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chemical engineering research and design 1 0 2 ( 2 0 1 5 ) 161–170

Contents lists available at ScienceDirect

Chemical Engineering Research and Design

j ourna l h omepage: www.elsev ier .com/ locate /cherd

n experimental investigation of phase separationf gas–liquid two-phase flow through a small break

achun Liang ∗, Lianmin Song, Yuan Sunollege of Pipeline and Civil Engineering, China University of Petroleum, No. 66 The Yangtze River West Road,ingdao 266580, Shandong, China

r t i c l e i n f o

rticle history:

eceived 26 March 2015

eceived in revised form 7 June 2015

ccepted 15 June 2015

vailable online 22 June 2015

eywords:

wo-phase flow

hase separation

reak

a b s t r a c t

This paper proposes a specially designed splitting device to study the phase separation of

gas–liquid two-phase flow through a small break. The inner pipe diameter of the main test

section is 40 mm. A small hole with 2.5 mm diameter was drilled at the main pipe wall to

simulate the break. Three break orientation angles were tested, including 0◦ (side), −45◦

(inclined) and −90◦ (bottom) from horizontal orientation. Experiments were conducted in

an air–water two-phase flow loop with a horizontal test section. Stratified wavy, annular

and slug flows were observed. Experimental results show that phase separation is affected

by the break location, flow pattern and gas and liquid superficial velocities. The fraction of

liquid taken off of slug flow is observed much larger than that of stratified wavy or annular

flows due to its particular flow behavior. A simplified correlation of break pressure difference

low pattern is proposed in terms of break outlet mass flow rate and gas quality.

© 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

through the break. Hence, knowledge of phase separation

. Introduction

as–liquid two-phase flow is widely found in a variety ofpplications such as power generation, chemical process,uclear energy, and hydrocarbon production industries. Whenas–liquid mixture is introduced into a dividing T-junction,neven distribution of the phases will inevitably take place,

.e., the qualities of the two outlets are different, which areot equal to that at the inlet (Roberts et al., 1997; Stacey et al.,000; Mak et al., 2006). This phenomenon is called phase sepa-ation. In the last several decades, extensive studies have beenarried out on phase separation at T-junctions (Seeger et al.,986; Shoham et al., 1987; Azzopardi, 1999; Mohamed et al.,011; Elazhary and Soliman, 2012; Chen et al., 2015).

When two-phase flow passes through a pipe with a break,evere phase separation may also occur, depending on theocation of the gas–liquid interface relative to the break (Weltert al., 2004; Bartley et al., 2010). If the break is submergedn liquid phase, liquid will preferentially flow into the break.he opposite is observed when the entrance is above the liq-

id level and the discharge is gas predominantly. Zuber (1980)

∗ Corresponding author. Tel.: +86 15053259392; fax: +86 053286981822.E-mail address: Liangfch@upc.edu.cn (F. Liang).

ttp://dx.doi.org/10.1016/j.cherd.2015.06.027263-8762/© 2015 The Institution of Chemical Engineers. Published by

reviewed the two-phase phenomena at a small branch on theside of a large reservoir containing stratified layers of gas andliquid fluid phases. He pointed out that, if gas/liquid inter-face was below the break, liquid may be entrained into the gaspredominating flow through the break. Similarly, gas may beentrained into the predominant liquid flow in form of vortexor vortex-free motion when the break is below the gas/liquidinterface.

Prediction of the discharged mass flow rates from asmall break is one of the most important safety issues intwo-phase flow systems (Castiglia and Giardina, 2010). Forinstance, light water nuclear reactors (LWRs) during a loss-of-coolant accident (LOCA), pipeline networks transferringhazardous fluid, offshore oil-well lines, and chemical batchor continuous reactors (Reimann and Khan, 1984). Owingto the inherent complexity of the two-phase flow, it is stilla challenge to accurately predict the discharged mass flowrate and quality. As mentioned above, phase separation hasa significant influence on the gas and liquid flow rates

phenomena at the break is essential for developing a model

Elsevier B.V. All rights reserved.

162 chemical engineering research and design 1 0 2 ( 2 0 1 5 ) 161–170

Table 1 – Experiments of two-phase flow through breaks.

Authors Simulant Flow pattern D (mm)a d (mm)b Break structure Orientation angle

Reimann and Khan(1984)

Air–water Stratified flow 206 6, 12, 30 Branch −90◦

Smoglie andReimann (1986, 1987)

Air–water Stratified flow 206 6, 8, 12, 20 Branch ±90◦, 0◦

Yonomoto andTasaka (1991)

Air water Stratified flow 190(square duct) 10, 20 Branch ±90◦, 0◦

Maier et al. (2001) Air–water Stratified flow 255 6.35 Branch 0◦, 10◦, 30◦, 60◦, 90◦

Lee et al. (2007) Air–water Stratified flow 184 16, 24.8 Branch 0◦, ±30◦, ±45◦, ±60◦, ±90◦

Bartley et al. (2008) Air–water Stratified flow 104 6.35 Branch 0, ±30◦, ±60◦, ±90◦

Bowden and Hassan(2011)

Air–water Stratified flow 50.8 6.35 Branch 0◦, −45◦, −90◦

a Main pipe diameter.

b Branch diameter.

that can predict the discharged mass flow rate and qual-ity.

Many experiments have been performed on two-phase flowdischarging through a small break in recent years, includinganalysis of air–water or steam–water flows through brancheswith different orientations and diameters, as well as variousoperating conditions. Typical experimental investigations aresummarized in Table 1.

These publications mainly focused on the onset of gas orliquid entrainment, but the phase separation mechanism ofgas–liquid two-phase flow at the break had not been studiedthoroughly. However, as mentioned above, the knowledge ofthe phase separation phenomena involved is extremely vitalfor the break prediction model. Besides, the break was sim-ulated by a T-junction, which consists of a small diameterbranch attached to a main pipe with larger diameter or a con-tainer. The length of the branch is usually several times ofits diameter. As well known, the branch resistance is differentfrom that of the break, which may affect gas–liquid two-phaseflow discharge characteristics of the break. The branch diam-eters in previous experimental studies were all larger than6 mm. Experimental data of break smaller than 2.5 mm is notavailable yet. In addition, most of the previous experimentalstudies focused on steady stratified flow, which don’t reflectthe real gas–liquid two-phase pipe flow where the annular andslug flows are common flow patterns. Therefore, through lit-erature review, studies about annular or slug flow dischargeseem to be unavailable at this moment.

The objective of the present study is to experimentally andtheoretically investigate the phase separation characteristicsof two-phase flow discharge through a small break at the pipewall. A splitting device was specially designed for experimen-tal study and a 2.5 mm hole was used as the break. The phaseseparation influencing factors, such as the break location, flowpattern and gas/liquid superficial velocities were studied. Inaddition, a correlation was developed to describe the relation-ship among break pressure difference, break outlet quality andmass flow rate based on gas–liquid two-phase orifice equation.

2. Experimental setup

2.1. The structure of the small break splitting device

The schematic of the small hole splitting device in the presentstudy is shown in Fig. 1. The splitting device mainly con-

sists of two sections: Sections 1 and 2. The inner diameterand wall thickness of the two sections are 40.0 mm and

5.0 mm, respectively. The front of Section 1 and the end ofSection 2 are connected to a gas liquid two phase flow loop.A circular hole with a 2.5 mm diameter is set on the wall of Sec-tion 2 to simulate the small break. The hole is surrounded byan annular fluid receiving room. When gas–liquid two-phaseflow passes through the test section, the fluid through the holewill be collected in the fluid receiving room and then enter theside branch. The side branch is connected to a metering sepa-rator, where the gas–liquid mixture is separated and metered.The Rosemount pressure transducer and pressure differencetransducers were used to monitor the pressure and pressuredrop at the small hole.

Sections 1 and 2 are connected by flange 1 and flange 2.A packing plate is placed between the two flanges to preventleakage. Section 2 can rotate around its axis, which results inthe whole range of angles, −90◦ ≤ � ≤ 90◦, could be covered. Anangle indicator is applied to indicate the current location ofthe break. The orientation angle of the break is determinedby the plumb line at the dial plate. Three orientation angles,including 0◦, −45◦ and −90◦, were experimentally investigatedin this study.

2.2. Gas–liquid two-phase flow loop

The fractions of gas and liquid taken off are often used todescribe the phase splitting behaviour of gas–liquid two-phaseflow. The fractions of gas taken off, KG, and liquid taken off,KL, are defined by the following equations:

KG = M3G

M1G(1)

KL = M3L

M1L(2)

where M is mass flow rate in the main pipe 1, kg/s; KG and KL

represent the fraction of gas or liquid taken off; subscripts 1and 3 denote main pipe 1 and break 3, respectively; subscriptsG and L represent gas and liquid phases.

Once M1G, M1L, M3G and M3L were measured, the fraction ofgas and liquid taken off could be easily obtained according toEqs. (1) and (2). Experiments were carried out in an air–watertwo-phase flow loop in order to obtain the fractions of gas andliquid taken off.

Fig. 2 presents the schematic of the gas–liquid two-phaseflow loop, mainly consisting of an experimental splittingdevice, an outlet tank, an air–water metering separator, a

water circulation pump, an air compressor, a water storagetank and pipelines.

chemical engineering research and design 1 0 2 ( 2 0 1 5 ) 161–170 163

Fig. 1 – The schematic diagram of the small break splitting device.

(mamw

The gas mass flow rate (M1G) and liquid mass flow rateM1L) of the main pipe were measured before mixing. M1G was

etered by a Yokogawa DY040-type vortex flow meter withn accuracy of ±0.75%; while M1L was measured by a Coriolis

ass flow meter made by Emerson’s Micro Motion Company,hich has an accuracy of ±0.5% of the full range.

The splitting device was horizontally installed in the testsection. A short transparent section of the pipe, made of plex-iglass, was mounted at upstream of the test section for flowpattern observation. When air–water two-phase flow entered

into the test section, the fluid was divided into two parts. Themain stream flowed downstream of the test section, while the

164 chemical engineering research and design 1 0 2 ( 2 0 1 5 ) 161–170

Fig. 2 – Air–water two-phase flow loop. (1) Compressor; (2) DY040-type vortex gas meter gas flowmeter; (3) mixer; (4) liquidflowmeter; (5) flow pattern observation section; (6) small break splitting device; (7) branch control valve; (8) meteringseparator; (9) liquid level control valve; (10) inverted U-tube; (11) electronic balance; (12) DY025-type vortex gas meter; (13)

k; (16) pump.

Fig. 3 – Experimental data on a flow pattern map.

main pipe control vale; (14) cyclone separator; (15) water tan

other small portion of the fluid was discharged through thesmall break. The main stream then entered a cyclone sepa-rator located at the end of the loop, where the gas and liquidwere separated. The gas was vented into the atmosphere andthe water flowed back into the water tank for circulation afterbeing measured.

The two-phase fluid flowing through the small hole wascollected in the fluid receiving room and then went into themetering separator. The metering separator was a vertical per-spex cylinder, whose inner diameter and height were 70 mmand 350 mm, respectively. A Yokogawa DY025-type vortex flowmeter at the top of the separator was used for gas flow ratemetering, with an accuracy of ±0.75% in the full range of0–803 L/min. The liquid mass flow rate through the break M3L,was determined based on weighting–time method. The timefor collecting water was within 120–600 s, depending upon thevelocity of the water.

All flow meters, temperature transducers, pressure andpressure difference transducers were calibrated beforehand.Experimental data are recorded by National Instruments dataacquisition system.

2.3. The test matrix and experimental procedure

Experiments were performed under the conditions of roomtemperature and the outlet pressure of the break closeto atmospheric pressure. Seven gas superficial velocities,USG = 3.0 m/s, 6.0 m/s, 9.0 m/s, 11.0 m/s, 13.0 m/s, 17.0 m/s and21.0 m/s, and three liquid superficial velocities, USL = 0.05 m/s,0.10 m/s and 0.15 m/s were investigated. Three orientationangles, � = 0◦, −45◦ and −90◦, were tested. Fig. 3 presents atheoretical air–water flow pattern map in a horizontal pipe,according to Taitel and Dukler (1977). As shown in Fig. 3, theflow patterns of the current test matrix are predicted as strat-ified wavy, slug and annular flows.

The experimental procedures are as follows:

(1) The break location was adjusted to a desired orientation

angle. Orientation angles of 0◦, −45◦ and −90◦ were testedin the current experiments,

(2) Gas and liquid flow rates were adjusted to desired values.The flow patterns were recorded when the flow becomesteady,

(3) Different split ratio was obtained by using the main pipecontrol valve and the branch control valve together,

(4) The data of M1L, M1G, M3L, M3G, P1 and �P13 were measuredand recorded,

(5) The fractions of gas and liquid taken off were calculatedusing Eqs. (1) and (2). Then the splitting characteristiccurves of the break were plotted and analyzed.

3. Results and discussions

3.1. Effect of break location on phase splitting

Fig. 4(a–i) shows the effect of break location on phase splitting.All data was plotted as the fraction of the gas taken off versusthe fraction of liquid taken off. The equal splitting line in Fig. 4means that the gas and liquid are distributed at the same pro-portion. In other words, the quality of fluid extracted into thebreak is identical to that of the main stream and no phaseseparation occurs, namely KG = KL. From Fig. 4, it is found thatthe gas and liquid are not discharged evenly and severe phase

separation occurs.

chemical engineering research and design 1 0 2 ( 2 0 1 5 ) 161–170 165

locat

clOtflb

Fig. 4 – Effect of break

When � = −45◦ and � = −90◦, it is observed that the splittingurves are above the equal splitting line. It means that theiquid is prone to enter the break with � = −45◦ and � = −90◦.n the contrary, as to � = 0◦ break, the splitting curve is below

he equal splitting line, which means that the gas prefers to

ow through the break with � = 0◦ and the split is characterizedy gas predominantly taken off.

ion on phase splitting

These phenomena can be explained by the “region of influ-ence” theory, which has been proposed for T-junction split(Azzopardi and Whalley, 1982; Castiglia and Giardina, 2010).As shown in Fig. 5, the fluid extracted into the small breakcomes from the zone in the vicinity of the break, which is

called as “region of influence”. The influenced region can bedivided into two sub zones: one is gas influence region and

166 chemical engineering research and design 1 0 2 ( 2 0 1 5 ) 161–170

Fig. 5 – Region of influence.

the other is liquid influence region. hb is the elevation dif-ference between the bulk water level, where the onset of theliquid entrainment or gas pull-through begins. hb determinesthe area of influence region and can be calculated with thefollowing equation (Smoglie et al., 1987):

hb = K(�εA3)0.4

g0.2

(�P13

�L − �G

)(3)

where � is the flow coefficient and ε is gas expansioncoefficient. A3 is the break sectional area. g is the gravity accel-eration. �L, �G is the density of liquid and gas, respectively.�P13 is the pressure difference across the break. K is constantand can be determined from the experiments.

Eq. (3) indicates that the area of the influence region isdependent on the pressure difference. hb will increase withthe increase of �P13, which leads to an increase of flow influ-ence area thus more fluid will be discharged through the break.Quality in the flow through the side break is determined bythe void fraction distribution in the influence zone. When h islarger than hb, there is no entrainment and only single contin-uous phase is discharged through the break. When h equals tohb, onset of entrainment begins. As h becomes smaller than hb,two phase flow with gas or liquid entrainment is dischargedthrough the break.

When � = 0◦, the influence region of gas is much larger thanthat of liquid. As a result, more gas flows through the breakand phase splitting data are below the equal splitting line. Onthe contrary, for � = −90◦and � = −45◦, the gas influence regionis smaller compared with that of the liquid, which results inmore liquid entering into the break and the splitting curve willbe above the equal splitting line.

3.2. Effect of flow pattern on phase separation

As mentioned in Section 2, stratified wavy flow, slug flow andannular flow were observed in the experiments. An interest-ing phenomenon can be observed that the split characteristicsin slug flow are extraordinary different from that of stratifiedwavy and annular flow, which was illustrated in Fig. 6. Whenthe fraction of gas taken off remains the same, the fraction ofliquid taken off in slug flow is much larger than that of wavyflow and annular flows. This phenomenon is mainly due tothe particular flow behavior of slug flow.

Slug flow is one of the most complex flow patterns. Fig. 7shows schematically the characteristic of a stable slug flow ina horizontal pipe. A slug unit is classified into two sections: theliquid slug zone and the film zone. The film zone consists of

a liquid film and an elongated gas bubble, which is similar tostratified flow. The slug is a section of liquid entrained by small

bubbles. Alteration of liquid slug and long gas bubble causeshigh fluctuations of pressure in slug flow. Generally pressurein the film zone is much lower compared with that in the liquidslug zone.

Considering the special flow characteristic of slug flow,the break discharge process can be divided into two stages,i.e. the liquid film leak stage and liquid slug leak stage. Dur-ing the liquid film leak stage, the split characteristic is similarto that of wavy flow. Only a small fraction of gas and liquidcan enter into the break at this stage due to the lower pres-sure difference of the break. However, when the liquid slugapproaches the break, the local pressure increases rapidly,which causes the pressure difference across the break ismuch larger than that of the liquid film stage. The slug con-taining small bubbles will fill the whole pipe cross sectionand causes large quantity of liquid flow through the breakat this stage. It was observed during the experiments thatthe liquid which enters the break mainly comes from theliquid slug leak stage. That is why there are more liquid dis-charge into the break than that of annular flow and wavyflow.

3.3. Effect of gas velocity on phase splitting

The effect of gas velocity on phase split of break is also shownin Fig. 6. For � = 0◦ break, under annular flow and wavy flow, itis found that the fraction of liquid taken off increases withincreasing gas superficial velocity. The distribution of gasand liquid in the main test section contributes to this phe-nomenon.

When gas velocity is small, the flow pattern is wavy flowand the liquid tends to flow at the bottom of the pipe, shown inFig. 8. With the increase of the gas velocity, the liquid film willcreep up the side wall of pipe (Zhang et al., 2003). The flowpattern may transform into semi-annular flow and furtherinto uniform annular flow at higher gas velocity. As describedabove, the fluid entering into the break is mainly from the zoneof influence. The liquid influence zone area will become largerat a higher superficial gas velocity than that at a lower gassuperficial velocity. Therefore, more liquid will be extractedinto the break with the increasing of gas velocity.

Contrarily, for � = −45◦ and � = −90◦ break, when the gassuperficial velocity increases, the fraction of liquid taken offdecreases. The � = −45◦ and � = −90◦ break were covered by alayer of water. When the superficial gas velocity grows up,the local liquid film will become thinner, reducing the areaof liquid influence zone.

3.4. Critical fraction of liquid taken off

As shown in Fig. 9, a critical value of the fraction of liquid takenoff is observed for −90◦ break. It can be seen that the fraction ofgas taken off remains zero until the fraction of liquid taken offreaches the critical value. At a given liquid superficial velocity,the critical fraction of liquid taken off is affected by gas super-ficial velocity. This critical value decreases with increasing gassuperficial velocity.

This phenomenon is related to the gas entrainment.According to Eq. (3), when the break pressure difference is low,the area of influence region is small. The break is below thegas/liquid interface and it is difficult for the gas to penetratethe liquid layer and enter the break .So the break outlet is filled

by continuous single phase liquid (see Fig. 10(a)). As the pres-sure difference of the break increase, the area of influence

chemical engineering research and design 1 0 2 ( 2 0 1 5 ) 161–170 167

Fig. 6 – Effect of flow pattern on phase splitting.

Fig. 7 – Schematic diagram of slug flow.

rafcii

og

egion increases too. When the distance between the breaknd the gas–liquid interface, h, equals to hb, a thin tube of gasorms and reaches the vicinity of the break (see Fig. 10(b)). Thisritical point is called the onset of gas entrainment. Furtherncrease of the pressure difference will cause the gas phasentensively discharge through the break (see Fig. 10(c)).

The local liquid film thickness decreases with the increase

f gas superficial velocity, which makes it more easily for theas to penetrate the liquid film and enter into the break. Hence,

with the increase of gas superficial velocity, the critical fractionof liquid taken off reduces.

3.5. The relationship among pressure difference, massflow rate and gas quality

The break can be regarded as an orifice. Accordingly, the pres-

sure difference across the break can be calculated based ongas–liquid two-phase flow orifice equation. Here, the orifice

168 chemical engineering research and design 1 0 2 ( 2 0 1 5 ) 161–170

Fig. 8 – Effect of gas superficial velocity on leak.

tion

Combining Eqs. (4) and (5), we could obtain the correlation

Fig. 9 – Critical frac

equation proposed by Lin (1982) is used to calculate the pres-sure difference at the break.

(�P13

�P0

)1/2= � + X3

((�L�G

)1/2− �

)(4)

where �P13 is pressure drop of the break; � is modificationfactor determined by the gas–liquid density ratio; X3 is orificeoutlet quality; �G and �L are the densities of gas and liquid,respectively. �P0 represents the pressure drop across orificeassuming total flow to be liquid, and is a function of total two-

phase flow rate M3 at the same time. �P0 can be calculated bythe following equation.

of liquid taken off.

(�P0)1/2 =M3

(1 − ˇ4

)1/2

CdA3(2�L)1/2

(5)

where M3 is the mass flow rate of gas–liquid two phase flowthrough the break; A3, ˇ, �L, Cd, and represent the breakflow area, the break diameter to pipe diameter ratio, the liquiddensity, the discharge coefficient and the thermal correctionfactor, respectively.

for pressure difference across the break, �P13, in terms of massflow rate, M3, and break outlet quality, X3.

chemical engineering research and design 1 0 2 ( 2 0 1 5 ) 161–170 169

F

(

w

A

bbe

mdqbi

0.0 0.2 0. 4 0.6 0.8 1.00

20000

40000

60000

80000

100000

120000

140000

160000

θ=0°θ=−45°θ=− 90°

(Δp 1

3)^(1

/2)/

M3

X3

(a)Effect of break position on Eq.(8)

0.0 0.2 0.4 0.6 0.8 1.00

20000

40000

60000

80000

100000

120000

140000

θ=−90 ° Ann wa vy slug

Sqr

t(Δp13)/M3

X3

(b)Effect of flow pattern on Eq.(8) √

ig. 10 – Illustration of mechanisms for gas entrainment.

�P13)1/2 =M3

(1 − ˇ4

)1/2

( CdA3) × (2�L)1/2

(� + X3

((�L�G

)1/2− �

))(6)

Eq. (6) can be rewritten as follows:

(�P13)1/2

M3=

(1 − ˇ4

)1/2

( CdA3) × (2�L)1/2�

+(

(�L/�G)1/2 − �)

×(

1 − ˇ4)1/2

( CdA3) × (2�L)1/2

X3 (7)

Then, Eq. (7) could be simplified to:

(�P13)1/2

M3= (A + BX3) (8)

here

=(

1 − ˇ4)1/2

( CdA3) × (2�L)1/2, B =

(((�L/�G)1/2 − �

)×

(1 − ˇ4

)1/2)

(( CdA3) × (2�L)

1/2)

Eq. (8) indicates that there exists a linear relationshipetween

√�P13/M3 and X3. The value of A, B is functions of

reak size, gas and liquid density and can be determined fromxperiments.

In the current experimental study, �P13 and M3 wereeasured and X3 was obtained from M3G and M3L. Fig. 11

emonstrates a good linear relationship between the breakuality X3 and

√�P13/M3. A = 9516.5, B = 144283.2 are selected

ased on least squares fitting and the corresponding R-squares 0.98. From Fig. 11(a) we can also observe that for � = 0◦, −45◦,

Fig. 11 – The relationship between �P13/M3 and X3.

and −90◦ break, all the experimental data fall in the samestraight line. It means that Eq. (8) is valid for all the breakorientation angles and the break location has no effect on therelationship between X3 and

√�P13/M3. Fig. 11(b) shows the

effect of flow pattern on Eq. (8). It is also found that Eq. (8) isnot affected by inlet flow patterns.

In fact, Eq. (8) constructs a relationship among pressuredifference, mass flow rate and gas quality. Once coefficient Aand B are known, Eq. (8) can be used with other correlationsto predict the quality and flow rate discharged into the break.

4. Conclusion

An experimental investigation has been carried out to studythe phase separation of a gas–liquid two-phase mixture flow-ing through a small break at the pipe wall. According to theexperimental results, the following conclusions can be drawn:

(1) The gas and liquid through the break mainly comes fromthe influencing region. The break location has a greateffect on the phase separation. The liquid prefers to enterinto the � = −45◦ and � = −90◦ break. On the contrary, thegas tends to flow through the � = 0◦ break and the split ischaracterized by gas taken off predominantly.

(2) The splitting characteristic of slug flow is extraordinarily

different from that of annular flow and wavy flow. The frac-tion of liquid taken off under slug flow is much larger than

170 chemical engineering research and design 1 0 2 ( 2 0 1 5 ) 161–170

that of annular flow and wavy flow due to its special flowbehavior.

(3) When the break is below the gas–liquid interface, thereexists a critical fraction of liquid taken off. When criticalfraction of liquid taken off is reached, the gas above theinterface can be entrained due to the pressure drop at theinterface produced by the water acceleration in the vicinityof the break.

(4) A simplified correlation is proposed for break pressuredifference (�P13) prediction based on Lin’s Orifice equa-tion. The pressure difference (�P13) is expressed in termsof break outlet mass flow rate (M3) and gas quality (X3).Experimental results confirmed there exists a good linearrelationship between X3 and

√�P13/M3.

Acknowledgements

The authors express their great thanks to National NaturalScience Foundation of China (grant no. 51006123), ShandongProvince Natural Science Foundation (grant no. ZR2010EQ016)and Fundamental Research Funds for the Central Universities(grant no. 14CX05028A) for financial support. The authors alsoexpress their great thanks to Dr.Jianjun Zhu for his carefullanguage revision.

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