Two-phase Flow in Microchannels

Two-phase flow in microchannels

Akimi Serizawa *, Ziping Feng, Zensaku Kawara

Department of Nuclear Engineering, Kyoto University, Yoshida, Sakyo, Kyoto 606-8501, Japan

Accepted 9 November 2001

Abstract

Gas–liquid two-phase flow patterns are visualized with a microscope for air–water flow in circular tubes of 20, 25 and 100 lm i.d.

and for steam–water flow in a 50 lm i.d. circular tube. The superficial velocities cover a broad range of JL ¼ 0:003–17.52 m/s and

JG ¼ 0:0012–295.3 m/s for air–water flows. Several distinctive flow patterns, namely, dispersed bubbly flow, gas slug flow, liquid ring

flow, liquid lump flow, annular flow, frothy or wispy annular flow, rivulet flow, liquid droplets flow and a special type of flow pattern

are identified both in air–water and steam–water systems, and their special features are described. It has been confirmed that two-

phase flow patterns are sensitive to the surface conditions of the inner wall of the test tube. It has been evidenced that a stable

annular flow and gas slug formation with partially stable thin liquid film formed between the tube wall and gas slugs appeared at

high velocities under carefully treated clean surface conditions. At lower velocities, dry and wet areas exist between gas slug and the

tube wall. The cross-sectional average void fraction was also calculated from photographs, showing a good agreement with the

Armand correlation for air–water flow in lager tubes.

Published by Elsevier Science Inc.

Keywords: Ultrasmall tube; Two-phase flow; Flow pattern; Visualization

1. Introduction

Two-phase flow in microchannels has recently at-tracted people’s concerns because of its wide applica-bility to modern and advanced science and technologiessuch as micro-electro-mechanical systems, electroniccooling, chemical process engineering, medical and ge-netic engineering, bioengineering and etc. For instance,The Research Committee on Heat Transfer and FluidFlow in Microchannel organized by the JSME recentlypublished a collection of papers in this area (Serizawa[1]). The knowledge of flow and heat transfer in micro-scale flow passages of the size less than 100 micronis thus strongly demanded. Specifically fundamentalknowledge of two-phase flow and its mechanisms insmall (of sub-mm order) or ultrasmall (of 1 lm order)flow passage, such as flow pattern, void fraction, pres-sure drop, liquid film thickness etc. are crucial for en-gineering design purposes as well as for evaluation ofpractical performance. Recent papers by Ghiaasiaan

and Abedel-Khalik [2] and Serizawa and Feng [3] ex-tensively reviewed the literatures on two-phase flow andheat transfer in microchannels. The related topics arealso included in conference proceedings (for example,International Conference on Heat Transfer and Trans-port Phenomena in Microscale, 2000). However ourcurrent knowledge is still quite limited and in realityonly a small number of literatures are available so farwhich deal with two-phase flow and heat transfer in suchvery small tubes (see Table 1, which lists some of theprevious studies on two-phase flow characteristics inmicrochannels).

One of our questions is whether or not two-phaseflow patterns in microchannels are different from thoseencountered in ordinarily sized tubes. In ordinarily sizedlarge tubes as well as in a few mm order microtubes,two-phase flow patterns are dominated in general bygravity with less surface tension effects. On the otherhand, in microchannels of the order of a few lm � to afew tens lm, two-phase flow is believed to be influencedmainly by surface tension, viscosity and inertia forces.However, no one knows yet in detail what two-phaseflow patterns are realized in such small tubes. The cri-terion for microchannel have been proposed by several

Experimental Thermal and Fluid Science 26 (2002) 703–714

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E-mail address: [email protected] (A. Serizawa).

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authors in different ways, but roughly given by theLaplace constant k or the Etovos number Eo, where

k ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

rgðqL � qGÞ

rð1Þ

Eo ¼ gDqD2

rð2Þ

Suo and Griffith [4] derived the following criterion fortube diameter D,

Confined number k=DP 3:3 ð3Þ

Here, the Laplace constant k calculates 2.7 mm for air–water flow at 0.1 Mpa. Another criterion was proposedby Brauner and Moalem-Maron [5] as given below.

Eo6 ð2pÞ2 ð4ÞHowever, two-phase flow patterns in a microchannelare not so simple to be identified by these equations.Moreover, the effects of surface roughness and wetta-bility (contamination) are our concerns.

Nomenclature

D hydraulic diameterEo Etovos numberg acceleration due to gravityj superficial velocityP pressureQ power input to preheaterR bubble radiusa cross-sectional average void fractionb gas volumetric flow ratio

k Laplace constantq densityDq density difference (¼ qL � qG)r surface tension

Subscripts

G gas phaseL liquid phase

Table 1

Some examples of previous works with microchannels

Investigator Hydraulic diameter

D (mm)

Confined no.

k=DOrientation Fluid Test

mode

Marchessault and Mason [17] 1–3 0.91–2.5 I Air/water, air/aqueous glycerol A

Taylor [18] 1.5–3 0.75–1.51 H Air/glycerine A

Suo and Griffith [4] 1–2 1.3–2.7 H Air/water, Heptane/He, A

Heptane/N2

Lazarek and Black [19] 3.1 0.33 V Vapor/R–113 D

Barnea et al. [20] 4 0.68 V Air/water A

Biwsas and Greenfield [21] 0.5–7.1 V (D) Air/water A

Fukano et al. [22] 1–4.9 H Air/water A

Kariyasaki et al. [11] 1, 2.4, 4.9 H Air/water A

Lin et al. [23] 0.66, 1.17 H Vapor/R12 D

Wambsganss et al. [24] 2.92 0.35 H Vapor/R–113 D

Barajas and Panton [10] 1.6 1.7 H Air/water A

Ide et al. [25] 0.5–6 V Air/water A

Fukano and Kariyasaki [13] 1, 2.4, 4.9 V(U&D),H Air/water A

Bao et al. [26] 0.7–3 0.91–3.9 H, V Air/water, air/glycerine A

Air/kerosene

Tran et al. [27] 2.46 0.44 H Vapor/R–12, vapor/R–134a D

Kew [28] 1.4–3.7 0.68–1.8 H Steam/water, vapor/R–141b D

Mudwar and Bowers [29] 0.4–2.5 1.0–6.3 V Steam/water D

Wattelet [30] 2.92 0.35 H Vapor/R–113 D

Umekawa et al. [31] 3, 4 0.63, 0.84 V Steam/water D

Inoue et al. [32] 0:209� 0:212 H N2/water (glycerine sol.) A

Kariyasaki et al. [33,34] 1, 2.4, 4.9 V(U&D) Air/water A

Mishima and Hibiki [14] 1–4 0.68–2.7 V Air/water A

Kew and Cornwell [35] 1.4–3.7 0.34–0.89 H R–141b D

Kureta et al. [36] 2–6 V Steam/water D

Triplett et al. [15] 1.09–1.49 H Air/water A

Lin et al. [37] 0.5–4 V Air/water A

Kuwahara et al. [38] 1.2 H HFC134a D

Feng and Serizawa [39] 0.025 H Air/water A

Feng and Serizawa [40] 0.05 H Stream/water D

(A: adiabatic, D: diabatic, H: horizontal, V: vertical, U: upward flow, D: downward flow).

704 A. Serizawa et al. / Experimental Thermal and Fluid Science 26 (2002) 703–714

With these backgrounds held in mind, we studiedexperimentally by visualization the two-phase flowpatterns in air–water two-phase flows in round tubesmainly of 20, 25 and 100 lm i.d. and in steam–watertwo-phase flow in a 50 lm round tube. The purpose ofthe present study is therefore to provide database ontwo-phase flow patterns and some characteristic featuresof the flow structures in microchannels of the order of afew tens lm to 100 lm. In this report, typical features oftwo-phase flows observed in ultrasmall channels aredescribed.

2. Experimental apparatus

Schematic diagrams of the test facility for air–waterand steam–water experiments are shown in Figs. 1 and2, respectively. The test section for air–water experi-ments consists of a transparent silica or quartz capillarytube with circular cross-section and positioned hori-zontally. The tube inner diameters we tested are 10, 20,25, 30, 50 and 100 lm. The whole length of the tuberanges from 10 to 14 mm in which 8–10 mm is visible,depending on the different designs of the mixing zoneleading to the test section. This paper deals with theresults obtained only with the test section of 20, 25 and100 lm in inner diameter.

A high-pressure gas bottle that also provides thepressure head for the water tank supplies the air. Aswater is driven with high-pressure air, no pump is used inthe present experiment to avoid pulsation by a pump andalso to avoid contamination by a pump. Prior to exper-imental runs the test section was cleaned either bydrawing ethanol through the test section or by beingtreated with the combination of mechanical cleaningwith a soft brush and ultrasonic vibration in a pool ofhigh purity distilled water, ethanol and dilute hydro-chloride acid. The two-phase flow was realized through

a mixer with different designs as typically shown in Figs.1 and 3. The air is injected into the mixer co-axially whilewater is introduced peripherally. The inlet pressure ofthe test section is varied to attain desired flow conditionsand was measured with a precise pressure gauge con-nected to the mixer, while the outlet pressure is roughlyequal to atmospheric pressure. In order to measure thetwo-phase flow rates, a precise injection syringes is in-stalled at the outlet of the test section through an injectorhead. This syringe can collect the two-phase flow comingfrom the test section and accurately reads out the volu-metric flow rates of both air and water phases. Becausethe volumetric flow rate is extremely small in the casessuch as dispersed bubbly flow, more than 24 hours arewaited to get enough fluid volume.

The visualization of the flow pattern was realizedthrough a precise microscope (NIKON, SMZ-U type),which can magnify the image up to 150 times as that ofthe original size. A high-speed camera system (FAST-CAM-Rabbit, CANON) with recording speed of 30–600frames per second and shutting speed of 1/30 to 1/10 000s was mounted together with microscope. The light beamused for the visualization is provided through an ad-justable light source under the test section. Two-phaseflow patterns were visualized mainly at the 7000–9000lm downstream part from the inlet of the test section.The steam–water test loop shown in Fig. 2 includes apre-heater and the test section. The pure water is em-ployed as working fluid and is pumped by high-pressureair. The pre-heater is a 370 lm i.d. stainless steel tubewith outer diameter of 630 lm. The heating power isprovided by an adjustable DC power source. The testsection is a 50 lm fused silicon capillary tube with360 lm o.d. and 10 000 lm length. A precise injectionsyringe was connected to the outlet of the test section tocollect liquid so as to measure the time averaged volumeflow rate. The visualization is realized through a SMZ-Utype (NIKON) microscope together with a high-speed

Fig. 1. A schematic diagram of the experimental facility for air–water flows.

A. Serizawa et al. / Experimental Thermal and Fluid Science 26 (2002) 703–714 705

video camera. A shutter speed of 1/10 000s and a re-cording frame rate of 600 fps were employed during thewhole test runs. The mean liquid velocity is maintained

at around 24.7 mm/s and the inlet pressure is 2.4 barthrough the whole test. The visualization is conducted at6000–8000 lm positions from the inlet of the test section.

Fig. 2. A schematic of steam–water experiment in a 50 lm circular tube.

Fig. 3. Mixing chamber (Type III).


3. Visual observation of two-phase flow patterns

Visual observations of two-phase flow patterns werecarried out mainly with Type I (20 and 25 lm i.d.) andType III test sections (100 lm) as shown in Fig. 3, whichare cleaned with ethanol alone, unless otherwise stated.The inlet pressure was varied at around 2.4 bars tocontrol flow conditions, whereas the exit pressure wasnearly atmospheric. The flow conditions we tested coverJL ranging from 0.0032–17.5 m/s, and JG ranging from0.0022–295.3 m/s. Two-phase flows in microchannels arerather unstable in general and the flow pattern tends tochange with time from one to another even at a givenset of time-averaged values of JG and JL. This unstableflow character may depend on the volume of the mixingzone [6].

3.1. Air–water two-phase flow in a 25 lm silica tube

Fig. 4 shows typical two-phase flow patterns observedin an air–water flow in a 25 lm silica tube at nearlyatmospheric pressure with Type I test section. For two-phase flow identification, we did not correct for refrac-tion of light in image construction. The bright bands

along the centerline of the tube in Fig. 4 are due torefraction in the silica tube.

3.1.1. Dispersed bubbly flowDispersed bubbles are observed when the gas flow

rate is very small such as JG ¼ 0:0083 m/s in case of air–water flow as well as steam–water flow. Two kinds ofbubbles are observed. The one is finely dispersed bub-bles with size smaller than the tube diameter. Anotherkind of bubble has a size of near to or a little larger thanthe tube diameter with spherical cap and tail, but thedistance between two consecutive bubbles is much longin many cases but not always, for example, longer thanten times of the tube diameter. This flow pattern isalso considered as a dispersed bubbly flow. It is veryoften in air–water flow that two kinds of bubbles ap-pear together as pairs of bubbles in which the small-sized bubbles follow the larger ones. The motion ofsmall bubbles is likely to be strongly influenced by thelarger bubble. The dispersed bubbles observed in thepresent experiments are always maintaining a sphericalshape, except those of the size equals to or a little largerthan the tube diameter. This is mainly due to the highinterfacial pressure difference and the strong rigidity of

Fig. 4. Two–phase flow patterns (Type I).


microsized bubble, which can be estimated by the well-known Laplace–Young equation

PG � PL ¼ 2rR

ð5Þ

Applying this equation to a 5 lm bubble we can findthat the pressure difference at bubble interface reachesalmost to 0.3 bar. This pressure difference is sufficientto maintain a bubble with high spherical shape and toprevent the bubble shape from being distorted. Thus,the coalescence between two moving bubbles is hardly tobe observed in such a small tube.

3.1.2. Slug flowThe rigidity of bubble is high in ultrasmall tubes, and

bubbles always keep spherical shape so that the coales-cence loses its base. This induces a totally differentmechanism from that in ordinary size tubes for thedispersed bubbly flow transiting into the slug flow.From the experimental observation it is quite clear thatthe occurrence of the slug flow is rather an entrancephenomenon than inducing from the tube inside. Slugflow occurs only if the gas volume flow rate is higher atthe tube entrance and the speed of long gas bubble is nothigh enough to overcome the strong surface tensionforce of the liquid bridge between them.

The pressure drop induced by slug flow is very high.This implies that the sliding between gas slug and thetube wall is suppressed and therefore a dry zone mayhave been developed underneath the gas slug due to astrong influence of surface tension. The surface tensionforce keeps liquid phase to a slug structure and preventsit from being dispersed as film. On the other hand, theultrasmall size of the gas slug will result in a highpressure on the gas–liquid interface. This pressure dif-ference pushes the gas slug to occupy the whole space ofthe tube cross-section, and it is thus difficult for theliquid film to exist underneath the gas slug. Becauseof the same reason, slug coalescence is also seldomobserved in the present experiment. This issue will bediscussed later.

3.1.3. Liquid ring flowFig. 4(c) is the typical liquid ring flow structure where

the liquid film on the wall is symmetrically distributed.From the experimental observation it is evident that theliquid ring flow transited from the slug flow when thegas velocity is high. Under low gas velocity conditions,the liquid ring firstly appears in the middle of a long gasslug. It seems that this liquid ring originates from aliquid bridge separating two consecutive gas slugs.When the gas velocity is high, many symmetrical liquidrings will appear on the tube wall with almost equaldistance. The liquid ring flow can develop from slug flowpattern when the gas flow rate increases to such an ex-tent that the liquid slug is too short to support a stable

liquid bridge between two consecutive gas slugs. In otherwords, when the gas flow rate is sufficiently high, thelength of the gas slug is increased while that of the liquidslug is decreased. At a certain gas flow rate, the fol-lowing gas slug will penetrate a liquid slug and thereforea liquid ring is constructed. The liquid ring flow hasnever been observed in conventional tubes under normalgravity conditions, however, is quite often with thepresent tube size. It is interesting to note that Rezkallah[7] observed under microgravity conditions frothy slug-annular which is similar to the present liquid ring flowbut it contains small liquid droplets in the gas phase.Triplett et al. [8] reported their observation of churnflow and slug–annular flow patterns in the 1.09 mm-hydraulic diameter semi-triangular test section with air–water, which clearly showed liquid ring type interfacedeformation with a few bubbles in the liquid film. In thepresent experiment, we did not identify any small liquiddroplets in the gas core, nor small bubbles in the liquidfilm.

Because of its peculiar shape, the force balance be-tween the viscosity force from the wall and the dragforce from the gas core determines the motion of theliquid ring. When the former is dominant, the liquid ringwill stick on the wall and grow up. This growth is sup-posed to be a result of liquid supply from liquid filmin both upstream and downstream regions. When theheight of the liquid ring approaches to a certain value,the drag force from the gas core becomes dominant andthe liquid ring moves to the downstream.

3.1.4. Liquid lump flowIf we further increase the gas flow rate in liquid ring

flow, a liquid lump flow, of which the high-speed coregas entrains the liquid phase and liquid lumps are slidingon the wall, will be developed, as shown in Fig. 4(d). Theshape of liquid lump is very similar to that of the wavyflow in a horizontal large tube. The liquid lump isshifting from side to side. When a liquid lump contactsthe tube wall, the strong surface tension force will pre-vent it from spreading into the liquid film.

3.2. Steam–water flow in a 50 lm silica tube

In steam–water experiment, we used a 10 mm long 50lm i.d. silica capillary tube for the test section connectedto a stainless steel tube that is heated by dc current.Pressuring the water reservoir circulates the pure waterinto the test section. The measurements were conductedat a low liquid velocity JL of 2.47 cm/s by changing heatinput to the pre-heater. Fig. 5 shows a typical exampleof two-phase flow pattern observation carried out insteam–water two-phase flow in a 50 lm silica tube.General trends observed are quite similar to those in air–water flow in a 25 lm tube. An exception was an ob-servation of liquid droplets flow. However, this is not an


essential difference between the two cases, since thereason we did not observe the liquid droplet flow in air–water flow is merely a problem of experimental condi-tions we tested.

3.3. Air–water two-phase flow in a 100 lm quartz tube

As demonstrated in Fig. 6, the result indicates thattwo-phase flow patterns observed in a 100 lm quartz

Fig. 5. Two-phase flow patterns in steam–water flow.

Fig. 6. Two-phase flow patterns in a 100 lm quartz tube.


tube (Type III) are almost similar to those observed in a25 lm silica capillary tube with several exceptions. Oneof such exceptions is that in slug flow encountered at lowvelocities, small liquid droplets in a gas slug are stickingon the tube wall (Fig. 7). This fact is evidence that noliquid film exists between the gas slug and the tube wall.This particular phenomenon was reported earlier byGeng et al. [9] in slug bubble oscillation experiment in a1-mm diameter vertical tube for pure water–air. Sec-ondly, we observed rather stable liquid ring flow withthin liquid film distributed uniformly at the tube wall.The growth of a liquid ring is supported by liquid supplyfrom surrounding liquid film. Thirdly, in liquid lumpflow, the liquid lumps are running in rather partiallycontinuous films than in separate discrete liquid lumpsjust like rivulets. At smaller liquid flow rates, partiallycontinuous liquid film flow changes to the rivulets oreven to large liquid droplets or discrete liquid lumps.The formation of multi-rivulets is reported to depend onthe contamination of the tube surface [10]. At higher gasvelocity, the liquid droplet flow was clearly identified.

One of our questions is whether the inner wall of thetest tube is wet or dry during the passage of a gas slug in

slug flow pattern. We checked this problem using a high-speed video and a high precision laser confocal dis-placement meter with accuracy of 0.4 lm in thickness.Fig. 8 shows a moving boundary between wet and dryareas on the tube wall during a passage of gas slugs. A

Fig. 7. Liquid droplets sticking onto the tube wall in a gas slug.

Fig. 8. Wet–dry boundary at the tube wall.

Fig. 9. Two-phase flow pattern transition.


detailed inspection of the signals from the laser confocaldisplacement meter suggests the existence of dry areaunderneath a gas slug as well. Kariyasaki et al. [11]measured the film thickness surrounding the large gasslugs in the tube diameter range 1–4.9 mm in air–water.They observed the existence of the liquid film betweenthe gas slug and the tube wall. Their results indicate thatthe film thickness decreases along the gas slug length.The difference between the present observation andKariyasaki’s observation may be attributed to the scaleeffects and surface wettability.

4. Flow pattern transition map

Fig. 9 shows a flow pattern map obtained for air–water two-phase flow in a 20 lm i.d. silica tube (Type I)at nearly atmospheric pressure. It is interesting to notein this figure that, if we ignore the difference betweenliquid ring flow and liquid lump flow both of which areseparated flows, the flow pattern transitions generallyfollow the lines predicted by the correlation of Mandh-ane et al. [12], although stratified flow and wavy flow arenot the case in the present study with negligible effect ofgravity. This result does not agree with the observationmade by Fukano and Kariyasaki [13] for air–water intubes of 1, 2.4 and 4.9 mm i.d.

5. Void fraction

Void fractions in microchannels have been measuredby Kariyasaki et al. [11], Mishima and Hibiki [14] andTriplett et al. [15] for air–water. Mishima and Hibikicorrelated their data as well as the data of Kariyasakiet al. and proposed a drift flux type correlation forbubbly and slug flows. Kariyasaki et al. used a constantcurrent method to measure void fraction in tubes of 1,2.4 and 4.9 mm i.d. They proposed the following cor-relations depending on the flow conditions in terms ofthe gas volumetric ratio b.

Here bA and bB are experimentally determined constantswhich depend on the tube diameter.

In the present work, the cross-sectional averaged voidfraction was calculated from high-speed video pictures,by assuming symmetrical shape of bubbles and gas

slugs, and thus the void fraction presented here concernsonly with bubbly flow and slug flow. Liquid ring flow isaxi-symmetrical, but not included in this figure. Resultsare demonstrated in Fig. 10. Although the present datascatter to some extent in nature, the present data arecorrelated with the Armand Correlation (Armand andTreschev [16]) for air–water flow. This trend does notcontradict to the data of Kariyasaki et al. [11] men-tioned above.

6. Effect of surface wettability

Two-phase flows in microchannels are more or lessaffected by surface tension force, and therefore bothsurface roughness and wettability between the tube walland the fluid are supposed to affect the two-phase flowpatterns or two-phase flow structures. In fact, Barajasand Panton [10] studied experimentally the effect ofwettability on two-phase flow patterns and their tran-sitions. Their results indicated that the general trends ofthe flow pattern transitions are not so significantly af-fected by the surface contamination, that is, the wetta-

bility, except for the fact that the rivulet/multi-rivuletflows are sensitive to the wettability effect.

In the present study, in order to examine how two-phase flow patterns in ultrasmall channels are sensitiveto surface contamination, we observed visually the flow

For b < bA: a ¼ bFor bB < b < 0:6ðintermittent flowÞ: a ¼ 0:833bFor bB < b; 0:6 < b < 0:95ðintermittent flowÞ: a ¼ 0:69b þ 0:0858For bB < b; 0:95 < bðannular and intermittent flowÞ: a ¼ 0:83 logð1� bÞ þ 0:633

Fig. 10. Cross-sectional average void fraction in air–water two-phase

flow in a 20 lm i.d. silica tube (Type I).


patterns formation using a 100 lm i.d quartz tube testsection (Type III) carefully cleaned by ultrasonic vi-bration in pools of high purity distilled water, ethanoland dilute hydrochloride acid solution. This cleaningprocedure is as follows:

(1) Mechanical cleaning with a soft brush.(2) After treating with high purity distilled water, the

test tube was cleaned by ultrasonic vibration inethanol (1 h).

(3) Place the tube in high purity hydrochloride solution(35 wt.%) for 1 h. Then, the ultrasonic vibration wasapplied for 5 min.

(4) Ultrasonic vibration applied in distilled water for10 min.

(5) The test tube was then kept in ethanol for 1 h.(6) Finally, the test tube was blown by filtered air and

installed to the experimental apparatus for measure-ments.

Fig. 11, showing very interesting various aspects oftwo-phase flow patterns, represents the results of ourobservation. It should be noticed from these picturesthat a variety of two-phase flow patterns were encoun-tered in a clean microchannel.

By comparing Figs. 6 and 11, we may notice that in avery clean tube, many small individual bubbles areflowing in a discrete way in the tube without coalescencein bubbly flow. The most interesting thing is the specialflow pattern given in Fig. 11(d) where several bubbleswith various shapes are connected in a series by the gasstems locating at the tube centreline like skewed bar-becue (Japanese Yakitori). The liquid ring flow was sobeautifully realized.

It is thus obvious that the tube wall is much easier tobecome wet under such conditions, and thus stable an-nular flow was realized. Even more, frothy or wispyannular flow was also observed where small bubbleswere trapped within the thin liquid film flow on the tubewall. This is because the water film is spreading and thefilm swallows up small bubbles without breaking it dueto improved wettability. However, when the flow wasvery low, we observed a formation of dry area betweengas slug and the tube wall similarly in contaminatedtubes. Fig. 12 shows bubbles with various shapes float-ing in stagnant water in the 100 lm i.d quartz tubecleaned with ethanol blowing through the tube. Theinterface of the bubble on the left side of the picture isdeformed at the upper wall surface due to the existenceof a liquid droplet there with no liquid film surroundingthe bubble. It has been thus confirmed that the two-phase flow structures in ultrasmall channel is more se-riously affected by the wettability between the tube andthe fluids, and thus the surface contamination is a keyparameter which dominates two-phase flow patterntransitions.

7. Conclusions

Air–water and steam–water two-phase flow patternsare visualized in a 20, 25 and 100 lm and in a 50 lmi.d. ultrasmall tube, respectively, through a microscope.More than several different flow patterns are observed,namely, dispersed bubbly flow, gas slug flow, liquid ring

Fig. 11. Air–water two-phase flow patterns in a 100 lm i.d. clean

quartz tube (Type III) treated with ultrasonic vibration in distilled

water, in ethanol and in dilute hydrochloride acid solution.

Fig. 12. Various shapes of bubbles floating in stagnant water in a 100

lm i.d. quartz tube cleaned only with ethanol.


flow, liquid lump flow, skewed barbecue (Yakitori)shaped flow, annular flow, frothy or wispy annular flow,rivulet flow and liquid droplets flow are identified bothin air–water and steam–water systems, and their detailedfeatures are described.

We also studied experimentally the effect of surfacecontamination and the wettability between the tube walland the fluids. It has been evidenced that a stable an-nular flow and gas slug formation with stable thin liquidfilm formed between the tube wall and gas slugs ap-peared at high velocities under carefully treated cleansurface conditions. However, at lower velocities, dry andwet areas exist between gas slug and the tube wall.

Two-phase flow pattern transition map was con-structed and compared with the Mandhane’s correla-tion. Although two-phase structures in microchannelswith a few tens lm to a few 100 lm i.d are supposed tobe essentially different in many points from those oftwo-phase flows encountered in ordinary scale tubes,general trends in the present case follow the Mandhane’sprediction.

The cross-sectional average void fraction was alsocalculated from photographs, showing a good agree-ment with the Armand correlation for lager tubes.

Although the present work has clarified several inter-esting findings about two-phase flow phenomena in mi-crochannels, we are still at the first stage of research in thisarea. More detailed database on two-phase structuresand characteristics should be accumulated for future.

Acknowledgements

The authors acknowledge the assistance from Mr. T.Fukami (Master Course Student at Department ofNuclear Engineering, Kyoto University) in calculatingvoid fractions.

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Two-phase Flow in Microchannels

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