experimental thermal and fluid...

Experimental Thermal and Fluid Science 34 (2010) 1375–1388

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Experimental Thermal and Fluid Science

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Bubble confinement in flow boiling of FC-72 in a ‘‘rectangular” microchannelof high aspect ratio

Jacqueline Barber a,b,*, David Brutin b, Khellil Sefiane a, Lounes Tadrist b

a School of Engineering, University of Edinburgh, The King’s Buildings, Mayfield Road, Edinburgh, EH9 3JL, UKb Aix-Marseille Université (UI, UII) – CNRS Laboratoire IUSTI, UMR 6595, 5 Rue Enrico Fermi, Marseille 13453, France

a r t i c l e i n f o

Article history:Received 20 February 2010Received in revised form 8 June 2010Accepted 12 June 2010

Keywords:Confined bubbleFlow boilingExperimentalHigh aspect ratio microchannelTwo-phase flow instabilities

0894-1777/$ - see front matter � 2010 Elsevier Inc. Adoi:10.1016/j.expthermflusci.2010.06.011

* Corresponding author at: Aix-Marseille UniversitéIUSTI, UMR 6595, 5 Rue Enrico Fermi, Marseille 1345

E-mail address: [email protected] (J. Ba

a b s t r a c t

Boiling in microchannels remains elusive due to the lack of full understanding of the mechanismsinvolved. A powerful tool in achieving better comprehension of the mechanisms is detailed imagingand analysis of the two-phase flow at a fundamental level. Boiling is induced in a single microchannelgeometry (hydraulic diameter 727 lm), using a refrigerant FC-72, to investigate the effect of channel con-finement on bubble growth. A transparent, metallic, conductive deposit has been developed on the exte-rior of the rectangular microchannel, allowing simultaneous uniform heating and visualisation to beachieved. The data presented in this paper is for a particular case with a uniform heat flux applied tothe microchannel and inlet liquid mass flowrate held constant. In conjunction with obtaining high-speedimages and videos, sensitive pressure sensors are used to record the pressure drop across the microchan-nel over time. Bubble nucleation and growth, as well as periodic slug flow, are observed in the microchan-nel test section. The periodic pressure fluctuations evidenced across the microchannel are caused by thebubble dynamics and instances of vapour blockage during confined bubble growth in the channel. Thevariation of the aspect ratio and the interface velocities of the growing vapour slug over time, are allobserved and analysed. We follow visually the nucleation and subsequent both ‘free’ and ‘confined’growth of a vapour bubble during flow boiling of FC-72 in a microchannel, from analysis of our results,images and video sequences with the corresponding pressure data obtained.

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1. Introduction

Microchannels show great aptitude in cooling applications dueto their ability to dissipate high heat fluxes. They can be used asmicro-cooling elements for laptop computer chips, electronic com-ponents [1] and aerospace avionics components, and in the designof compact evaporators and heat exchangers [2,3]; in these situa-tions their compact size and heat transfer capabilities are unparal-leled. Channels in heat transfer applications have been gettingincreasingly smaller in dimensions due to their enhanced heattransfer performance. However, this heat transfer performance isaccompanied by high pressure drops per unit length. The balanceof these two factors is vital when designing a microchannel coolingsystem for electronics components etc. Flow boiling in microchan-nels is even more attractive than single-phase flow, due to an in-creased heat transfer coefficient, and even greater heat removalcapability for a given mass flowrate of coolant, due to the latentheat involved in flow boiling flows being generally greater than

ll rights reserved.

(UI, UII) – CNRS Laboratoire3, France.rber).

specific heat capacities for single-phase flows. Boiling flows requireless pumping power than single-phase liquid flows to achieve a gi-ven heat removal. It has been reported by various researchers thatthe heat transfer process and hydrodynamics occurring in micro-channels are distinctly different than that in macroscale flows[2,4–7]. This implies that only some of the available macroscaleknowledge can be applied to the microscale, and hence newknowledge is required to solve microscale heat transfer. For exam-ple, the condition that triggers the critical heat flux (CHF) in largerchannels is postulated to be when the liquid film vanishes (dry-out) at the heated channel wall [8]. This is believed to be similaras the cause of CHF in microchannels, where the onset of dry-outcauses CHF. It is thought that the controlling heat transfer mecha-nism in microchannels is the evaporation of the thin liquid filmaround the bubbles inside microchannels [9,10]. There are severalgeneral literature reviews on flow boiling heat transfer in micro-channel geometry [11,12].

The development and the progression of a liquid–vapour inter-face through a microchannel have all been well documented, butthe mechanisms defining these observations are still unclear.Physical phenomena such as bubble confinement and thin filmevaporation have been recorded by researchers, and subsequently

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Nomenclature

a height length scale in aspect ratio, mb width length scale in aspect ratio, mCo confinement number, –dh channel hydraulic diameter, md channel diameter, mE power, J s�1

g gravitational constant, m s�2

h heat transfer coefficient, W m�2 K�1

l channel heated length, mm liquid mass flowrate, kg s�1

P pressure barQ heat flux density, W m�2

r thickness of glass channel, mt time period, sT temperature, �CU velocity, m s�1

w channel width, m

Greeka thermal diffusivity, m2 s�1

DP pressure drop (DP = Pin–Pout) bar

e channel emissivity, –l viscosity, kg m�1 s�1

q density, kg m�3

r surface tension, N m�1

x Stefan–Boltzmann constant, 5.67 � 10�8 W m�2 K�4

Subscriptsavg average valuec convectiveelec electricali inner dimensionin inlet conditionso outer dimensionout outlet conditionssat saturationwall wall conditionL liquidV vapour

1376 J. Barber et al. / Experimental Thermal and Fluid Science 34 (2010) 1375–1388

attempts have been made to explain these observations. It isthought that surface tension, capillary forces and wall effects aredominant in small diameter channels. Various phenomena are ob-served as the bubble diameter approaches the channel diameter;that is as the bubbles become more confined. The channel’s diam-eter can become so confining that only one bubble exists in thecross-section, sometimes becoming elongated. This is in stark con-trast to flows seen in macrochannels, where numerous bubbles canexist at one time.

Kew and Cornwell observed three flow patterns, including iso-lated and confined bubbles, and annular slug flow. Their researchwas carried out using R-113 in parallel rectangular minichannels.They presented their confinement number equation (Eq. (2)), basedon transition criterion of confinement for boiling applications [13].

Co ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffir=ðgðqL � qV ÞÞ

p

dð1Þ

To fully understand the high heat transfer potential of boilingflows in microchannels, it is vital to first understand the mecha-nisms occurring in these small diameter channels. It has been gen-erally accepted that both nucleate boiling and convective boilingmechanisms exist in micro and minichannels; it is however thedominant mechanism that remains inconclusive.

Flow visualisation was made with a high-speed camera byBrutin et al. [14]. The flowing liquid enters the microchannel andnucleation begins. The wall superheat allows the vapour bubbleproduced to grow rapidly, coalescing and forming a vapour slug.Several investigators [15] have considered this vapour slug as anelongated bubble. This rapid bubble growth is greater than the ratethe vapour can exit the channel. There is hence a vapour build-upin the channel, with a thin liquid film at the channel walls. Theover-pressure produced by the vapour slugs reduces the upstreamboiling flowrate. Bubbles growing before the vapour slug slowdown, and quickly the whole channel cross-section is filled. The va-pour needs to expand, and the liquid–vapour interface at bothsides of the vapour slug is pushed upstream and downstream. Thisleads to the inflowing liquid being pushed back to the channel en-trance, i.e. the vapour bubbles recoil, creating a reverse flow in themicrochannel. It has been also noted by Brutin et al. [16] that pres-

sure oscillations accompany the (afore mentioned) visual observa-tions of flow reversal in a microchannel.

Fluctuations in the pressure drop across the channel exist dur-ing flow boiling in microchannels; with flow accelerations to refillthe vapour spaces. When the channel is empty, and the pressuredrop across the channel has been re-established, the liquid willonce again begin to flow into the channel. Bubbles will be formedagain quickly, due to uniform heat flux applied at the channelwalls, and the phenomena will be repeated. Physical quantitiessuch as flowrate and pressure drop, as well as the visual observa-tions, will be vital in understanding and predicting flow patternsand flow instabilities. Several references referred to in this paperare concerning boiling experiments in a single microchannel. It isalso important to note that in the literature there are many similarexperiments conducted on multiple, parallel microchannels. How-ever, since the experimental campaigns conducted in this researchwork are for flow boiling in a single microchannel only, our litera-ture review reflects this.

In the literature there have been several researchers that usenon-uniform heating in their microchannel research, i.e. with aheater on a base face(s) of the microchannel, with one face of theirchannels transparent for visualisation purposes. Several examplesof this in the literature follow. Kenning et al. [17] had a single rect-angular minichannel of cross section 2 � 1 mm, heated on threesides, with the fourth side used as a flow visualisation window.Brutin and Tadrist [14] used a single rectangular minichannel, ofcross-section 0.5 � 8.0 mm, with a heater adhered to the back faceof the channel with a transparent plexi-glass face for visualisationpurposes. The authors typically use two identically dimensionedminichannels; one which is used to gather heat transfer data, withthermocouples placed along the flow length of the channel, and an-other that is used purely for flow visualisation with no thermocou-ples present. This is a common trend among researchers.

Also in the literature there are many researchers who provideuniform heating to their microchannels, typically via electricalresistance to metal tubing. Kew and Cornwell [2] used as theirminichannels, stainless steel circular tubing of inner diameterrange 1.39–3.69 mm, heated via direct current. Wambsgansset al. [6] used a circular minichannel of inner diameter 2.92 mm,

J. Barber et al. / Experimental Thermal and Fluid Science 34 (2010) 1375–1388 1377

constructed of stainless steel and heated via direct current. Tranet al. [18] used two different geometry minichannels; one circularof inner diameter 2.46 mm and the other rectangular of hydraulicdiameter 2.40 mm, both channels were heated by means of electri-cal resistance and direct current. Even though these researchersmanage to provide uniform heating to their mini and microchan-nels, they are not able to visualise the flow inside the channelssimultaneously to heating it. A more detailed literature review offlow boiling in microchannels, uniform and non-uniform heatingmethods and the associated liquid–vapour flow instabilities duringflow boiling can be found in Barber et al. [19].

In the literature there has already been researchers who studiedperiodic pressure drop and temperature fluctuations during flowboiling in a single microchannel. Huh et al. [20] used deionisedwater in a single horizontal microchannel, the periodic fluctuationsthey observed matched the transition of two alternating flow pat-terns inside the microchannel.

There are several researchers in the literature who have con-ducted boiling experiments with FC-72. Chih Kuang et al. con-ducted experiments of pool boiling heat transfer on micro-cavitysurfaces using FC-72 [21]. Yen et al. experimentally investigatedboiling heat transfer in tubes of diameter 0.19–0.51 mm with FC-72 and HCFC-123 [22]. Muwanga and Hassan investigated the flowboiling of FC-72 in a microtube, paying particular attention to thetemperature profiles across their channel during flow boiling [23].An interesting work by Mukherjee and Mudawar in 2002 [24], com-mented on the dramatic difference in bubble departure diameterbetween water and FC-72 based on the Fritz [25] Bond number cor-relation. These differences between water and FC-72 exist mainlydue to the difference in the surface tensions of the two liquids.

The objective of this work is to look in detail at the liquid–va-pour interface behaviour and dynamics of FC-72 during flow boil-ing under set experimental conditions, using various data; namelypressure sensors measuring the channel pressure drop and a high-speed camera imaging the flow. These data will be analysed and

Table 1Physical property data of FC-72.

Boiling point (1 atm) 56 �CLiquid density (25 �C) 1680 kg m�3

Vapour density (56 �C) 13.24 kg m�3

Liquid viscosity (25 �C) 0.64 mPa sVapour viscosity (56 �C) –Surface tension (25 �C) 0.0105 N m�1

Liquid specific heat capacity (25 �C) 1.1 kJ kg�1 K�1

Latent heat of vaporisation (25 �C) 88 kJ kg�1

Liquid thermal conductivity (25 �C) 0.057 W m�1 K�1

Liquid thermal diffusivity (25 �C) 3.08 � 10�8 m2 s�1

di

r

wi

Fig. 1. Schematic representation of Vitrotubes™ from VitroCom�, where di is the internglass, and l the channel heated length.

discussed in detail, paying particular attention to the onset of bub-ble confinement in the microchannel and its associated pressurefluctuations.

2. Experimental apparatus

2.1. Working fluid

The fluid chosen for the experimental campaign is perfluoro-hexane, a Fluorinert™ Electronic Liquid and is commerciallynamed FC-72. The main physical properties of this dielectric liquidare reported in Table 1. FC-72 is thermally and chemically stable,compatible with sensitive materials, non-flammable, non-toxic,colourless, and has no ozone depletion potential. This combinationof properties, together with its particularly low viscosity, makesFC-72 appropriate for applications such as heat sinks for electroniccomponents. It is also important to note here that the latent heat ofFC-72 is significantly higher (88 kJ/kg) than its specific heat capac-ity (1.1 kJ/kg K). This backs up the theory mentioned earlier, that itis more advantageous to have a boiling flow system than a single-phase flow system, since FC-72 can carry larger amounts of ther-mal energy through the latent heat of vaporisation.

The novel aspect of this research is the simultaneous data acqui-sition and flow visualisation. This has been achieved due to a uni-form, transparent, metallic deposit of Tantalum on the exteriorwalls of the rectangular microchannels investigated. This tantalumdeposit is both conductive and transparent at the thickness (nm)sputtered, hence enabling simultaneous uniform heating and visual-isation of the microchannel flow. The cross-section and sizing of theparticular rectangular microchannel used can be seen schemati-cally in Fig. 1, where for a hydraulic diameter microchannel727 lm, the dimensions are: di = 400 lm, wi = 4000 lm andr = 400 lm, with a cross-sectional aspect ratio of 10, and approxi-mate heated channel length, l, of 80 mm. The majority of the geom-etries in real world applications of micro and minichannels are notcircular. This is why our results are interesting and possibly morerelevant to real world applications of flow boiling in microchannels.

An experimental apparatus has been designed to induce boiling,to measure parameters across the channel such as the pressure drop,and to visualise and record the phenomena occurring inside themicrochannel test section. The final flow loop consists of an injectionsystem providing a constant liquid mass flowrate into the system, aninterchangeable microchannel test section, a condensation system, aflow visualisation system using a high-speed camera, and a dataacquisition system, see Fig. 2. This is all housed inside a temperatureregulated box of volume 1 m3, at a regulated temperature of34 �C (at atmospheric pressure) which is below the saturation

l

al diameter of the channel, wi the internal width of channel, r the thickness of the

Microchannel Test Section

Condenser

Heating Device P=IV

Syringe Pump

Peltier and Heat Sink

P

P

Pout (t)

Pin (t)

Degassing Valve

Air Flow Box Air Flow

Heat Regulated Box T(t)

Labview Data Acquisition,

Flow Visualisation,IR Imaging T(t)

Fig. 2. Schematic representation of the experimental loop and heat regulated box.

Microchannel

Fluid flow

Fluid (vapour) flow

Electrical current

(a)

Pressure sensors

Electrodes

Thermocouples

(b)

Liquid flow direction through

microchannel

Fig. 3. (a) Schematic diagram of the microchannel test section and (b) photograph of the microchannel inserted in the flow loop with the corresponding instrumentation andthe electrodes used for the electrical heating supply.


temperature (56 �C) of FC-72. The pressure sensors used (Honeywell24PC differential series) are accurate to ±0.25% span. The microchan-nel test section, in which the microchannel is placed vertically, canbe seen in more detail in Fig. 3. Further descriptions and diagramsof the experimental apparatus can be found in Barber et al. [19].

2.2. Experimental procedure

The FC-72 liquid is considered clean; due to the high purity itwas purchased at, however it was still degassed before entry tothe flow loop to remove any non-condensables. The degassing is


achieved by boiling the FC-72 liquid in the reservoir vigorouslyusing an imbedded 320 W cartridge heater for one and a half hours,which is over five times the required time to boil the volume of FC-72 liquid present in the reservoir. It should be noted here that thedegassing system is a separate unit from the experimental flowloop, and hence does not appear in Fig. 2. The inlet liquid massflowrate is held constant by the syringe pump during an experi-mental run. It is also possible to apply a range of heat flux to thetest section via electrical resistance and the power regulator thatis connected to the microchannel. Steady state is reached afterabout 30 min in each test run. The barrel size of the syringe usedlimits the operational time of the liquid flow to just under an hour,hence implying that once steady state is reached (after approxi-mately 30 min), there is still 27 min remaining to record steadystate data. Bubble nucleation and growth, slug flow and other phe-nomena can be observed in the test section with the use of a high-speed camera and macro lens. Observations of the nucleation of avapour bubble, its growth, and subsequent blockage of the micro-channel cross-section are recorded in the test section. Pressure andtemperature readings at the inlet and outlet of the microchannel inconjunction with flow visualisation images obtained providesimultaneous data.

2.3. Pressure drop data

The pressure sensors are located upstream and downstream ofthe microchannel, and so the measured pressure drop is the sum-mation of the pressure drops across the inlet and outlet manifolds,the microchannel, and the pressure drop resulting from the inletand outlet contraction and expansion. The acquisition frequencyfor all the pressure data is 133 Hz.

2.4. Heat transfer data

A thorough heat transfer reduction has been performed. Initiallythe IR thermography data was analysed during single-phase flowconditions in the microchannel. In this way we could be confidenton the uniformity of the resistive metallic deposit on the exterior ofthe microchannels, and we observed the constant temperatureacross and along the microchannel wall before flow boiling tookplace.

Heat losses, both convective and radiative were calculated, andit was found that the maximum heat loss was <5% for all heat andmass flux cases investigated. Thus implying that over 95% of thepower provided at the microchannel wall was transmitted directlyto the flowing liquid inside the microchannel.

The power provided to the fluid (Ep) was calculated based onthe power applied to the microchannel deposit (Eelec) and correctedfor losses to the environment (Elosses), i.e. Ep = Eelec � Elosses. Theselosses include both convective and radiative heat transfer. The con-vective losses (Ec) were calculated using Eq. (3), where hc is theconvective heat transfer coefficient. The value obtained for Ec is0.059 W.

Ec ¼ hcAðTwall � TinÞ ð2Þ

The radiative losses (Erad) were calculated using Eq. (4), where eis the emissivity of the microchannel deposit (e = 0.76), and x isthe Stefan–Boltzmann constant (5.67 � 10�8 W/m2 K4). The valueobtained for Erad is 0.082 W.

Erad ¼ exAðT4wall � T4

inÞ ð3Þ

The power provided to the fluid inside the microchannel (Ep)could then be calculated based on Eq. (1):

EP ¼ Eelec � Ec � Erad ð4Þ

The power actually provided to the fluid via the deposit at themicrochannel wall is Ep � 3.00 W. This translates to a heat fluxdensity of Q = 4.26 kW/m2.

Percentage losses from the microchannel could then be calcu-lated based on: Ep/Eelec. These losses were of the magnitude of 5%or less, for a worst case scenario. Hence implying that the majorityof the heating provided at the microchannel exterior wall (over95%) was transmitted directly to the flowing liquid inside themicrochannel, with only small losses to the surroundings.

3. Results

Presented in the following section are results illustrating peri-odic flow boiling, in terms of both pressure data and high-speedflow imaging. The conditions of the case presented are: a singlemicrochannel of hydraulic diameter 727 lm, rectangular cross-section 0.4 � 4.0 mm, heated channel length 80 mm, uniform heatflux (Q) applied to the microchannel is 4.26 kW/m2 and inlet liquidmass flowrate (min) is held constant at 1.33 � 10�5 kg/s (injectionspeed of inlet liquid = 4.95 � 10�3 m/s). Several interesting fea-tures are noted during the high-speed flow imaging, namely; bub-ble nucleation and growth, slug flow and vapour blockage in themicrochannel leading to pressure build-up in the channel. Due tospace and time limitations only bubble confinement and its effecton the microchannel pressure drop is analysed here.

3.1. Periodic pressure fluctuations and bubble dynamics

Pressure fluctuations during two-phase flow boiling are re-corded in the microchannel test section, see Fig. 4. The pressuredrop (DP) is simply the difference between the inlet and outletpressure across the microchannel, and is measured in mbar. Thepressure signal fluctuations are correlated to the passage of vapourbubbles and slugs inside the microchannel, see Figs. 5 and 6 forthese correlations. The experimental conditions for all figures inthis paper are for the same heat and mass flux case, noted previ-ously, where the flow has already passed the onset of boiling con-dition for the hydraulic diameter channel given, based on previousdata obtained. In Fig. 4 the pressure fluctuations shown are thoseduring periodic flow boiling.

In the top plot of Fig. 4, data over a time period of 700 s is pre-sented. Large temporal fluctuations can be seen in the pressuredata, for both the inlet and outlet pressure of the microchannel.It can be seen in the pressure data of Fig. 4, that there exists bothlarge positive amplitude fluctuations (approximately + 106 mbar),and also large negative amplitude fluctuations (approximately�66 mbar) with similar time periods of approximately 40–50 s.Thanks to the high sensitivity of our pressure measurements, wecould pick out fluctuations at much smaller timescales, as seen inthe bottom two plots of Fig. 4. The middle plot of Fig. 4 is over atime period of 57 s, which corresponds to the period oscillationshighlighted in the top plot of Fig. 4. This time period shown inthe middle plot of Fig. 4, draws attention to the constant pressurereadings of both the inlet and outlet pressures between pressurefluctuations. This time period between pressure fluctuations mightbe related to several effects, for example a nucleation waiting timeperiod, or the microchannel wall heating time period and/or the li-quid heating time period. These effects are currently under inves-tigation. It is also interesting to note that inside these periodicoscillations, exists smaller frequency oscillations as presented inthe bottom plot of Fig. 4, which shows the fluctuations of the pres-sure drop across the microchannel over a time period of 10 s. Sim-ilar observations by Barber et al. [19] of flow instabilities duringflow boiling of n-pentane were recorded, again with pressure

Fig. 4. Data measurements of the temporal variation in the microchannel pressuredrop during flow boiling of FC-72 in a microchannel over a time period of 700 s,with close-up plots over 56 and 10 s. Experimental conditions: Q = 4.26 kW/m2,min = 1.33 � 10�5 kg/s and dh = 727 lm.

A

B

C

D

G

E

Channel starts to refill with liquid

F

H

Fig. 5. Close-up of Pin (blue line) and Pout (red line) pressure fluctuations duringflow boiling of FC-72, taken over 500 ms from the data in Fig. 4, annotated withcorresponding high-speed imaging. Experimental conditions: Q = 4.26 kW/m2,min = 1.33 � 10�5 kg/s, dh = 727 lm.

1 For interpretation of color in Figs. 1–3, 5, 6, 10, 11, 14 the reader is referred to theweb version of this article.


fluctuation periods ranging from hundreds of seconds to just a fewmilliseconds.

During the pressure data collection process, the boiling phe-nomena are captured simultaneously by a high-speed camera.From analysis of the high-speed imaging these small-amplitude/short-period fluctuations appeared to have been caused by thebubble dynamic instabilities during the two-phase flow period,as can be seen presented in Figs. 5 and 6. It should be noted herethat the boiling flows observed in the microchannel are complex.

The flow conditions of Fig. 5 are an inlet liquid speed of FC-72 of4.95 mm/s, as supplied by the syringe pump, flowing through arectangular geometry microchannel (dh = 727 lm). A small bubbleis nucleated in the centre of the microchannel; the entire sequenceof this bubble nucleation to slug flow can be seen in Fig. 7 and inthe associated video sequence online, but it suffices to say herethat the growth period from a small radial bubble to an elongatedvapour slug, filling the microchannel cross-section and length, isvery quick indeed, approximately 340 ms. This fast growth rateand confinement of the vapour bubble leads to sharp pressure fluc-tuations at both the microchannel inlet and outlet. Various pointsA–H in the pressure fluctuation data are highlighted in Fig. 5, pri-marily annotated for the inlet pressure signal, Pin (blue line).1

Before point A in Fig. 5, there is primarily liquid flow in themicrochannel and the baseline of the pressure sensor at the chan-nel inlet is approximately +33 mbar, and +3 mbar for the channeloutlet. A vapour bubble begins to grow in the microchannel radi-ally at first, then expanding sideways until it completely fills thewidth of the microchannel. (A discussion of this confinement intwo dimensions will be discussed in greater detail followingFig. 7.) At point A there is a sharp increase in the pressure at boththe microchannel inlet and outlet. The pressure at the inlet risesdue to the growing vapour bubble effectively blocking the inlet li-quid flow through the microchannel, see the attached high-speedimages. The peak reaches a maximum at point C. The points A–Cof the Pin pressure signal can be related to the bubble growthand subsequent bubble confinement leading to vapour blockagein the microchannel, corresponding high-speed images are pre-sented at these points. After point C, the slug quickly expandsalong the microchannel length; this sudden vapour expansionpushes the fluid in the microchannel towards the inlet and the out-let as the vapour slug shows elongated growth, with a maximum(C) for both Pin and Pout of approximately +106 mbar. A sharp de-crease in the measured pressure at the microchannel inlet, backto the original baseline value at approximately +33 mbar, can beseen at point D, during which period the vapour slug starts to be

liquidlnlet

flow

Fig. 6. Close-up plots of DP, and Pin and Pout pressure fluctuations during flow boiling of FC-72, taken over 80 ms from the data in Fig. 5, with corresponding high-speedimaging presented.


purged from the microchannel, as the slug nose reaches the chan-nel outlet and the slug tail expands to the channel inlet. After this,fresh inlet liquid is once again admitted into the microchannel atthe inlet, as the tail of the vapour slug begins to once again enterat the microchannel inlet and proceed through the microchannel.The liquid refilling stage continues until the two pressure values

Pin and Pout have stopped oscillating and have both returned totheir original constant baseline values of +33 mbar and +3 mbarrespectively.

The pressure signal at the outlet follows much the same patternas that at the inlet until point C. After point C there is a more pro-nounced decrease in the Pout signal than the Pin signal, with the Pout

Channel inlet

0.28 0.34s Liquid Flow

0.15s

s 0.29s 0.30s 0.31s 0.32s 0.33s

0s 0.03s 0.06s 0.09s 0.12s 0.18s 0.21s 0.24s 0.27s Liquid Flow

Channel inlet

Fig. 7. Images taken with a high-speed camera, acquisition frame rate = 1000 fps, with liquid flow direction and timescales as illustrated, namely a time step of 30 ms in thetop row of images and a time step of 10 ms in the bottom row of images, and channel inner width wi = 4.0 � 10�3 m (associated high-speed video sequence is also availableonline).


signal decreasing to approximately �66 mbar (E), which is past itsoriginal baseline value of +3 mbar. The decrease in Pout occursapproximately 7 ms before the decrease in Pin occurs, althoughthe minima for Pin and Pout are reached at equivalent times att = 44.21 s. The 7 ms advance in the minima for Pout can be ex-plained, by assuming that the liquid–vapour interface at the va-pour slug nose reaches the channel outlet before the interface atthe slug tail reaches the channel inlet. This is understandable, sincethe slug nose is advancing in the same direction as the inlet liquidflow, whilst the slug tail is receding towards the channel inlet andso will face more resistance against the inlet liquid flow.

Again it is reiterated here that the onset of bubble confinementin the channel leads to an over-pressure in the microchannel, asthe confined bubble effectively blocks the inlet liquid flow intothe channel. The sudden vapour expansion of the bubble to a va-pour slug, leads to the liquid–vapour interfaces of the slug tailand nose expanding towards both the channel inlet and the chan-nel outlet simultaneously. It can be seen that the sudden increasein pressure at both the inlet and outlet of the channel is related tothe bubble dynamics and the associated vapour blockage in thechannel during confined bubble growth. The negative pressure isindeed related to the flow reversal, i.e. that the liquid–vapourinterface at the slug tail recedes towards the channel inlet duringfast vapour expansion of the bubble to a vapour slug.

The two peaks labelled as F and G, in Fig. 5, can be accounted foras subsequent liquid–vapour expansions in the microchannel, fol-lowing the initial sudden vapour expansion of the confined bubbleas illustrated in the high-speed images. After these two furtheroscillations there is constant pressure in the microchannel, withboth Pin and Pout remaining at their respective baseline values. Once

the pressure in the channel is again sufficient, a liquid front prop-agates and the cycle starts over again, the next pressure fluctuationcan be identified at t = 81.8 s in the top plot of Fig. 4.

The growth and expansion of the FC-72 vapour bubble is extre-mely quick. To enable further comprehension of the bubbledynamics and subsequent vapour blockage causing the pressurefluctuations seen in Fig. 5, Fig. 6 has been presented over a timeperiod of 80 ms; this time period corresponds to the black dashedrectangle highlighted in Fig. 5.

The top plot in Fig. 6 highlights the variation in the pressuredrop (DP) across the microchannel during 80 ms of the bubbledynamics and vapour blockage phenomenon. The middle plot ofFig. 6 shows the variation in both the Pin (black line) and Pout (greyline) sensors during the same time period of 80 ms. The corre-sponding high-speed imaging illustrates the nature of the bubbledynamics that, due to the confined bubble growth, creates the va-pour blockage in the microchannel and hence relates back to theaforementioned pressure fluctuations observed at both the micro-channel inlet and outlet.

3.2. High-speed imaging of bubble dynamics

Bubble nucleation and growth can be seen clearly in the top rowof frames of Fig. 7, and also in the corresponding video sequenceonline. A small bubble enters the microchannel inlet at t = 0 s, pos-sibly nucleated at the superheated electrode, it grows radially(spherically) very quickly over 0.15 s; it is unconfined by the chan-nel during this initial growth, i.e. the bubble diameter is signifi-cantly smaller than the channel diameter, and the growth is‘free’. By t = 0.18 s the bubble starts to depart from its spherical,


symmetrical shape, and by t = 0.24 s it begins to deform to a non-symmetrical bubble expanding sideways into the available widthwi (from the channel dimension definition as given in Fig. 1) ofthe channel as it grows. By timeframe t = 0.27 s, it has a distinctbell-shape to its form, and has filled the horizontal cross-sectionof the channel, i.e. the bubble diameter is now approximately equalto the channel inner width. The bell-shaped tail of the vapour slugseen between t = 0.27 and 0.30 s is thought to be due to entrain-ment at the curved microchannel walls. This curvature at the cor-ners of the rectangular microchannel can be seen in Fig. 1.

It is important to note here that the confinement provided bythe microchannel acts in two directions, y and z directions (withthe x-direction representing the channel length). As shown pictori-ally in Fig. 1, the microchannel has an internal cross-section(di � wi), where di = 400 lm and wi = 4000 lm for this rectangularmicrochannel. From analysis of the high-speed videos, it is possibleto follow the aspect ratio of the bubble, with a departure fromunity being an indication of confinement of the bubble. As previ-ously mentioned, at t = 0.18 s the bubble begins to exhibit an as-pect ratio greater than unity. The bubble diameter at this point isstill significantly less than the width (wi) of the channel but ithas already exceeded the value of the breadth (di) of the channel.The confinement in the z-direction, i.e. when the diameter of thebubble equals the smallest cross-section dimension of the channel(namely di), occurs at t = 0.03 s when the diameter of the bubbleequals 400 lm. It can hence be assumed that this is the start ofthe confinement effect of the microchannel on the growing bubble,i.e. partial confinement. Unfortunately this is difficult to see visu-ally, due to the set-up angle of the high-speed camera. The high-speed videos taken here are aligned, so as to focus on the widthof the channel only, y-direction, making the confinement effectin the z-direction difficult to obtain.

The second rows of frames of Fig. 7 shows the elongated growthof the bubble into a vapour slug filling both the cross-section andthe length of the channel in the x-direction, and the bubble ishence said to be fully confined. The distinct bell-shaped tail ofthe vapour slug begins to disappear as the vapour nose progressessteadily through the channel, so that by t = 0.32 s the tail of the va-pour slug is simply a curved interface much like the nose of theslug. As the vapour slug nose progresses through the channel, itis interesting to note that the slug tail begins to recede and asthe slug grows, the tail is pushed back towards the microchannel

I) RadialGrowth

y

z

II) ElongatGrowth

x

Fig. 8. A pictorial representation illustrating the two types of growth seen in the microcshape representations seen in the microchannel geometry.

inlet. The liquid–vapour interface at the slug tail exhibits flowreversal.

3.3. Bubble growth and confinement

The two distinct growth patterns, i.e. radial (in both y and zdirections) and elongated growth (in x-direction), occurring insidethe microchannel can be seen pictorially in Fig. 8. During flow boil-ing in a macrochannel, bubbles grow symmetrically and retaintheir spherical shape whilst expanding and coalescing; this is be-cause they are not confined in any direction. Multiple bubblescan also exist at any one time in a macrochannel cross-section.The growth of the FC-72 bubble, as described previously in Figs. 7and 8, behaves in a different way partly due to confinement effects.It is also clear from the high-speed imaging analysis that thechange from partial bubble confinement to complete bubble con-finement in the microchannel is not a smooth transition.

As stated earlier in the introduction to this paper, Kew andCornwell [13] have proposed a confinement number (Co) whichcan be utilised when the bubble diameter approaches the channeldiameter, and is thus ‘confined’.

The transition criterion for the use of the confinement numberis when Co P 0.5, and Co is defined in Eq. (1). Hence the confine-ment number for this case is Co = 3.5, which being greater than0.5, implies that confinement effects are present, and differencesfrom macrochannel phenomena will be seen. What is importantto note here is that Kew and Cornwell carried out their researchand hence derived their Co equation for data found using a circu-lar channel. For our case we have used the hydraulic diameter(dh) of our rectangular microchannel as the channel diameter din Eq. (1).

The bubble initially grows radially and symmetrically as illus-trated in Fig. 8 and 9ii, and as the bubble diameter approachesthe channel diameter, or channel width in our case, the bubblesgrowth is less symmetrical (Fig. 9iii). When confinement effectsare at a maximum (i.e. when the bubble is confined in both the yand z directions by the channel breadth and width), a bell-shapedbubble is produced (Fig. 9iv); which grows larger and larger in thedirection of the inlet liquid flowrate (x-direction), until it fills theentire cross-section and length of the microchannel. It is interest-ing to note in Fig. 7 and in the illustration in Fig. 9(iv) that thisbell-shape representation is particularly notable at the tail end of

ed

hannel: (I) radial growth and (II) elongated growth, with the corresponding bubble

(i) (ii) (iii) (iv)

b

a

b

aa

b

Fig. 9. A schematic representation of bubble growth in a microchannel during flow boiling. (i) Illustrates a small, spherical, nucleated bubble in the microchannel, (ii) showsthat initially the bubbles growth is radial in all directions, (iii) illustrates the bubbles departure from symmetry due to uneven radial growth rates, and (iv) shows the finalbell-shaped tail of the vapour slug (lengths a and b are parameters used to define the aspect ratio of the bubble/slug).


the bubble, which is perhaps due to the inlet liquid flow accelera-tion and vortex at the slug tail. There will also be significant capil-lary effects and liquid entrainment at the microchannel walls dueto the channel wall curvature as highlighted in Fig. 1.

3.4. Aspect ratio

The growth rate and the departure time of the bubble fromsymmetry are important variables to analyse. The latter can beinvestigated by considering the aspect ratio of the bubble overtime. The parameters to calculate the aspect ratio are defined as

Fig. 10. Graph to show the relationship between the aspect ratio of the FC-72 bubble/sattached.

indicated in the schematic of Fig. 9. Where a is the diameter/lengthof the bubble as it grows vertically into a slug in the direction (x-direction) of the inlet liquid flow, and b is the diameter/width ofthe bubble as it grows horizontally into the confined microchannelwidth (y-direction) (see Eq. (5)).

Aspect ratio ¼ ab

ð5Þ

From the plot shown in Fig. 10, it is easy to identify the depar-ture time of the bubble from symmetry as t = 0.15 s. Betweent = 0.15 and 0.25 s, there is a negative gradient of the plot, this is

lug, shown in Fig. 7, with the pressure data, and corresponding high-speed images


due to a decrease in the aspect ratio of the bubble. Initially, thebubble grows to a greater extent horizontally in the y-direction,(parameter b is larger than parameter a), growing radially in thedirection of the channel walls. However, the bubble quickly be-comes constrained by the channel walls, and so as it continues togrow it does so, but this time in a vertical direction towards thechannel outlet, elongated bubble growth in the x-direction. Thisgreatly increases the aspect ratio, since parameter a can becomeas large as the channel length. This growth, expansion and con-straint of vapour bubbles, as well as the vapour slug formation,can also be seen pictorially in Fig. 10, with high-speed images nextto the relevant values of their aspect ratios.

It is also possible to correlate the variation of the aspect ratio tothe temporal pressure fluctuations in the microchannel. Thesemeasurements were taken simultaneously, and a correlation be-tween the two parameters was achieved. Constant inlet and outletpressures can be seen over the initial time period, equivalent tot = 0.26 s, this is logical to have a constant pressure in the micro-channel during primarily liquid only flow, with unconfined, smallradial bubble growth. After this time, there is a sharp increase inboth the inlet and the outlet pressures, reaching a peak at approx-imately + 106 mbar for both pressures. This sharp increase can bedirectly correlated to the full confinement of the bubble in thechannel, and hence channel vapour blockage. As the vapour bubble

Fig. 11. Graph showing the relationship between the velocity of the liquid–vapour interFig. 7, with the corresponding pressure fluctuations over time. Note: velocity profile is bvelocity indicates a change in direction of the interface. The inlet liquid velocity as suprelative speed of the interfaces as they either recede or advance through the microchan

has grown radially until it fills the horizontal cross-section of thechannel, there is still a constant inlet liquid mass flowrate intothe channel as supplied by the syringe pump. The fact that the bub-ble is now confined by the channel walls, and it is effectively block-ing the inlet liquid from entering the channel, it is plausible toassume that the pressure would suddenly start to increase drasti-cally in the microchannel at both the inlet and outlet. After thissharp peak, there is a decrease in both the inlet and outlet pres-sures, as the vapour bubble starts to expand and becomes an elon-gated vapour slug, expanding towards both the channel inlet andoutlet. The inlet pressure returns to its baseline value (+33 mbar),whilst the outlet pressure drops significantly to �66 mbar, whichis below its baseline value of +3 mbar. This can be explained bythe bubble dynamics. The pressure at the outlet decreases pastits initial baseline as the vapour slug is effectively purged at theoutlet. The inlet channel pressure would not decrease lower thanits initial value due to the constant inlet liquid flowrate providedto the channel at the inlet. As the vapour slug is purged and freshliquid gradually begins to enter at the channel inlet, there aresmaller peaks in the microchannel inlet and outlet pressures(although they are not seen here in Fig. 10).

Another interesting parameter to plot is the velocity of theadvancing and receding liquid–vapour interfaces of the bubble asit grows to become a vapour slug over time. The progression of

Slug Nose

Slug Tail

face at the nose (black line) and at the tail (grey line) of the FC-72 vapour slug fromased on the instantaneous velocity of the slug nose and tail; a change in sign of theplied by the syringe pump, 4.95 � 10�3 m/s, has been highlighted to indicate the

nel over time.


the bubble’s instantaneous velocity over time, Eq. (6), is calculatedat both the nose and at the tail.

Ububble ¼slug distance

timeð6Þ

It is possible to observe the evolution of the liquid–vapourinterface velocities over time, and to see how this relates to thepressure fluctuations, see Fig. 11.

The interface velocities at both the slug nose and slug tail ini-tially increase at the same gentle, constant rate over the first0.18 s. After this point there is a slight change in the gradients ofthe velocities, and the slug tail starts to recede towards the channelinlet. A dramatic change in the velocity profiles can be seen aftert = 0.27 s, as the slug expands quickly towards the channel inletand outlet. As identified previously in Fig. 10, the vapour bubblewhen fully confined by the microchannel in two dimensions att = 0.24 s, proceeds to block the channel cross-section causing anincrease in pressure at both the channel inlet and outlet. Att = 0.28 s, the velocity of the interface at the slug tail is in fact zero.After this point, the sharp increase in the slug nose velocity ismatched by a sharp increase in the slug tail velocity, albeit in a dif-ferent direction, during sudden elongated slug growth in themicrochannel. These velocity profiles are also evidenced in theflow images shown previously in Fig. 7. The top line of flow imagesin Fig. 7 show the bubble’s initial radial growth, correspondinghere to the initial constant gradient of the plots in Fig. 11. The shar-per gradients for both the vapour nose and the vapour tail, includ-ing the inflection point in the vapour tail velocity profile,correspond well to the second row of flow images seen in Fig. 7,where vapour blockage in the channel causes an increase in Pin

and Pout pressures, followed by a sharp decrease in the Pout pressureas the vapour bubble expands to become a vapour slug. It is alsointeresting to note here that the velocities of the interface at theslug nose and tail are more than an order of magnitude greaterthan the inlet liquid velocity as provided by the syringe pump, inthis particular case 4.95 mm/s. The vertical positions of the slugnose and the slug tail during the initial phases of radial bubblegrowth and then slug growth can be seen in Fig. 12. It is also inter-esting to note in Fig. 12 the almost stationary position of the slugtail at the onset of vapour blockage, and then its sudden inflectionas it recedes towards the channel inlet during elongated sluggrowth.

Fig. 12. Graph showing the vertical position of both the nose (black line) and

To summarise the bubble dynamics, the liquid–vapour inter-faces at both the slug nose and at the slug tail proceed towardsthe outlet and inlet of the microchannel respectively during elon-gated vapour bubble growth. As the vapour slug is purged fromthe channel, the pressure at both the channel inlet and outlet de-creases. Figs. 11 and 12 illustrate the velocity and the position ofboth the slug nose and tail over time during the bubble expansionprocess. Whilst the bubble is confined and before elongatedgrowth has taken place, the slug nose and tail have approximatelythe same velocity and their vertical position in the channel in-creases at the same constant rate.

3.5. Reoccurance of bubble growth

In Fig. 13, the sequence of images shows a vapour bubble enter-ing the microchannel, and its subsequent growth and expansionalong the microchannel length. The first 60 ms of this high-speedsequence are during the initial growth period of the bubble, as itfirstly expands spherically until it is confined by two dimensionsof the microchannel, with the only available dimension left forexpansion being along the channel length. This is a similar obser-vation to that described previously in Fig. 7.

The fact that the bubble dynamics (bubble nucleation, bubblegrowth, bubble confinement and elongated slug growth) recur inthe same high-speed sequence emphasises the cyclic nature ofthe phenomena, and gives an indication as to the periodicity ob-served earlier in the pressure fluctuations of Fig. 4. The initialgrowth of the bubble, as presented in Fig. 13, covers a time periodof approximately 60 ms. This initial period can be split even furtherand a plot of the aspect ratio of the bubble over time can be deter-mined for these first few images of the sequence, see Fig. 14. Thiswill allow the confinement of the bubble to be seen more easily. Aspreviously described, the rectangular geometry of the microchan-nel, with an internal cross-section (di � wi) of (0.4 � 4.0) mm, pro-vides two different length scales of confinement. This implies thatthe bubble can expand and grow spherically until it reaches thefirst confinement in the di direction at 400 lm, similarly, it can ex-pand further across the width of the channel wi, until it reaches4000 lm, i.e. at t = 0.05 s in Fig. 13. After this point, the growthof the vapour bubble is only possible along the channel length, l.This elongated slug growth is evident from t = 0.06 s onwards inFig. 13, where the bubble is fully confined.

Slug Nose

Slug Tail

tail (grey line) of the FC-72 vapour bubble/slug, from Fig. 7, over time.

0s 0.01s 0.02s 0.03s 0.04s 0.05s 0.06s 0.07s 0.08s

0.09s 0.10s 0.11s 0.12s 0.13s 0.14s 0.15s 0.16s 0.17s

Liquid Flow

Channel inlet

Liquid Flow

Channel inlet

Fig. 13. High-speed imaging of bubble growth of FC-72 in a microchannel, frame rate = 1000 fps, with liquid flow direction and time step 10 ms, and channel inner widthwi = 4.0 � 10�3 m. Experiment conditions: min = 1.33 � 10�5 kg/s, Q = 4.26 kW/m2 and dh = 727 lm.

Fig. 14. The variation of the aspect ratio of the FC-72 bubble during the first 55 msof growth in Fig. 13.


Focusing in greater detail on the variation of the aspect ratio asthe spherical bubble starts to deform and to develop into an elon-gated vapour slug in Fig. 13, reveals oscillations in the aspect ratiooccurring at small timescales. Fig. 14 is a plot of the aspect ratioover time, for data taken at time intervals of 2 ms.

These oscillations in the aspect ratio over small time scales canalso be observed when watching the corresponding high-speed

imaging, as the small vapour bubbles appear to be buoyant inthe flow, before becoming constrained. Once the bubbles becomeconfined they appear to be almost anchored in place whilst the va-pour bubbles expand along the microchannel length as vapourslugs.

4. Conclusion

The mechanisms of flow boiling in microchannels and associ-ated instabilities are still not fully elucidated. The novel aspect ofthis work is the simultaneous measurement of the local flow visu-alisation and pressure drop measurements of FC-72 across themicrochannel. This was achieved through the use of a novel, trans-parent, conductive metallic deposit on the exterior of the micro-channels, and led to the comprehension of the sharp pressurefluctuations witnessed during confined bubble growth in amicrochannel.

It is hoped that this paper has demonstrated some fundamentalresults possible from detailed analysis of quality, high-speed imag-ing. Flow boiling phenomena can be observed experimentally inrectangular microchannels (here presented dh = 727 lm). The flowphenomena are captured in terms of pressure fluctuations, as wellas visually with a high-speed camera. High-speed imaging gaveevidence for bubble nucleation, growth and slug evolution overtime. The rectangular geometry channels investigated led to con-finement in two dimensions; initially the internal diameter (di)dimension and afterwards the internal width (wi) dimension. Three


main stages of bubble growth were observed; namely uncon-fined bubble growth (dbubble� di), partial bubble confinement(dbubble � di) and full bubble confinement (dbubble � wi). The con-finement in these two dimensions produced two growth patterns,both radial growth and elongated growth. It was possible toexplain the periodic pressure fluctuations in terms of the FC-72bubble dynamics. The confined growth of vapour bubbles in themicrochannel led to instances of channel vapour blockage andresulted in sharp pressure fluctuations at both the microchannelinlet and outlet. The correlation of the aspect ratio to the pressurefluctuations over time was possible, with a sharp increase in pres-sure occurring at equivalent times to fully confined vapour growthin two-directions (vapour blockage) in the microchannel, with asharp decrease in pressure as the two-dimensionally confinedbubbles show elongated slug growth. The quantification of the as-pect ratio over short time frames of 2 ms, permitted oscillations ofthe vapour bubble to be observed which resulted in fluctuations inthe aspect ratio until an aspect ratio of 1.0 was again achieved.After the aspect ratio had passed unity, there was an exponentialgrowth in a/b due to elongated slug growth in the microchannel.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.expthermflusci.2010.06.011.

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