viscosity on the flow structure and pressure.pdf

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chemical engineering researchand design 9 0 ( 2 0 1 2 ) 10191030

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Chemical Engineering Research and Design

j o u rn a l hom ep age : www.e l sev i e r. com/ loca t e / che rd

Effect of oil viscosity on the ow structure and pressuregradient in horizontal oilwater ow

N. Yusuf a , Y. Al-Wahaibi a , , T. Al-Wahaibi a , A. Al-Ajmi a , A.S. Olawale b,I.A. Mohammed ba Department of Petroleum and Chemical Engineering, Sultan Qaboos University, P.O. Box 33, Al-Khoud, P.C. 123, Omanb Department of Chemical Engineering, Ahmadu Bello University, Zaria, Nigeria

a b s t r a c t

The ow patterns and pressure gradient of immiscible liquids are still subject of immense research interest. Thisis partly because uids with different properties exhibit different ow behaviours in different pipes congurationsunder different operating conditions. In this study, a combination of oilwater properties ( = 20.1mN/m) not previ-ously reported was used in a 25.4mm acrylic pipe. Experimental data of ow patterns, pressure gradient and phaseinversion in horizontal oilwater ow are presented and analyzed together with comprehensive comments. Theeffect of oil viscosity on ow structure was assessed by comparing the present work data with those of Angeli andHewitt (2000) and Raj et al. (2005) . The comparison revealed several important ndings. For example, the water veloc-ity required to initiate the transition to non-stratied ow at low oil velocities increased as the oil viscosity increasedwhile it decreased at higher oil velocities. The formation of bubbly and annular ows and the extent of dual contin-uous region were found to increase as the oilwater viscosity ratio increased. Dispersed oil in water appeared earlier

when oil viscosity decreased.The effect of oil viscosity on pressure gradient was also investigated by comparing the results with Angeli and

Hewitt (1998) and Chakrabarti et al. (2005) . One of the main ndings is the large difference between the pressuregradient results which is attributed to the difference in oil viscosity. The differences between the results becomebigger at higher oil velocities. The largest difference in pressure values was observed in ow region where oil is thecontinuous phase. On the contrary, for dispersed oil in water (Do/w), the pressure gradient values observed at thesame conditions are approximately the same. A simple correlation was developed to predict the pressure gradientin this regime. The correlation was validated using new experimental data.

Finally, the effect of oil viscosity on pressure gradient prediction was investigated using the two ow model forstratied ow and the homogenous model for oil dispersed in water. Both models showed better prediction for thelow oil viscosities.

2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Keywords: Oilwater ow; Phase inversion; Flow pattern map; Flow pattern transition; Pressure gradient

1. Introduction

The ow of two immiscible liquids in pipes is a challenging subject that is rich in physics and practical applications. Itis encountered in many industries such as oil and chemicalindustries. When a mixture of oil and water ows simultane-ously in a channel, the two uids can distribute themselvesin numerous congurations that are largely dependent on the

Corresponding author . Tel.: +968 99358758.E-mail address: [email protected] (Y. Al-Wahaibi).Received26 March2011;Receivedin revisedform31 May 2011;Accepted14 November 2011

physical properties of theuids and theoperatingparameters.For instance, ow congurations of two immiscible liquidswith large density difference are expected to defer from thoseof two liquids with small or same density difference.

In many applications such as articial lift methods,corrosion technology, production strings in oil wells, theunderstanding of oilwater ow behaviour is of signicantimportance. The understanding of oilwater ow in pipes can

0263-8762/$ see front matter 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.doi:10.1016/j.cherd.2011.11.013
http://www.sciencedirect.com/science/journal/02638762http://www.elsevier.com/locate/cherdmailto:[email protected]://localhost/var/www/apps/conversion/tmp/scratch_1/dx.doi.org/10.1016/j.cherd.2011.11.013http://localhost/var/www/apps/conversion/tmp/scratch_1/dx.doi.org/10.1016/j.cherd.2011.11.013mailto:[email protected]://www.elsevier.com/locate/cherdhttp://www.sciencedirect.com/science/journal/02638762


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1020 chemical engineering researchand design 9 0 ( 2 0 1 2 ) 10191030

Pressure switch

Non return valve

Ball valve

Pressure gauge

OilTank

Water Tank

Viewingarea

S e p a r a t

i o n

T

a n k

Acrylic Testsection

Pump PressureTank

Pressure ports

Flow- meter

Mixing point

Fig. 1 Schematic diagram of the oilwater experimental ow facility.

be crucial in determining the amount of free water in contactwith the pipe wall that could cause corrosion problems. Theperformance of separation facilities and multiphase pumps isa strong function of the ow pattern. Understanding of theow structure in liquidliquid ow in horizontal pipes will goa long way in developing predictive models that could aid inthe design and construction of ow equipments.

Quiet large number of studies is currently available in theliterature on ow pattern and pressure drop in horizontaloilwater ow ( Lain and Oglesby, 1976; Oglesby, 1979; Cox,1985; Scott, 1985; Arirachakaran et al.,1989; NdlerandMewes,1997; Valle and Kvandal, 1995; Trallero, 1995; Fairuzov et al.,2000;Angeliand Hewitt, 2000; Raj etal., 2005; Al-Wahaibi et al.,2007; etc.). However, no generalized ow pattern map can beconstructed from themsince different researchers useduidswithdifferent properties in different pipes congurations andunder different operatingconditions. Very fewstudies tried tounderstandtheeffect ofsomeoftheseparameters onow pat-terns andpressure drop( Angeli andHewitt, 1998, 2000;Mandalet al., 2007; Sotgia et al., 2008 ). All of these works have focusedon theinuenceof pipe geometries or materialson eitherowpatterns or pressure drop.

There is currently no work available in the literature onoil viscosity effect on ow patterns and pressure gradientfor horizontal oilwater ow. Investigating the effect of uidproperties, pipe geometries and materials under differentoperating conditions will help us to obtain a clear picture andunderstanding on liquidliquid behaviour. Suchstudiescan belinked at the end to obtain a generalized ow pattern map forliquidliquid ow.

This paper describes theeffectofoilviscosity onow struc-ture and pressure gradient in horizontal oilwater ow. Thisis achieved by combining the results obtained in this studyusing the 12 cp oil viscosity with those reported by Angeliand Hewitt (1998, 2000) , Raj et al. (2005) and Chakrabarti et al.(2005). These systems were selected because they used pipessimilar in diameter and material to the current work.

2. Experimental set-up

The experimental studies on ow patterns and pressure dropwere carried out in the liquidliquid ow facility shownschematically in Fig. 1. Oil and water were used as test u-idswith properties given in Table1 . Each uidwas transferredfrom their storage tank with a pump to the test section made

Table 1 Properties of oil and water used in this study.Parameters Mineral oil Water

Density (g/cm 3) 0.875 0.998Viscosity (cP) 12 1Interfacial tension (mN/m) 20.1

up of 25.4mm acrylic pipe that consists of two 8m long sec-tions joined by a U-bend. The two uids entered the testsection from two pipes via a Y like-junction. The water phasewas allowed to enter from thebottom while theoil joinedfromthe top to reduce the effect of mixing. Two ow meters with

maximum capacity of 20,000l/h and30 l/min were attachedtoeach of the ow lines (water and oil) which were regulatedthrough pin. The ow meters were calibrated with the u-ids with accuracy of 0.5% full scale. The mixture returns via aPVC pipe to a separator tank which allows the two phases toseparate and hence return to their respective storage tanks.

High-speed camera (Fastec Troubleshooter) and visualobservation were used to identify the different ow patternsand the transition from one pattern to another. The camerawas located 6.5 m from the rst 8 m part of the test section. Atthis point the ow is fully developed as preliminary investi-gation showed. The camera was a Troubleshooter system thatcanrecordup to1000 fps. In thiswork,500 fps wasselectedand

the images were then transferred and analyzed using MiDAS4.0 express software. Pressure gradient experiment was con-ducted in the test section by measuring the pressure dropbetween two points 1 m apart along the ow line 7 m from theentry point. The pressure drop was measured with a Dywer490 digital differential manometer.

3. Results and discussion

3.1. Flow pattern map

Theowpatternsidentied in this work forthe rangeof super-cial oil and water velocities investigated are presented inFig. 2. They are classied into six patterns namely; stratied(stratied smooth, SS, and stratied wavy, SW), bubbly (Bb),dual continuous (DC), annular, (AN), dispersed oil in water(Do/w) and dispersed water in oil (Dw/o). They are dened asfollows:


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chemical engineering researchand design 9 0 ( 2 0 1 2 ) 10191030 1021

0.1

1

0.05 0.5

U s w ,

m / s

Uso, m/s

ST Bb AN DC Do/w Dw/o

Fig. 2 Flow pattern map constructed for this study(25.4 mm ID pipe, =12cP, =0.87g/cm 3 , = 20.1 mN/m).

Stratied(stratiedsmooth, SS, and stratiedwavy, SW): wherethe two uids ow in separate layers at the top and bottom of the pipe according to their densities.

Dual continuous (DC): where both oil andwater form contin-uous layers at the top and bottom of the pipe respectively butdrops of one phase appear into the continuum of the otherphase.

Annular (AN): wherewater formsanannular lmat thewalland oil ows in the pipe core.

Bubbly (Bb):where the oil appears in the form of elongateddrops (slightly longer than the pipe diameter) within watercontinuum.

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

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

3.2. Visual observation

3.2.1. Stratied owStratied owappeared at lowoil andwater velocities becauseat these velocities gravity force dominated while momentuminstabilitieswere minimal. Stratied owwas observedwithinthe range of supercial oil velocity ( Uso ) of 0.060.33 m/s andsupercial water velocity ( Usw ) of 0.10.48m/s. Stratied owwasinitially characterizedby a smooth interfacewith no dropsand waves (stratied smooth (SS) ow). The ow changed tostratied wavy ow as supercial water velocity increased.The amplitude of the waves grew with increase in supercialwater velocity. For example, at Uso =0.06m/s and 0.22m/s, thewave amplitudes increased in sizeas supercial water velocityincreased as shown in Fig. 3a and b.

Stratied ow was observed to transform into two types of owpatterns as supercial water velocityincreased.These aretransition to bubbly anddual continuous ows. Stratiedowchanged to bubbly ow at Uso less than 0.1 m/s. This is largelydue to the increase in turbulence of the water phase at highwater velocity and because the layer of the oil phase is thin atthese oilvelocities, theprobabilityof thewaves at theinterfacebreaking 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 than the pipediameter).

Onthe other hand, transition from stratiedto dual contin-uousow occurredat Uso >0.1m/s.Thisisattributedtothefactthat at these ow conditions, the oil layer was thick enoughto resist the turbulence of the water phase from breaking its continuous ow. Instead of breaking the continuity of thelayer, the relative movement between the phases increased,which increased the amplitude propagation eventually break-ing the interfacial tension between the oil and water, hencedrop formed at the interface.

For the effect of supercial oil velocity on stratied ow,the area of the pipe occupied by the oil phase increased whilethe smooth interface turned wavy (see Fig. 4 at Usw = 0.42 and0.1m/s). Thewaveamplitude increased assupercial oilveloc-ity increased. Stratied ow extended to higher supercial oilvelocities at lower supercial water velocities. For example,at Uso =0.33 m/s, stratied ow had already transformed todual continuous ow for Usw =0.42m/s, while the ow wasstill stratied for Usw =0.1m/s.

3.2.2. Non-stratied ow3.2.2.1. Bubbly ow. Bubbly ow pattern occurred at lowsupercial oil velocities ( Uso =0.060.1 m/s) and moderatesupercial water velocities ( Usw =0.540.95m/s). This isbecause at low oil velocity, the oil layer was thin while theturbulence of the water layer increased as the water velocityincreased. This created instability at the interface and brokethe thin oil layer; hence, bubbles of oil were formed in thewater continuum.

Theincreasein supercial water velocitycaused a decreasein the bubble length (see Fig. 5). The elongated bubbleswhich were initially slightly longer than the pipe diameterat Uso =0.1m/s and Usw =0.6m/s decreased to less than thepipe diameter as Usw increased from 0.6 to 0.95m/s. Furtherincrease in supercial water velocity turned the oil bubblesinto drops in the water continuous phase.

On the other hand, as supercial oil velocity increased, thebubble length increased due the increase in the bubbles coa-lescence rate. Further increase in the oil velocity made the oillayer to preserve its continuity. Hence a transition to eitherannular or dual continuous ow occurred depending on thesupercial velocity of water (see Fig. 6). At Usw =0.60m/s, atransition to dual continuous ow occurred at Uso =0.14m/s.At this condition the oil layer was thick enough to with-stand the effect of the turbulence caused by the water layer.Instead of breaking the layer, it caused increase in the relativemovement between the phases that increased the interfaceinstability and eventually drop formation.

At higher supercial water velocity ( Usw = 0.80 m/s), transi-tion from bubbly ow to annular ow occurred at the samesupercial oil velocity. This is because the oil layer was thickenough to retain its continuity and the water velocity thoughhigh butnot strong enough to disperse the oil layer. Due to thehigh difference in velocity, the water was swept up along thepipe wall until it wetted the whole circumference of the pipe,while the oil continuous layer ew in the core of the pipe.

3.2.2.2. Dual continuous (DC) ow. Dual continuous ow wasfound to occur within the range of supercial oil velocity of 0.140.90m/s and water supercial velocity of 0.10.95m/s.Dual continuous ow isa functionof therelative velocityofthetwo phases and wave amplitude. At low supercial oil veloci-ties only few drops of oil were initially dispersed in the waterphase (see Fig. 7). As the water velocity increased, more of the oil was dispersed into the water phase. Further increase


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Fig. 3 Effect of increasing water supercial velocity at (a) U so = 0.06m/s, (b) U so =0.22m/s.

Fig. 4 Effect of increasing oil supercial velocity on stratied ow at (a) U sw =0.42m/s and (b) U sw =0.1m/s.

in water velocity caused the DC ow to transform into annu-lar ow and then to dispersion of oil in water ow (Do/w). Thewaterphase didnotdisperse into theoil phaseat low oilsuper-cial velocity. This is may be because the region near the wallhad relatively higher shear stress due to high velocity gradi-ents. At higher supercial oil velocities, as supercial watervelocity increased, both phases dispersed into the other andthe interfacial mixing extended more into the oil continuousphase until transition from DC to Do/w ow occurred. This is

likely due to the increase in instability as a result of the com-bined effect of the turbulence force caused by the velocitiesand the viscosity resistance due to the thick oil layer in thepipe, thereby causing both phases to disperse into the other.Fig. 7 shows that at the same supercial water velocities, themixing at the interface is larger for Uso =0.44m/s compared toUso =0.21m/s.

Fig. 8 shows the effect of supercial oil velocity ondual continuous ow at low (e.g. Usw =0.16m/s) and high

Fig. 5 Effect of increasing supercial water velocity on bubbly ow at U so =0.1m/s.


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Fig. 6 Effect of increasing supercial oil velocity on bubbly ow at (a) U sw =0.60m/s and (b) U sw =0.80m/s.

supercial water velocities (e.g. Usw = 0.60 m/s). AtUsw =0.16m/s dual continuous ow appeared as waterdrops dispersed into the oil continuous phase and a contin-uous water layer owing at the bottom of the pipe. As the oilsupercial velocity increased, more water dispersed into theoil phase while the thickness of the water layer was found todecrease until the whole water dispersed in the oil phase at

Uso =0.74m/s. This is considered as the transition from DCow to dispersion of water in oil (Dw/o).

At Usw =0.6m/s, both the oil and water phases were dis-persed in the continuum of the other phase. As supercialoil velocity increased, the mixing of one phase into theother phase increased while the thickness of the water layerdecreaseduntil a transition to dispersion of waterin oil (Dw/o)

Fig. 7 Effect of increasing supercial water velocity on DC ow at (a) U so =0.21m/s and (b) U so = 0.44 m/s.


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Fig. 8 Effect of increasing supercial oil velocity on DC ow at (a) U sw = 0.16m/s, (b) U sw = 0.60 m/s.

occurred at Uso =0.90m/s. Thus, it can be said that as thesupercial water velocity increased, the supercial oil velocityat which the transition from dual continuous to dispersion of water in oil appeared also increased.

3.2.2.3. Annular ow. Annular ow can be classied as a typeof dual continuous ow. In this study, this pattern occurred atslightly higher supercial oil velocity than that of bubbly ow.It appeared within Uso =0.140.33m/s and Usw = 0.731.14m/s.A thin layer of water was seen at the top of the pipe and theinterface was disturbed with oil drops clearly seen within thewater.

As shown in Fig. 9, as the water supercial velocityincreased, the disturbance at the interface also increased;therefore, more of the oil owing in the core of the pipe wasseen to disperse into the water phase until the whole oilcollapsed into droplets. This is attributed to the increase inturbulence due to the increase in water velocity.

For the effect of supercial oil velocity on annular ow,the thickness of the core oil layer increased while the thinwater layer at the top decreased as the supercial oil velocityincreased (see Fig. 10). Further increase in oil velocity changedthe annular pattern to dual continuous ow.

Annular ow where oil is the wall-wetting phase didnot occur. Considering the results of other researchers(Arirachakaran et al., 1989 ; Charles et al., 1961; Valle and

Kvandal, 1995; Al-Wahaibi et al., 2007 ; and Sotgia et al., 2008 ),it can be said that in annular ow pattern, the phase with thesmaller density is the one that forms the core ow.

3.2.2.4. Dispersion of oil in water (Do/w). Dispersion of oil inwater occurred at high supercial water velocity throughoutthe supercial oil velocity investigated. At low supercial oilvelocities, Do/w was transformed from bubbly ow while atmoderate oil supercial velocities; it is dual continuous owthat changed to Do/w. At higheroil supercial velocities, Do/wwas inverted from dispersion of water in oil.

Fig. 11a and b shows that at low and moderate oil super-cial velocities (e.g. Uso =0.06m/s and 0.33m/s respectively),the oil was initially dispersed in the water continuum at thetop of the pipe with a continuous layer of water owing atthe bottom. As the water velocity increased, the area occu-pied by the dispersed oil extended to the entire pipe crosssectional area. However, at higher oil supercial velocities(Uso =1.2m/s), increase in supercial water velocity did nothave signicant effect on the dispersion (see Fig. 11c).

At relatively high supercial water velocities (e.g.Usw =0.9m/s), transition from dispersion of oil in waterto dispersion of water in oil occurred. Though visual obser-vation and the pictures obtained from the camera are nolonger sufcient to differentiate between these ow patterns(dispersion of oil in water and dispersion of water in oil),


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Fig. 9 Effect of increasing supercial water velocity on annular ow at U so =0.14m/s.

Fig. 10 Effect of increasing supercial oil velocity on annular ow at U sw =0.80m/s.

but from the graph of the pressure gradient experiment, it isobserved that a sharp increase in pressure gradient occurredat about 0.30 water input fraction, which corresponds to thephase inversion point as different researchers (e.g. Ndlerand Mewes, 1997; Angeli and Hewitt, 1998; Ioannou et al.,2005) who used conductivity probe conrmed that at phaseinversion point, there is a sharp increase in pressure gradient

before a sharp decrease is observed. This has been attributedto the fact that when water is the continuous phase in the

ow, the pressure gradient is lower than when oil is thecontinuous phase because of the viscosity of the oil whichwill cause higher skin pressure drop.

3.2.2.5. Dispersion of water in oil (Dw/o). Unlike thedispersionof oil in water that was observed throughout the oil super-cial velocities investigated, the dispersion of water in oil

(Dw/o) appeared from supercial oil velocity of 0.63m/s. Assupercial water velocity increased, the dispersion of water

Fig. 11 Effect of increasing supercial water velocity at (a) U so = 0.06m/s, (b) U so = 0.33m/s, and (c) U so =1.2m/s.


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Fig. 12 Effect of increasing supercial water velocity on dispersion of water in oil at (a) U so =0.63m/s and (b) U so =1.4m/s.

in oil metamorphosed to two different types of ow patternsnamely; dual continuous ow anddispersionof oil in water. Atmoderate supercial oil velocities, as water velocity increased,the ratio of oil to water in the pipe decreased until the waterlayer was thick enough to form a continuous phase at thebottom of the pipe with the water dispersed in the oil con-tinuum at the top. Hence, dual continuous ow occurred. Athigh supercial oil velocities, as the water supercial velocityincreased, the ratio of oil to water decreased until the waterphase which is denser than the oil became the continuousphase and oil dispersed in its continuum. At moderate super-cial oil velocity, the turbulence of the ow was not strong enough to disperse the whole water; hence, the water formeda continuous layer at the bottom. While at high oil velocity,the turbulence of the ow was high enough to keep the twophases dispersed in one another. Fig. 12 shows the effect of increasing supercial water velocity on dispersion of water inoil at Uso =0.63m/s, Uso =1.4m/s.

3.3. Pressure gradient results

Pressure gradient due to friction was measured in this studyover a broad range of supercial oil and water velocities rang-ing from 0.1 to 2.0m/s and 0.1 to 2.6m/s respectively. Theresults averaged over at least three measurements are pre-sented in Fig. 13 in terms of supercial water velocity. Atlow supercial oil velocities where the ow transformed fromstratied to dual continuous and then to dispersed ow withincreasing supercial water velocity, (e.g. Uso = 0.14 m/s), pres-sure gradientincreased assupercialwater velocity increased.Also, at moderate supercial oil velocities where the owtransformed from dual continuous to dispersed ow withincrease in supercial water velocity (e.g. Uso = 0.52m/s), thepressure gradient followed similar trends to that observed atlow supercial oil velocities. However, at moderate and highsupercial oil velocities where transition from dispersion of waterin oil to dispersion of oil in wateroccurred with increasein supercial water velocity (e.g. Uso =1.08 and 1.78m/s), thepressure gradient was observed to initially increase gradu-ally before a sharp increase was observed followed by a sharp

0

1000

2000

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4000

5000

6000

7000

8000

9000

10000

0 0.5 1 1.5 2 2.5 3

d p

/ d x ,

P a / m

Usw , m/s

Uso =0.14 Uso =0.33 Uso =0.52 Uso =0.7

Uso =1.08 Uso =1.4 Uso =1.78 Uso =2.27

Fig. 13 Effect of increasing supercial water velocity onpressure gradient at different supercial oil velocities.

decrease forming a hump, then a gradual increase in pressuregradientoccurred.The peak of thehump occurred close to thesupercial water velocity where transition from dispersion of water in oil to dispersion of oil in water appeared. This pointis termed as the phase inversion point.

Phase inversion from oil continuous to water continuousdispersed owoccurredat around 2530% input water volumefraction. The peak in pressure gradient accompanying phaseinversion was observed in the present work as illustrated inFig. 13. Fig. 14 shows the phase inversion points for all theconditions investigated. These are taken as the water volumefraction at which the maximum pressure gradient appeared.It is clear that the water volume fraction decreased as themixture velocity increased.

The semi-empirical equations developed by Arirachakaranet al. (1989) and Brauner and Ullamann (2002) were comparedwith the experimental data. Arirachakaran et al. (1989) modelpredicts the phase inversion at 38% water volume fractionwhich is higher than this study results (2530% water volumefraction). On the other hand, Brauner and Ullamann (2002)


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0.2

0.25

0.3

0.35

0.4

1.5 2 2.5 3 3.5

W a t e r V o l u m e F r a c t i o n , -

U m , m/s

Fig. 14 Phase inversion at different mixture velocities.

0.1

1

10

0.01 0.1 1 10

U s w , m

/ s

Uso, m/sST Bb AN

DC Do/w Dw/oAngeli & Hewitt

Do/w Dw/o

DC

ST

Fig. 15 Effect of oil viscosity on ow patterns: comparisonwith the ow pattern boundaries of Angeli and Hewitt (2000).

predict the phase inversion at 30% water volume fractionwhich is close to those reported in this work.

3.4. Effect of oil viscosity

The effect of oil viscosity on ow pattern and pressure gradi-ent was investigated by comparing the current experimentalresultsat12cp with those reportedin theliterature forviscosi-ties between 1.2and1.6 cp (see Table2 ). All thedata presentedin Table 2 were obtained for horizontal oilwater ow using pipe geometry and material similar to the one used in thepresent study.

0.1

1

10

0.01 0.1 1 10

U s w , m

/ s

Uso, m/s

ST Bb AN DC Do/w Dw/o Raj et al

Do/w

Dw/oDCBb

ST

Fig. 16 Effect of oil viscosity on ow patterns: comparisonwith the ow pattern boundaries of Raj et al. (2005) .

3.4.1. Effect on ow patternsFigs. 15 and 16 describe comparisons between the currentwork ow pattern map at the 12cp with those of Angeli andHewitt (2000) and Raj et al. (2005), respectively. The ow pat-tern maps were reconstructed in such a way that all the owpatterns observed by various researchers are classied intostratied, bubbly, annular, dual continuous, dispersion of oilin water, and dispersion of water in oil ows.

3.4.1.1. Stratied ow. Similar to thisstudy, Angeli andHewitt(2000) and Raj et al. (2005) observed stratied ow patternin their studies. The only difference is the extent to whichthe ow pattern extends. Two regions were obtained whencomparing the transition from stratied to non-stratied owof systems of different oil viscosities. In the rst region (atlower oil velocities), the water supercial velocities needed forthe transition to non-stratied ow increased as oil viscosityincreased. In the second region (for the higher oil velocities),earlier transition to dual continuous ow was observed as oilviscosity increased.

3.4.1.2. Bubbly ow. Compared to this study, Raj et al. (2005)observed bubbly ow withina wider rangeof supercialwaterand oil velocities ( Usw = 0.380.7m/s and Uso = 0.030.14m/s)while Angeli and Hewitt (2000) did not observe any bubblyow in their study although they used an oil viscosity approx-imately similar to that used by Raj et al. (2005). This can beattributed to the decrease in interfacial forces in Angeli andHewitt (2000) system. Raj et al. (2005) used about that sameoil viscosity but with higher interfacial tension. As a resultintermittent ow was observed in their system. In the cur-rent study, although the value of interfacial tension was closeto those used by Angeli and Hewitt (2000) , the oilwater vis-cosity ratio is high which increased the interface instabilityand as a result bubble formed at low oil ow rates. Thus it

Table 2 Data used to investigate the effect of oil viscosity on ow pattern and pressure gradient.

Authors Pipe ID (mm) Pipe material Measured parameter Oil properties

(mPas) (kg/m 3) (mN/m)

Angeli and Hewitt (1998) 25.4 Acrylic Pressure gradient 1.6 801 17.0Angeli and Hewitt (2000) 25.4 Acrylic Flow pattern 1.6 801 17.0Chakrabarti et al. (2005) 25.4 Acrylic Pressure gradient 1.2 787 45Raj et al. (2005) 25.4 Acrylic Flow pattern 1.2 787 45Present work 25.4 Acrylic Flow pattern and pressure gradient 12 875 20.1


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0

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100

150

200

250

300350

0 0.1 0.2 0.3 0.4 0.5

d p

/ d x , P

a / m

U sw, m/s

Angeli & Hewitt (1998) , Uso =0 .11m/sChakrabarti et al. (2005), Uso = 0.12m/sPresent study, Uso = 0.12m/s

Fig. 17 Effect of oil viscosity on pressure gradient in thestratied ow region: comparison between present study,Angeli and Hewitt (1998) and Chakrabarti et al. (2005) .

can be deduced that systems of oilwater ow with relativelyhigh viscosityratioand high interfacial forceswill have higherchance for intermittent ows to form (i.e. bubbles and slugs).

3.4.1.3. Dual continuous ow. In this study, dual continu-ous ow pattern occurred within supercial water and oilvelocity of 0.10.95m/s and 0.140.95m/s, respectively. BothAngeli and Hewitt (2000) and Raj et al. (2005) had reportedDC ow in their studies. Compared with the results of thisstudy, at low oil velocities, Angeli and Hewitt (2000) and Rajet al. (2005) reported early formation of DC ow as supercialwater velocity increased. The extent of dual continuous pat-tern reported by Angeli and Hewitt (2000) and Raj et al. (2005)system (lower oil viscosity system) is smaller when compared

to the present study results (higher oil viscosity system). Thiscould be related to the higher oilwater viscosity ratio used inthis study.

3.4.1.4. Annular ow. Annular ow was not reported by nei-ther Angeli andHewitt (2000) nor Raj et al. (2005). This is likelydue to the low viscosity oil used in their studies. As reportedby Grassi et al. (2008) , high oil viscosity favours the formationof annular ow pattern.Thus, as theoil viscosityincreases theprobability to have an annular ow increases.

3.4.1.5. Dispersion of oil in water and water in oil (Do/w andDw/o) ows. The dispersion of oil in water or water in oil is afunction of turbulence of the ow. Once the viscous and therelative velocity forces which cause instability overcome theinterfacial forces, the ow will eventually transform to dis-persion of oil in water or dispersion of water in oil depending on which one the uids is the continuous phase. Similar tothis study, both Angeli and Hewitt (2000) and Raj et al. (2005)observed dispersed ow patterns. Compared to this study,early formation of Do/w was noticed in both works whichmay be attributed to the lower oil viscosity implemented inboth studies. For the Dw/o, Angeli and Hewitt (2000) reportedan early transition to Dw/o. This is likely due to the entrycondition of the uids. Raj et al. (2005) introduced their u-ids through a mixer comprising of two concentric pipes withtheoil introduced through theannulus andwater through thetube. This method prevented lateral mixing of the two uidsnear the entry point. On the other hand, Angeli and Hewitt(2000)used a T-junction with a 90 elbow immediately down-stream andbefore thetestsectionto introduce their uidsinto

0

200

400

600

800

1000

0.4 0.5 0.6 0.7 0.8 0.9 1

d p / d x ,

P a / m

U sw , m/s

Chakrabarti et al. (2005), Uso = 0.12m/sPresent study, Uso = 0.12m/s

Fig. 18 Effect of oil viscosity on pressure gradient in the bubbly ow region: comparison between present study andChakrabarti et al. (2005) .

0

200

400

600

800

1000

1200

0 0.2 0.4 0.6 0.8 1

d p

/ d x ,

P a / m

Usw, m/s

Present study, Uso = 0.52m/s Present study, Uso = 0.70 m/sChakrabarti et al., Uso =0.50m/s Chakrabarti et al., Uso =0.60m/sAngeli & Hewitt, Uso = 0. 55m/s Angeli & Hewitt, Uso = 0. 66m/s

Fig. 19 Effect of oil viscosity on pressure gradient in theDC ow regime: comparison between present study, Angeliand Hewitt (1998) and Chakrabarti et al. (2005) .

0

1000

2000

3000

4000

0 0.1 0.2 0.3 0.4 0.5 0.6

d p

/ d x ,

P a / m

U sw , m/s

present study, Uso = 1.1m/s Angeli & Hewitt, Uso = 1.1m/s present study, Uso = 1.4m/s Angeli & Hewitt, Uso = 1.4m/s

Fig. 20 Effect of oil viscosity on pressure gradient in theDw/o ow regime: comparison between present study andAngeli and Hewitt (1998) .

the testsection. Thisincreased thepossibility of lateralmixing of the uids at the entry point, hence causing early transition.

3.4.2. Effect on pressure gradientThe effect of oil viscosity on pressure gradient is shownin Figs. 1721. The data of Angeli and Hewitt (1998) andChakrabarti et al. (2005) are plotted against those obtainedin this study in terms of pressure gradient against super-cial water velocity for some Uso and for different owpatterns. The pressure gradient values obtained in this studyare greater than those reported by Angeli and Hewitt (1998)and Chakrabarti et al. (2005) at similar supercial oil and water


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0

1000

2000

3000

4000

5000

6000

0 0.5 1 1.5 2 2.5 3

d p / d x , P a / m

Usw, m/s

Present study, Uso = 0.88m/s Angeli & Hewitt (1998), Uso = 0.88 m/s

Fig. 21 Effect of oil viscosity on pressure gradient in theDo/w ow regime: comparison between present study andAngeli and Hewitt (1998) .

y = 435.1x 2.0

0

1000

2000

3000

4000

5000

6000

1 1.5 2 2.5 3 3.5 4

d p / d x , P a / m

U m , m/s

Fig. 22 Pressure gradient correlation for Do/w ow regime.

velocities as shown in Figs. 1720. This is because the viscos-ity of the oil used in their studies is lower than that used inthis work (1.6 and 1.2 cP, respectively, compared to 12cP forthis study). Since the oil is in direct contact with the pipe wallespecially during stratied, dual continuous and dispersionof water in oil ow regimes, the system with higher oil vis-cosity is expected to have higher drag, hence greater pressuredrop. From the gures, it is clear that the differences in pres-sure gradient results become bigger with higher oil velocities.The largest difference in pressure values was observed in owregion where oil is the continuous phase as shown in Fig. 20.

On the other hand, in ow patterns where water formsthe continuous phase (Do/w) the pressure gradient valuesobserved at similar conditions are approximately the same.Fig. 21 is an example of pressure gradient comparison in theDo/w region. It should be noted that the most interesting behaviour presented in Fig. 21 is that all the results followedthesame trend.Thus, regression analysiscan be performedforthese data to obtaina correlation that canpredictthe pressuregradient for a ow of dispersed oil in water (Do/w). The datain Fig. 21 can be replotted as a function of mixture velocityto describe the pressure gradient at any set of condition (seeFig. 22). The resultingcorrelation for the pressure gradientcanbe written in terms of mixture velocity as

dpdx

= 435.1U2m (1)

where Um is the mixture velocity of oil and water.

0

1000

2000

3000

4000

5000

6000

7000

0 7000600050004000300020001000

( d p

/ d x )

p r e

d , P

a / m

(dp/dx) exp , Pa/m

+20 %

-20 %

Fig. 23 Comparison between the experimental pressuregradient results and those predicted by Eq. (1) f or Do/w owregime.

0

50

100

150

200

250

300

350

0.00 0.10 0.20 0.30 0.40 0.50

d p / d x ,

P a / m

Usw, m/s

Exp_Present Study Pred_Present study

Exp_Angeli & Hewit t (1998) Pred_Angeli & Hewit t (1998)

Exp_Chakrabarti et al. (2005) Pred_Chakrabarti et al. (2005)

Fig. 24 Effect of oil viscosity on the prediction of pressuregradient data using the two-uid model at U so = 0.12 m/s.

0

1000

2000

3000

4000

5000

6000

0 0.5 1 1.5 2 2.5 3

d p / d x ,

P a / m

Usw, m/s

Exp_Present Study Exp_Angeli & Hewitt (1998)

Pred_Present study Pred_Angeli & Hewitt (1998)

Fig. 25 Effect of oil viscosity on the prediction of pressuregradient data using the homogenous model, Dukler et al.(1964), at U so =0.88m/s.

The correlation was validated by comparing the predic-tion with new experimentaldata (see Fig. 23). The comparisonrevealed that the equation was able to correlate the datawithin 20%.

3.4.3. Effect of oil viscosity on the accuracy of the pressuredrop modelThe effect of oil viscosity on the accuracy of the pres-sure gradient model was examined by comparing theexperimental pressure gradient results of the present work,Angeli and Hewitt (1998) and Chakrabarti et al. (2005) with


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the two-uid and the homogeneous models. At low super-cial velocities the ow is stratied and the phases are in twolayers separated by an interfaceacrosswhich momentum canbe transferred. Hence, the two-uid model is considered forthis regime. At high supercial velocities (dispersed phase),the homogenous model which considers the two uids as apseudo-uid and used the average properties of the uids isappropriate for testing the pressure gradient data obtainedexperimentally at this regime.

The comparison with the two-uid model is shown inFig. 24 at Uso =0.12m/s for different supercial water veloc-ities. The model was found to better predict the pressuregradient data at low oil viscosity ( Angeli and Hewitt, 1998and Chakrabarti et al., 2005 ). The differences between the pre-dicted and experimental results increased as the oil viscosityincreased.

For the dispersed ow regime, the homogeneous model asdened by Dukler et al. (1964) was used for the comparison. Itpredicts the data of Angeli and Hewitt (1998) with an averageerror of 6%, while it overpredicts the present work data withanaverageerrorof 35%. Thus,it canbe concluded that that thehomogenous model works better as the oil viscositydecreases(see Fig. 25).

4. Conclusions

Flow structure, pressure gradient and phase inversion wereobtained using a combination of oilwater properties not pre-viously reported in a 25.4mm acrylic horizontal pipe. Theeffect of oil viscosity on these parameters was investigated bycomparing the experimental results of the present work withthose of Angeli and Hewitt (1998, 2000) , Raj et al. (2005) and

Chakrabarti et al. (2005) .

Based on this, the following conclu-sions can be drawn:

1. Six ow patterns were identied namely; stratied, bub-bly, annular, dual continuous, dispersion of oil in water,and dispersion of water in oil ow. At Uso < 0.1m/s, strati-edow transformedto bubbly as watervelocity increasedwhile at Uso > 0.1 the ow changed to dual continuous owas water velocity increased.

2. At low oil velocities, the water velocity required to initiatethetransitionto non-stratied ow increasedas theoil vis-cosity increased while it decreased at higher oil velocities.Also, it is observed that the formation of bubbly and annu-

lar ows

and the extent of dual continuous ow increasedas the oilwater viscosity ratio increased. Dispersed oil inwater formedat lower water velocities for certain oil veloc-ities as the oil viscosity decreased.

3. Large difference in pressure gradient data was obtained.The differences in the results become larger as the oil vis-cosities and velocities increased. The largest difference inpressure values was observed in ow region where oil isthe continuous phase.

4. A simple correlation was developed to predict the pres-sure gradient of oil dispersed in water. The correlation wasable to correlate the pressure gradient results within 20%when validated using experimental data.

5. The effect of oil viscosity on pressure gradient predictionwas investigated using the two ow model for stratiedow andthehomogenous model for oil dispersed in water.

Both models showed better prediction for the low oil vis-cosities.

6. The phase inversion point obtained experimentally wasfound to be around water cut of 0.250.30 which is closeto the one predicted by Brauner and Ullamann (2002) .

References

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Angeli, P., Hewitt, G.F., 1998. Pressure gradient in horizontalliquidliquid ows. International Journal of Multiphase Flow24 (7), 11831203.

Angeli, P., Hewitt, G.F., 2000. Flow structure in horizontaloilwater ow. International Journal of Multiphase Flow 26 (7),11171140.

Arirachakaran, S., Oglesby, K.D., Malinowsky, M.S., Shoham, O.,Brill, J.P., 1989. An analysis of oil/water ow phenomena inhorizontal pipes. In: SPE Proc. Prod. Operation Symp.Oklahoma City, SPE Paper 18836.

Brauner, N., Ullamann, A., 2002. Modelling of phase inversionphenomenon in two-phase pipe ows. International Journalof Multiphase Flow 28, 11771204.

Chakrabarti, D.P., Das, G., Ray, S., 2005. Pressure drop inliquidliquid two phase horizontal ow: experiments andprediction. Chem. Eng. Technol. 28, 10031009.

Cox, A.L., 1985. A study of horizontal and downhill two-phaseoilwater ow, M.S. Thesis. The University of Texas.

Dukler, A.E., Wicks, M., Cleveland, R.G., 1964. Pressure drop andhold-up in two-phase ow. AIChE Journal 10, 3851.

Fairuzov, Y.V., Arenas-Medina, P., Verdejo-Fierro, J.,Gonzalez-Islas, R., 2000. Flow pattern transitions in horizontalpipelines carrying oilwater mixtures: full-scale experiments. Journal of Energy Resources Technology 122, 169176.

Grassi, B., Strazza, D., Poesio, P., 2008. Experimental validation of

theoretical models in two-phase high-viscosity ratioliquidliquid ows in horizontal and slightly inclined pipes.International Journal of Multiphase Flow 34, 950965.

Ioannou, K., Nydal, O.J., Angeli, P., 2005. Phase inversion indispersed liquidliquid ows. Experimental Thermal andFluid Science 29 (3), 331339.

Lain, G.C., Oglesby, K.D., 1976. An experimental study on theeffects of ow rate, water fraction and gasliquid ratio onairoilwater ow in horizontal pipes, B.S. Thesis. TheUniversity of Tulsa.

Mandal, T.K., Chakrabarti, D.P., Das, G., 2007. Oil water owthrough different diameter pipes: similarities and differences.Chemical Engineering Research and Design 85 (8), 11231128.

Ndler, M., Mewes, D., 1997. Flow induced emulsication in theow of two immiscible liquids in horizontal pipes.

International Journal of Multiphase Flow 23, 5568.Oglesby, K.D., 1979. An experimental study on the effects of oilviscosity, mixture velocity, and water fraction on horizontaloilwater ow, MS Thesis. The University of Tulsa.

Raj, T.S., Chakrabarti, D.P., Das, G., 2005. Liquidliquid stratiedow through horizontal conduit. Chemical Engineering andTechnology 28, 899907.

Scott, G.M., 1985. A study of two-phase liquidliquid ow atvariable inclinations. MS. Thesis. The University of Texas.

Sotgia, G., Tartarini, P., Stalio, E., 2008. Experimental analysis of ow regimes and pressure drop reduction in oilwatermixtures. International Journal of Multiphase Flow 34 (12),11611174.

Trallero, J.L., 1995, Oilwater ow patterns in horizontal pipes,PhD. Thesis. University of Tulsa.

Valle, A., Kvandal, H., 1995. Pressure drop and dispersioncharacteristics of separated oil/water ow. Two-Phase FlowModelling and Experimentation, 583592.

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