Gas–liquid two-phase flow splitting at microchannel junctions withdifferent branch angles

Jinfang Chen, Shuangfeng Wang n, Hongfeng Ke, Songheng Cai, Ying ZhaoKey Laboratory of Enhanced Heat Transfer and Energy Conservation of the Ministry of Education, School of Chemistry and Chemical Engineering,South China University of Technology, Guangzhou 510640, China

H I G H L I G H T S

� The effect of flow patterns on phase split at microchannel junctions was studied.� The effect of branch angles on phase split was clarified.� The effect of channel diameter on phase split was studied.

a r t i c l e i n f o

Article history:Received 6 April 2013Received in revised form27 September 2013Accepted 5 October 2013Available online 18 October 2013

Keywords:Multiphase flowPhase splitMicrochannel junctionsVisualizationMomentum transferInterface

a b s t r a c t

In the present work, the phase split occurring at microchannel junctions with the square cross-sectionof 0.5�0.5 mm2 was investigated experimentally under adiabatic condition. Microchannel junctionswith five different branch angles varying from 301 to 1501 were manufactured. Two different methods ofcontrolling gas/liquid division from the run/the branch were employed, suitable for acquiring splittingdata for a wide range of flow patterns covering slug flow, slug–annular flow and annular flow. The inletsuperficial velocities varied from 0.8 to 21.3 m/s for gas phase (nitrogen), and from 0.019 to 0.356 m/sfor liquid phase (pure water). The high speed recording technique was utilized to elucidate the fluiddynamics of two-phase flow splitting at microchannel junctions. Data analysis revealed that the phasesplitting curves did not simply take on a transitional characteristic when the inlet flow pattern changedfrom slug flow to annular flow. Furthermore, the liquid taken off from the branch did not decrease withincreasing branch angle for all kinds of inlet flow patterns. Finally, by comparing the present data withthose of mini- and macro-scale research, it was found that more liquid was taken off from the branch atmicrochannel junctions.

& 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Microchemical technology, due to advantages such as highefficiency, safety, flexibility and high degree of integration (Bayraktarand Pidugu, 2006; Tu et al., 2010), has been an attention-catchingsubject since it was brought forward. Meanwhile, scaling down thesystem greatly increases the specific surface area, and thereforeenhances the mass and heat transfer (Thulasidas et al., 1995). As aspecial microstructure that can be easily fabricated, microchanneljunctions with different branch angles (T-shape and Y-shape) arecommonly applied in precisely controlled microbubbles generation(Xu et al., 2006), microdroplet coalescences (Sarrazin et al., 2007),solvent microextractions (Choi et al., 2010), biochemical processes(Köhler and Kirner 2005), and mixings/reactions (Bothe et al., 2006;

Salman et al., 2007; Weinmueller et al., 2009). In addition to theabove applications for different inlet fluids merging into one, the useof microchannel junctions to divide one inlet two-phase flow intotwo outlet streams is also an important issue in cooling microelec-tronic circuits (Dang et al., 2007; Li et al., 2010; Cho et al., 2010),compact chemical plants (Chen et al., 2012), microbiological situa-tions (Ody et al., 2007), and other related fields. On the one hand,when a gas–liquid flow passes through branching conduits, unevendistribution of the phases will inevitably take place between theoutlets, which could cause downstream equipment to malfunction.On the other hand, the phase maldistribution feature can also bemade use of as a compact and inexpensive partial phase separation(Azzopardi et al., 2002; Wren and Azzopardi, 2004; Baker et al.,2007).

Up till now, extensive studies on phase maldistribution havebeen carried out in normal diameters larger than 5 mm whereinertial force and gravitational force dominated. According to theresearch in literature, phase split in branching conduits was

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Chemical Engineering Science

0009-2509/$ - see front matter & 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.ces.2013.10.013

n Corresponding author. Tel.: þ86 20 22236929.E-mail address: sfwang@scut.edu.cn (S. Wang).

Chemical Engineering Science 104 (2013) 881–890

primarily affected by the orientations of inlet and outlet sides(Saba and Lahey, 1984; Mak et al., 2006; Geraci et al., 2007), theflow patterns upstream of the junction (Azzopardi and Whalley,1982; Shoham et al., 1987), inlet phasic superficial velocities (Wrenet al., 2005), the diameters of inlet and outlet sides (Walters et al.,1998; Marti and Shoham, 1997), inlet pressure (Das et al., 2005)and branch angles (Hwang et al., 1988). However, there are nouniversal correlations or models capable of accurately predictingphase split for arbitrary branching conduits due to various geo-metric parameters as well as inlet flow conditions. Accordingly,more phase splitting data are urgently needed, particularly inmicrodiameter conduits where investigations are rarely seen.

In microscale system, gas–liquid two-phase flow behaves quitedifferent from that with diameters larger than 1 mm because of thedominant effect of surface tension and the diminished gravitationaleffect. The work of Serizawa et al. (2002) suggested that surfacetension dominated when channel diameter dr

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffis=gðρL�ρGÞ

p,

where s is the surface tension, g is the gravitational acceleration,and ρL and ρG represent the liquid and the gas densities, respectively.This gives, for example, a critical d of 0.82 mm for the air–liquid two-phase system. The greater effect of surface tension results in thedisappearance of a stratified flow pattern (Fukano and Kariyasaki,1993), the emerging of a slug–annular flow (Triplett et al., 1999), andthe disappearance of both the liquid entrainment in gas core and thebubbles captured in liquid film of annular flow. Furthermore, thesymmetrical distribution of film thickness along the periphery of thechannel in annular flow indicates that the gravity force is negligiblein microchannels, as pointed out by Feng and Serizawa (2000). Andthe diminished gravitational effect may greatly weaken the influenceof the orientation on phase split. As per the above analysis, it can befound that the flow behaviors in microchannels have changed in abig way on account of the scaling effect by diameter reduction.Consequently it is considered that there may be significant differ-ences for phase split feature of two-phase flow at microchanneljunctions.

According to literature review, only limited research on phasesplit of two-phase flow with diameter less than 1 mm could befound. And all of the studies were focused on the T-shapedjunctions. Azzi et al. (2010) firstly extended surveys to a T-junction with diameter of 1 mm at inlet of slug flow. They foundthat pressure had no influence on phase split, which was incon-sistent with the results at a 5 mm diameter T-junction by Das et al.(2005), who suggested that an increase in pressure led to adecrease of liquid taken off from the branch. Hong et al. (2010)

investigated the influence of the slug length on the flow splittingbehavior with the square cross-section of 0.6�0.6 mm2. Theyhighlighted that for the long slug, the flow split was not muchaffected by the superficial velocities of phases as reported byothers in larger size (mm-order) tubes. Wang et al. (2011) studiedthe phase split of nitrogen/non-Newtonian fluid two-phase flow ata horizontal square cross-section micro-T-junction with diameterof 0.5 mm. They noticed that the liquid taken off from the branchfor nitrogen gas/non-Newtonian liquid system was found to behigher than that for nitrogen gas/Newtonian liquid system. Theycontributed it to the fact that increasing the effective viscosityincreased the wall shear stress, which in turn decreased the liquidmomentum flux. He et al. (2011b) investigated the effect of surfacetension on phase split of nitrogen–water flow at a micro-T-junction with 0.5 mm diameter. When the inlet was annular flow,they found that the phase split was remarkably affected by surfacetension, which contributed positively to liquid taken off from thebranch.

Owing to the inherent complexity of the gas–liquid two-phaseflow, the phase split characteristics in microchannel branchingconduits are still in the infancy and exploration stage at present.Many works are still needed, such as the studies of the role ofbranch angles and specific two-phase flow behaviors in micro-channels. Therefore, the present work experimentally investigatesthe phase split that occurs at 0.5 mm diameter horizontal junc-tions with the aid of the high speed recording technique to makein-depth analysis on the fluid dynamics of two-phase distributionat microchannel junctions. Phase split of three commonly appear-ing flow patterns in microchannels (slug, slug–annular, andannular flows) is studied and the effect of branch angles evenlyvarying from 301 to 1501 is clarified. Finally, the experimentalresults are compared with data of previous studies to identify thechanges that occur as a result of the reduction in channel diameter.

2. Experiment

2.1. Experimental procedure

A schematic diagram of the experimental setup is shown inFig. 1. Feeding of nitrogen was adjusted by a mass flow controller(CMQ-V9500, Yamatake Corporation, Japan) coming from a pres-surized gas tank, and water was driven by a syringe pump togenerate two-phase flow through a mixer. A pressure transducer

Fig. 1. Schematic diagram of the experimental setup.

J. Chen et al. / Chemical Engineering Science 104 (2013) 881–890882

was inserted at the inlet of the test section and the pressure wasmaintained at 12075 kPa in all cases. Each test section (T-shapeand Y-shape) was fabricated by the laser technique in a 3 mmthick transparent polymethyl methacrylate (PMMA) substrate forvisualization. Each arm of the test section with the square cross-section of 0.5�0.5 mm2 was 50 mm long to assure fully developedflow before splitting at junctions. A blank cover plate was bondedto the substrate according to the method introduced by Hsieh et al.(2004). The Y-shaped test sections had branch angels of 301 and601, which could be reversed to also give branch angles of 1201and 1501. The flow behaviors at and around the junction wereviewed with a high-speed video camera (American Fastec corpora-tion, model Troubleshooter HR) set at 500 ftp with a shutter speedof 1/10,000 and a resolution of 640(H)�480(V). Leaving the testsection, each of the two outlet streams flowed through anadjustable-length flexible tube with diameter of 1 mm into a50 ml separator. The liquid mass flow rates off the separatorswere decided by using electronic balances (12070.0001 g,BS124S, Sartorius, German) to weigh the discharging liquid col-lected over a measured period of time. The gas flow rates wereeither adjusted using mass flow controllers or measured by bubblemeters (Chen et al., 2012) under different inlet flow conditions,which would be further interpreted in Section 2.3.

2.2. Inlet conditions and flow patterns observed

The apparatus was operated under the following conditions:inlet gas superficial velocities, USG, ranging from 0.8 to 23 m/s andinlet liquid superficial velocities, USL, ranging from 0.019to 0.356 m/s. All tests were carried out at ambient pressureand temperature of 20 1C. Seven cases (Table 1) were selected inthree different regions based on the flow pattern map presentedby Chung and Kawaji (2004), as illustrated in Fig. 2. The flowpattern map was generated from experiments conducted with anitrogen–water two-phase flow in a circular channel of 0.53 mmdiameter which is close to the test section in this work. Visualinvestigations were carried out at the inlet of the junction to verifythe inlet flow patterns. Slug flow is constructed mainly by liquidslugs and gas slugs flowing alternatively. Slug–annular flow, aunique flow pattern in microchannels, behaves as alternativeappearances of different sub-flow patterns. For annular flow, thegas flows along the center of the channel and the liquid flows as asymmetrical liquid film on the wall of the channel. As to bubblyflow and churn flow, none of our selected data falls into thesecategories.

The following calculations consider the fluid to be laminar flow.At inlet of slug flow, the slug velocity is given by the Bendiksen(1984) equation

Uslug ¼ 1:2ðUSGþUSLÞþ0:54ffiffiffiffiffiffigd

pð1Þ

For inlet of annular flow, the velocities of gas and liquid aredetermined with the following formulas:

UG ¼USG=ε ð2Þ

UL ¼ USL= 1�εð Þ ð3Þwhere ε is the void fraction, which is decided based on the annularflow void fraction model for a rectangular channel with diameterof 1.02 mm, developed by Niño (2002). Then the Reynolds num-bers (Re¼ρUd/μ) of gas and liquid are estimated to be 50–800 and60–700, respectively, thus the flow was confirmed to be laminarflow in the experiment.

2.3. Definition of phase split

The division of gas/liquid flows at a dividing junction relieson the outlet pressure, the resistances in the run and the branch,as well as the physical phenomena which affect the phase split.In practical applications, these resistances are attributed to theequipment downstream of the junction. In experimental studieshere, two combined methods were employed according to differ-ent inlet flow patterns, to study the phase split with gas/liquidflows separated out through the branch from low taken off to hightaken off. For inlet of slug–annular flow and annular flow, thedivision of the gas flow was conducted by synchronously adjustingthe gas mass flow rates of the run and the branch using two MFCs(CMQ-V9500, Yamatake Corporation, Japan). At the beginning, thepressure in each separator was about atmospheric pressure, andthe gas taken off from the branch/the run was not equal to thevalue set by the MFC. Therefore, the system pressure was changedwith time and the liquid mass flow rates of the branch and the runalso varied. However, since both slug–annular flow and annularflow were formed at high gas volumetric flow rate, the pressure inthe branch/the run was easy to reach balance, which could bejudged from the steady increase of the liquid mass flow ratesmeasured by the electronic balances. Then the gas taken off fromthe branch/the run was the same as the setting by the MFC. Andthe liquid mass flow rate of the branch/the run could be deter-mined by measuring the mass of liquid collected over a measuredperiod of time. Therefore, for each gas taken off, we could get aliquid taken off. This controlling method was also adopted bymany researchers (Hong, 1978; Chien and Rubel, 1992; Wang et al.,2011). Nevertheless, as for inlet of slug flow formed at low gasvolumetric flow rate, the above-mentioned controlling methodwas not adequate again. It was attributed to the fact that thesystem pressure was hard to reach balance at low gas volumetricflow rate. However, because of the high total mass flow rate andthe pressure loss induced by capillary force (Chen et al., 2012), thepressure drop per unit length is relatively high for slug flow. So it

Table 1Gas and liquid superficial velocities and the corresponding flow patterns.

Case USG (m/s) USL (m/s) Flow pattern

1 0.8 0.356 Slug flow2 2.0 0.356 Slug flow3 4.667 0.189 Slug–annular flow4 7.333 0.104 Slug–annular flow5 13.33 0.053 Slug–annular flow6 23 0.033 Annular flow7 23 0.019 Annular flow

Fig. 2. Inlet flow conditions plotted in flow pattern map of Chung and Kawaji(2004), “Line” by Chung and Kawaji (2004) and “ ” presents experimental data.

J. Chen et al. / Chemical Engineering Science 104 (2013) 881–890 883

was easy to change gas/liquid taken off through the branch/therun by adjusting the length ratio of the flexible tubes connectingthe exit of the run/the branch to the inlet of the respectiveseparators. Then the liquid flow rates off the separators weredetermined by two electronic balances and the gas flow rates were

measured by two bubble meters based on the time taken for thebubbles to move up at a set distance. So for each length ratio, wecould get one pair of gas taken off and liquid taken off. Addition-ally, to confirmwhether different controlling methods affect phasesplit or not, we selected the experimental data of slug–annularflow (Case 3 in Fig. 2) to carry out the phase split with bothmethods. It is because slug–annular flow is a transitional flow thattwo controlling methods are both applicable. Based on the experi-mental results, negligible difference on the phase split was observed,indicating that different resistance controlling methods downstreamhave no effect on the phase split at junctions.

The data measured for phase split were presented in terms ofthe fractions of gas and liquid taken off through the branch. Theywere defined as the following formulas:

fraction of gas taken off ¼ gas mass flow rate of the branchgas mass flow rate of inlet

ð4Þ

fraction of liquid taken off ¼ liquid mass flow rate of the branchliquid mass flow rate of inlet

ð5ÞAll the possible errors in the experiment were taken into

consideration carefully. The uncertainties of inlet liquid mass flowrate controlled by syringe pump was within 71% and inlet gasflow rate controlled by MFC was within 71%. The reproducibilityof data was tested thrice under the identical inlet conditions, andFig. 3. Phase split data for 901 junction.

Fig. 4. Time trace of liquid phase before and after the junction for each sub-flow pattern, fraction of gas taken off¼0.5.

J. Chen et al. / Chemical Engineering Science 104 (2013) 881–890884

the data measured with difference exceeding 72% were dis-carded. For each inlet condition, mass balances between the inletand outlet were checked. For liquid, the balances fell within 71%,and for gas, the balances were kept within 72%.

3. Results and discussion

3.1. Effect of inlet flow patterns on phase split

Fig. 3 displays the phase split results of nitrogen/water two-phase flow at 901 T-junction. The abscissa and the ordinate are thefractions of gas and liquid taken off through the branch, respec-tively. The diagonal line represents that gas and liquid are equallysplit, above which is the liquid rich zone while the zone below isrich in gas. The figure shows that the phase split characteristics arehighly dependent on inlet flow patterns.

When the inlet is slug flow (Cases 1 and 2), most of theexperimental data are located in the gas rich zone, except thosefalling in the liquid rich zone at low taken off. It can be analyzedfrom two aspects as follows. Firstly, the liquid separated out fromthe branch mostly comes from the liquid slug, and the gas is fromthe gas slug. The liquid slug and the gas slug are traveling at thesame velocity. Accordingly, the liquid has a higher momentum flux(ρu2) than the gas, thus it is much more difficult for the liquid slugto change direction and exit through the branch compared to thegas slug. Secondly, the liquid film clinging to the channel wall hassmall kinetic energy due to viscous shear. So the liquid filmapproaching the branch is unsupported by the wall and preferen-tially separated out than the gas in the case of low taken off.Particularly, when the gas superficial velocity USG increases from0.8 m/s to 2.0 m/s leaving the liquid superficial velocity USL

constant, the growing length of gas slug makes an increasingproportion of liquid traveling as liquid film on the channel wall,resulting in moving up of the phase split curve at low taken off.

Contrary to the slug flow, the data points of annular flow (Cases6 and 7) basically lie in the liquid rich zone, indicating that thereis a preference for the liquid phase to exit through the branch.

For annular flow, liquid is present in the form of liquid filmbetween the gas core and channel wall. The liquid existing closeto the branch will be preferentially separated out upon losingsupport of the channel wall. Moreover, it is obvious that with USL

decreasing from 0.033 m/s to 0.019 m/s at fixed USG, the phase splitcurve is getting farther away from the equal split line. Thisindicates that the gas entering the branch will tend to drag theliquid film with it into the branch as well. Because the inertialforce of the liquid decreases with the decrease of USL, more liquidis diverted into the branch in the form of secondary flow and lessliquid flows straight through the junction.

As mentioned above, slug–annular flow (Cases 4–6) liesbetween slug flow and annular flow in the flow pattern map,suggesting that the phase split curves of slug–annular flow maytake on a transitional characteristic. However, in reality, it does notbehave like that as expected. As firstly observed by Kawahara et al.(2002), the slug–annular flow is more complex behaving as thesimultaneous occurrence of different sub-flow patterns in thechannel. In the current study, four kinds of sub-flow patterns arecategorized as “liquid slug” flow, “liquid lump” flow, “liquid ring”flow and “liquid film” flow based on the distribution manner ofthe liquid in the channel. As a side note, “liquid lump” flow ischaracterized by the ratio of liquid length to channel diameter lessthan 3. Fig. 4 depicts the time trace of liquid phase before and afterthe junction for each sub-flow pattern. It can be seen that thedistribution ratios of liquid phase of the run and the branch arechanged with different sub-flow patterns. For “liquid slug” flowin Fig. 4A1 and A2, the split of liquid phase takes on a similarcharacteristic to that of slug flow as discussed above. To be specific,the coming liquid slug at junction splits into two daughter liquidslugs, and the liquid slug in the run is longer than that in thebranch. However, in the case of “liquid lump” flow as illustratedin Fig. 4B1 and B2, a significant proportion of liquid phase flowsdirectly through the junction without splitting into two daughterliquid lumps, and the rest flow into the branch in the form ofliquid film. The phenomena may be explained by the facts that thetime of the existence of the liquid phase at the entrance to thebranch is insufficient for the liquid lump to flow into the branch.

Fig. 5. Probability of appearance of different sub-flow patterns of slug–annular flow for Cases 3–5.

J. Chen et al. / Chemical Engineering Science 104 (2013) 881–890 885

Fig. 4C1 and C2 documents a liquid ring flowing through thejunction. It can be seen that almost no liquid is dragged into thebranch and the liquid ring travels along the run. When “liquidfilm” flow approaches the junction as shown in Fig. 4D, film stophappens due to the increase of pressure along the run caused bythe deceleration of the mainstream. Then as a result of phasic slip,the gas turning to the side arm produces a centripetal force andthus draws partial liquid phase into the side arm, namely second-ary flow. From the analysis above, the liquid taken off through thebranch of different sub-flow patterns is sorted in descending orderas: “liquid film” flow4“liquid slug” flow4“liquid lump” flow4“-liquid ring” flow. Besides, only under “liquid film” flow conditioncan the liquid phase preferentially flow into the branch whilemore liquid tends to the run for the other sub-flow patterns.Therefore, for better understanding the phase split of slug–annularflow, the probability of appearance of different sub-flow patternsneeds to be well studied.

The probability of appearance of different sub-flow patterns isobtained for Cases 3–5 by counting the number of images contain-ing each flow type from 2000 picture sequences captured at500 ftp, as typically shown in Fig. 5. When the inlet conditionshifts from slug flow (Case 2) to slug–annular flow (Case 3), thephase split curve moves down as shown in Fig. 3. There aretwo reasons for this. Firstly, some of the liquid slugs convert toliquid lumps and liquid rings resulting from the increase of gas

superficial and decrease of liquid superficial. Secondly, comparedwith slug flow (Case 2), in slug–annular flow (Case 3) much moremomentum of gas is transferred to the liquid, resulting in a higherliquid velocity. Then the time of the existence of the liquid slug atthe entrance of the branch gets shorter, and thus more liquidtravels straight along the run. When the inlet condition transformsto Case 4, the probability of the appearance of “liquid slug” reducesby half and much more liquid exists in the form of “liquid lump”flow, reaching a probability of 6.5%. As discussed before, the higherappearing probability the liquid lump is, the smaller liquid takenoff in the branch. Therefore, the phase split curve gets farther awayfrom the equal split line, as seen in Fig. 3. However, when the inletcondition changes from Case 4 to Case 5, the occurrence of “liquidfilm” flow increases and reaches over 90% while the other threesub-flow patterns decreases, particularly the liquid slug flow isalmost not observed. A higher proportion of liquid is prone to beseparated out from the branch when it comes to liquid film flow.As a result, the phase split curve moves up towards the equal splitline. Continuing the transition to annular flow, more and moreliquid appear in the form of liquid film, thus the phase split curvekeeps moving up, shown as Cases 5 and 6 in Fig. 3.

Since the phase split results of the other junctions of 301, 601,1201 and 1501 branch angles take on a similar characteristic to thatof 901 junction with changing inlet condition from Case 1 to Case7, they are not presented here.

Fig. 6. Effect of branch angle on phase split at inlet of slug flow: (A) Case 1 and(B) Case 2.

Fig. 7. Effect of branch angle on phase split at inlet of annular flow: (A) Case 6 and(B) Case 7.

J. Chen et al. / Chemical Engineering Science 104 (2013) 881–890886

3.2. Effect of branch angle on phase split

The effect of branch angle on phase split was investigated byincreasing it from 301 to 1501 for different inlet flow patterns.Fig. 6A and B depicts the effect of branch angle on phase split atinlet of slug flow (Cases 1 and 2). The figure shows that with theincrease of branch angle from 301 to 1501, the phase split curvemoves down gradually, indicating that less and less liquid wasseparated out from the branch for a set gas taken off. These datatrends are reasonable if we recall that at slug flow the liquid phasehas a higher momentum flux (ρu2) than the gas phase and thus itis harder for it to change direction and exit through the branch,which is particularly true for large branch angle. But the effect ofbranch angle on phase split for low taken off region (o0.1) isrelatively small. This can be well explained by the fact that in thisregion the liquid taken off from the branch is mostly from thatclinging to the channel wall, which has a very low momentum flux(ρu2). Therefore, in low taken off region (o0.1), the fraction ofliquid separated out from the branch has little relationship tobranch angles.

However, when the inlet flow pattern shifts to annular flow, theeffect of branch angle on phase split differs from that of slug flow.Fig. 7A exhibits that at the same gas taken off, the liquid separatedout from the branch of 901 junction is the maximum, followedby 601, 301, 1201, and 1501 orderly. As aforementioned, in the caseof annular flow, film stop happens opposite to the branch side. Inaddition to the liquid existing close to the branch side, the otherliquid is separated out from the branch in the form of secondaryflow, as depicted in Fig. 8. In general, the bigger the branch angleis, the less liquid is taken off from the branch as a result of theinertia effect. However, as illustrated in Fig. 8, the streamlines ofsecondary flow from the film stop to the branch are quite differentfor varied branch angles. 901 Junction has a shortest flow line ofd¼0.5 mm, followed by 601 and 1201 junctions of ð2=

ffiffiffi3

pÞd, and the

longest flow line for 301 and 1501 junctions are both 2d. The longerthe flow line is, the greater frictional resistance needs to beovercome, and thus the less obvious the secondary flow is. Since

901 junction has the strongest secondary flow, the amount of theliquid dragged into the branch is the largest. Moreover, the phasesplit data of 601 and 1201 junctions fall above those of 301 and 1501junctions, as shown in Fig. 7B. It can be analyzed from two aspects.Firstly, the effect of liquid inertia diminishes when the inlet liquidflow rate decreases (Case 7). Secondly, due to the shorter flow line,the secondary flow of 601 and 1201 junctions is stronger than thatof 301 and 1501 junctions. Additionally, although under the sameflow line of 601 and 1201 junctions, the liquid taken off from thebranch of 601 junction is still more than that of 1201 junctionowing to the inertia effect. And the same analytical approach isapplicable to the comparison of 301 and 1501 junctions.

Fig. 9 displays the influence of branch angle on phase split atinlet of slug–annular flow. Overall, the liquid separated out fromthe branch decreases as the branch angle gets larger. As the inletcondition shifts from Case 3 to Case 5, the liquid separated outfrom the branch of 901 junction firstly exceeds that of 601 junction,as shown in Fig. 9A and B, and eventually takes the first place inFig. 9C. This is because at Case 3, most of the liquid exists in theform of liquid slug, and the inertia force is dominant. But when theinlet condition moves towards annular flow (Case 5), the “liquidfilm” flow is predominant as already indicated. In this case, theeffect of inertia force weakens while secondary flow tends to playa more significant role. As discussed above, 901 junction has thestrongest secondary flow, so its phase split data fall above those of301 and 601 junctions at Case 5.

3.3. Comparison with previous data

In order to examine the effect of pipe diameter on phase split,the current experimental results have been compared with thepublished data in larger diameter pipes at the most similar inletflow conditions. Fig. 10 illustrates that the current phase split datashow similar tendencies with those by Wren et al. (2005) andArirachakaran (1990), at inlet of slug flow. Most of the data pointslocate in the gas rich zone, and at low taken off the phasemaldistribution is small while at high taken off there is a large

Fig. 8. Captured images of film stop at junctions with different branch angles and the corresponding sketches of secondary flow.

J. Chen et al. / Chemical Engineering Science 104 (2013) 881–890 887

phase maldistribution. However, the degree of phase maldistribu-tion is different for various diameter pipes. At micro-T-junction,the liquid taken off from the branch is larger than that at mini- ormacro-scale T-junction. On the one hand, the liquid taken off ofslug flow at the junction with square section is slightly higher than

that at the circular junction discussed by He et al. (2011a). On theother hand it may be mainly contributed to the special internalrecirculation within the liquid slugs in microchannel whichpromotes the radial mixing of fluids, as shown in Fig. 11, leadingto a greater proportion of liquid separated out from the branchonce the liquid slugs lose the support of the channel wall atT-junction.

Data for flow rates similar to those used here are available forpipe diameters of 9.5 mm (Hong, 1978), and 51 mm (Shoham et al.,1987) to study the effect of pipe diameter at inlet of annular flow.Fig. 12 illustrates data with a gas superficial velocity of �24 m/sand liquid superficial velocities of �0.033 and �0.018 m/s. Aspresented by He et al. (2011a), there is a lower liquid taken off in asquare section junction than that in a junction with circularsection at annular flow. However, Fig. 12 shows that more liquid

Fig. 9. Effect of branch angle on phase split at inlet of slug–annular flow: (A) Case 3;(B) Case 4; and (C) Case 5.

Fig. 10. Comparison between present data (0.5 mm diameter square T-junction),Wren et al. (2005) (5 mm diameter circular T-junction) and Arirachakaran (1990)(50 mm diameter circular T-junction), at inlet of slug flow.

Fig. 11. Sketch of internal recirculation within the liquid slug.

Fig. 12. Comparison between present data (0.5 mm diameter square T-junction),Hong (1978) (9.5 mm diameter circular T-junction) and Shoham et al. (1987)(51 mm diameter circular T-junction), at inlet of annular flow.

J. Chen et al. / Chemical Engineering Science 104 (2013) 881–890888

is split to the branch with the smaller T-junction at a set gas takenoff, with a square cross-section being used here. At macro-scaleT-junction of 51 mm, the liquid phase at inlet of annular flow canbe divided into the liquid film where the momentum flux is low(high density and low velocity) and liquid drops entrained in gasflow with high momentum flux (high density and high velocity).The momentum fluxes of the two items are approximately in theratio of 1:2500 (Azzopardi and Whalley, 1982). In this case mostof the liquid film is diverted from the branch whereas the liquiddrops are expected to carry straight on. Hence, the phase splitcurves of macro-scale T-junction locate below those of smallersized pipes. As for the mini- and microscale T-junction, the liquidfilm Reynolds number stays below the critical value for liquidentrainment, and little amount of the entrained liquid flowsthrough the run along with the gas stream. With the reductionin channel diameter from 9.5 mm to 0.5 mm, We numberdecreases from 16 to 0.3 for USL of �0.033 case and from 5 to0.1 for USL of �0.019 case by the following calculation:

We¼ ρLU2Ld

sð6Þ

where ρ is the density, s is the surface tension, and subscript “L”represents the liquid phase. It indicates that surface force over-whelms liquid inertia force at micro-T-junction, and will affectthe phase split. Considering the curved arc interface opposite tothe branch side, the Laplace pressure across the interface is infurtherance of the secondary flow, which facilitates the liquid atthe film stop to be dragged into the branch, as illustrated in Fig. 13.Therefore, more liquid is taken off at micro-T-junction than that atmini- T-junction at similar flow condition for inlet annular flow.

4. Conclusions

An experimental investigation has been conducted to studygas–liquid two-phase flow splitting at microchannel junctionswith different branch angles. Test range covers slug, slug–annularand annular flows. With experimental data presented and visua-lization analyzed, the significance of the experimental results canbe emphasized as follows:

(1) Data analysis shows that the phase split at microchanneljunctions is very dependent on inlet flow patterns. When theinlet is slug flow, gas phase preferentially flows into the branchwhile at inlet of annular flow more liquid is diverted into thebranch, which is consistent with the previous study of mini-and macro-size junctions. However, for slug–annular flow,its phase split curve does not simply take on a transitional

characteristic. Specifically, it gets farther away from the equalsplit line at first but moves up towards the liquid rich zonegradually.

(2) The liquid taken off from the branch does not decrease withincreasing of branch angle for all kinds of inlet flow patterns.At inlet of slug flow, due to liquid inertia, as the branch anglesincrease from 301 to 1501, less and less liquid is separated outfrom the branch. Nevertheless, at inlet of annular flow, theliquid separated out from the branch of 901 junction has themaximum value owing to its strongest secondary flow. As toslug–annular flow, the effect of branch angle displays atransitional characteristic since the liquid inertia becomes lessand less effective while the effect of secondary flow graduallystrengthens.

(3) Comparing the present data with those of mini- and macro-scale research, it is found that more liquid is taken off from thebranch at microchannel junctions at inlet of slug flow andannular flow. It indicates that the phase maldistribution atmicrochannel junctions is smaller for slug flow but gets worseat inlet of annular flow compared with larger diameterjunctions.

Acknowledgments

The authors gratefully acknowledge the financial support forthis project from the National Natural Science Foundation of China(Grant no. 51276068) and International Cooperation and ExchangeProgram from the Ministry of Science and Technology of China(Grant no. 2013DFG60080).

References

Arirachakaran, S., 1990. Two-Phase Slug Flow Splitting Phenomenon at a RegularHorizontal Side-Arm Tee (Ph.D. thesis). University of Tulsa.

Azzi, A., Al-Attiyah, A., Liu, Q., Cheema, W., Azzopardi, B.J., 2010. Gas–liquid two-phase flow division at a micro-T-junction. Chem. Eng. Sci. 65, 3986–3993.

Azzopardi, B.J., Whalley, P.B., 1982. The effect of flow patterns on two-phase flow ina T junction. Int. J. Multiph. Flow 8, 491–507.

Azzopardi, B.J., Colman, D.A., Nicholson, D., 2002. Plant application of a T-junctionas a partial phase separator. Chem. Eng. Res. Des. 80, 87–96.

Baker, G., Clark, W.W., Azzopardi, B.J., Wilson, J.A., 2007. Controlling the phaseseparation of gas–liquid flows at horizontal T-junctions. AIChE J. 53, 1908–1915.

Bayraktar, T., Pidugu, S.B., 2006. Review: characterization of liquid flows inmicrofluidic systems. Int. J. Heat Mass Transfer 49, 815–824.

Bendiksen, K., 1984. An experimental investigation of the motion of long bubbles ininclined tubes. Int. J. Multiph. Flow 10, 467–483.

Bothe, D., Stemich, C., Warnecke, H.J., 2006. Fluid mixing in a T-shaped micro-mixer. Chem. Eng. Sci. 61, 2950–2958.

Chen, J.F., Wang, S.F., Cheng, S., 2012. Experimental investigation of two-phasedistribution in parallel micro-T channels under adiabatic condition. Chem. Eng.Sci. 84, 706–717.

Chien, S.F., Rubel, M.T., 1992. Phase splitting of wet steam in annular flow through ahorizontal impacting tee. SPE Prod. Eng. 7, 368–374.

Cho, E.S., Choi, J.W., Yoon, J.S., Kim, M.S., 2010. Experimental study on microchannelheat sinks considering mass flow distribution with non-uniform heat fluxconditions. Int. J. Heat Mass Transfer 53, 2159–2168.

Choi, Y.H., Song, Y.S., Kim, D.H., 2010. Droplet-based microextraction in the aqueoustwo-phase system. J. Chromatogr. A 1217, 3723–3728.

Chung, P.M.-Y., Kawaji, M., 2004. The effect of channel diameter on adiabatic two-phase flow characteristics in microchannels. Int. J. Multiph. Flow 30, 735–761.

Dang, M., Hassan, I., Muwanga, R., 2007. Adiabatic two phase flow distribution andvisualization in scaled microchannel heat sinks. Exp. Fluids 43, 873–885.

Das, G., Das, P.K., Azzopardi, B.J., 2005. The split of stratified gas–liquid flow at asmall diameter T-junction. Int. J. Multiph. Flow 31, 514–528.

Feng, Z.P., Serizawa, A., 2000. Measurement of steam–water bubbly flow in ultra-small capillary tube. In: Proceedings of the 37th National Heat TransferSymposium of Japan. Japan.

Fukano, T., Kariyasaki, A., 1993. Characteristics of gas–liquid two-phase flow in acapillary tube. Nucl. Eng. Des. 141, 59–68.

Geraci, G., Azzopardi, B.J., van Maanen, H.R.E., 2007. Effect of inclination oncircumferential film thickness variation in annular gas/liquid flow. Chem.Eng. Sci. 62, 3032–3042.

Fig. 13. Analysis of the Laplace pressure in furtherance of secondary flow at micro-T-junction.

J. Chen et al. / Chemical Engineering Science 104 (2013) 881–890 889

He, K., Wang, S.F., Huang, J.Z., 2011a. The effect of flow pattern on split of two-phaseflow through a micro-T-junction. Int. J. Heat Mass Transfer 54, 3587–3593.

He, K., Wang, S.F., Huang, J.Z., 2011b. The effect of surface tension on phasedistribution of two-phase flow in a micro-T-junction. Chem. Eng. Sci. 66,3962–3968.

Hong, K.C., 1978. Two-phase flow splitting at a pipe tee. J. Pet. Technol. 30, 290–296.Hong, J.H., Lee, S.Y., Lim, J.S., 2010. Two-phase plug flow distribution at a meso-scale

T-junction-effect of plug bubble length. In: Proceedings of the 7th InternationalConference on Multiphase Flow. USA.

Hsieh, S.S., Lin, C.Y., Huang, C.F., Tsai, H.H., 2004. Liquid flow in a micro-channel.J. Micromech. Microeng. 14, 436–445.

Hwang, S.T., Soliman, H.M., Lahey, R.T., 1988. Phase separation in dividing two-phase flows. Int. J. Multiph. Flow 14, 439–458.

Kawahara, A., Chung, P.M.-Y., Kawaji, M., 2002. Investigation of two-phase flowpattern, void fraction and pressure drop in a microchannel. Int. J. Multiph. Flow28, 1411–1435.

Köhler, J.M., Kirner, T., 2005. Nanoliter segment formation in micro fluid devices forchemical and biological micro serial flow processes in dependence on flow rateand viscosity. Sensors Actuators A: Phys. 119, 19–27.

Li, J., Wang, S.F., Cai, W., Zhang, W.J., 2010. Numerical study on air-side performanceof an integrated fin and micro-channel heat exchanger. Appl. Therm. Eng. 30,2738–2745.

Mak, C.Y., Omebere-Iyari, N.K., Azzopardi, B.J., 2006. The split of vertical two-phaseflow at a small diameter T-junction. Chem. Eng. Sci. 61, 6261–6272.

Marti, S., Shoham, O., 1997. A unified model for stratified-wavy two-phase flowsplitting at a reduced T-junction with an inclined branch arm. Int. J. Multiph.Flow 23, 725–748.

Niño, V.G., 2002. Characterization of Two-Phase Flow in Microchannels (Ph.D.thesis). University of Illinois.

Ody, C.P., Baroud, C.N., de Langre, E., 2007. Transport of wetting liquid plugs inbifurcating microfluidic channels. J. Colloid Interface Sci. 308, 231–238.

Saba, N., Lahey, R.T., 1984. The analysis of phase separation phenomena inbranching conduits. Int. J. Multiph. Flow 10, 1–20.

Salman, W., Gavriilidis, A., Angeli, P., 2007. Axial mass transfer in Taylor flowthrough circular microchannels. AIChE J. 53, 1413–1428.

Sarrazin, F., Prat, L., Di Miceli, N., Cristobal, G., Link, D.R., Weitz, D.A., 2007. Mixingcharacterization inside microdroplets engineered on a micro-coalescer. Chem.Eng. Sci. 62, 1042–1048.

Serizawa, A., Feng, Z.P., Kawara, Z., 2002. Two-phase flow in microchannels. Exp.Therm. Fluid Sci. 26, 703–714.

Shoham, O., Brill, P., Taitel, Y., 1987. Two-phase flow splitting in a tee junctionexperiment and modelling. Chem. Eng. Sci. 42, 2667–2676.

Thulasidas, T.C., Abraham, M.A., Cerro, R.L., 1995. Bubble-train flow in capillaries ofcircular and square cross section. Chem. Eng. Sci. 50, 183–199.

Triplett, K.A., Ghiaasiaan, S.M., Abdel-Khalik, S.I., Sadowski, D.L., 1999. Gas–liquidtwo-phase flow in microchannels: part I. Two-phase flow pattern. Int. J.Multiph. Flow 25, 377–394.

Tu, S.T., Yu, X., Luan, W., Löwe, H., 2010. Development of microchemical, biologicaland thermal systems in China: a review. Chem. Eng. J. 163, 165–179.

Walters, L.C., Soliman, H.M., Sims, G.E., 1998. Two-phase pressure drop and phasedistribution at reduced tee junctions. Int. J. Multiph. Flow 24, 775–792.

Wang, S.F., Huang, J.Z., He, K., Chen, J.F., 2011. Phase split of nitrogen/non-Newtonian fluid two-phase flow at a micro-T-junction. Int. J. Multiph. Flow37, 1129–1134.

Weinmueller, C., Hotz, N., Mueller, A., Poulikakos, D., 2009. On two-phase flowpatterns and transition criteria in aqueous methanol and CO2 mixtures inadiabatic, rectangular microchannels. Int. J. Multiph. Flow 35, 760–772.

Wren, E., Azzopardi, B.J., 2004. The phase separation capabilities of two T-junctionsplaced in series. Chem. Eng. Res. Des. 82, 364–371.

Wren, E., Baker, G., Azzopardi, B.J., Jones, R., 2005. Slug flow in small diameter pipesand T-junctions. Exp. Therm. Fluid Sci. 29, 893–899.

Xu, J.H., Li, S., Chen, G.G., Luo, G.S., 2006. Formation of monodisperse micro-bubblesin a microfluidic device. AIChE J. 52, 2254–2259.

J. Chen et al. / Chemical Engineering Science 104 (2013) 881–890890