Nucleate pool boiling in microgravity: recent progress and future prospects

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Nucleate pool boiling in microgravity: recent progress

and future prospects

Catherine Colin, Olivier Kannengieser, Wladimir Bergez, Michel, Thomas

Lebon, Julien Sebilleau, Michaël Sagan, Sébastien Tanguy

To cite this version:

Catherine Colin, Olivier Kannengieser, Wladimir Bergez, Michel, Thomas Lebon, Julien Sebilleau, et

al.. Nucleate pool boiling in microgravity: recent progress and future prospects. Comptes Rendus

Mécanique, Elsevier Masson, 2017, Basic and applied researches in microgravity – A tribute to Bernard

Zappoli’s contribution, 345 (1), pp.21-34. �10.1016/j.crme.2016.10.004�. �hal-01472139�

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Contents lists available atScienceDirect

Comptes

Rendus

Mecanique

www.sciencedirect.com

Basic and applied researches in microgravity/Recherches fondamentales et appliquées en microgravité

Nucleate

pool

boiling

in

microgravity:

Recent

progress

and future

prospects

Catherine Colin

∗

,

Olivier Kannengieser,

Wladimir Bergez,

Michel Lebon,

Julien Sebilleau,

Michaël Sagan,

Sébastien Tanguy

Institut de mécanique des ﬂuides de Toulouse, Université de Toulouse (INP–UPS–CNRS), 2, allée du Professeur-Camille-Soula, 31400 Toulouse, France

a

r

t

i

c

l

e

i

n

f

o

a

b

s

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r

a

c

t

Article history:

Received8April2016 Accepted1June2016

Availableonline9November2016

Keywords:

Poolboiling Bubbledynamics Heattransfer Microgravity

Pool boilingonflat platesinmicrogravityhasbeenstudiedformorethan50 years.The resultsofrecentexperimentsperformedinsoundingrocketarepresented andcompared topreviousresults.Atlowheatflux,theverticaloscillatorymotionoftheprimarybubble is responsibleforthe increaseinthe heattransfercoefficientinmicrogravity compared to groundexperiments.The effectof anon-condensable gas onthe stabilisation ofthe large primarybubbleonthe heaterispointedout. Experimentsonisolatedbubblesare alsoperformedongroundandinparabolicflight.Theeffectofashearflowonthebubble detachmentishighlighted.Aforcebalancemodelallowsdetermininganexpressionofthe capillaryforceandofthedragforceactingonthebubble.

1. Introduction

Liquidvapourﬂowsexistinawidevarietyofapplicationsinbothnormalgravityandreducedgravityenvironments.As itisusually thecase, thereare manybeneﬁts anddrawbacksinthe useoftwo-phasesystemsand, consequently, serious considerations are neededbefore decidingon whetheror not toproceed withthe design,construction and useofthese systems,particularlyinareduced-gravitycontext.

In normal gravity, or terrestrial applications, gas–liquid flows have been traditionally studied by the petroleum and nuclearindustries.Thepetroleumindustryhasfocusedmostoftheireffortsonflowthroughlongpipelineswiththeintent oftransferringamixtureofcrudeoilandnaturalgasfromthewellandthenperformingtheseparationofthecomponents or products atthe refinery. The nuclearindustry hasbeen concerned withsystem stability andsafety withthe primary intent ofpreventingdry out ofthe nuclearreactor througheither a heattransfer/fluid flow instability orloss ofcoolant accidentastheheatenergyistransferredfromthereactortotheturbines.Thechemicalindustrieshaveutilisedgas–liquid contactors to increase interfacial heat and mass transfers in absorption, stripping and distillation processes that involve two-phaseflowthoughcomplexgeometries.

Ina reducedgravity environment,theprinciplesremain thesame.The applicationsconcernthe thermalmanagement systemsforsatellites,thepowermanagementssystemsforlongtimemissionsormannedspaceplatforms,andﬂuid man-agementfromthestoragetanksthroughthelinestotheengine.Thermalmanagementsystemstransferheatfromasource

*

Correspondingauthor.

E-mail address:colin@imft.fr(C. Colin).

http://dx.doi.org/10.1016/j.crme.2016.10.004

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(resistanceheatfromelectronicequipment)toasink,typicallythrougharadiatorpanel.Differentdevicesareused depend-ingonthepowertobetransferred:heatpipes,loopheatpipe,single-phasemechanicalpumpedloop.

Anotherimportantproblemconcernsﬂuid management:the behaviourofthepropellant inthetanksofthelaunchers andthetransferfromthetanktotheenginesthroughthesupplylines.Thecryogenicliquidsarepressurisedbytheirvapour or anon-condensablegas. Duringthedifferentphasesof themission(propelled phase,ballistic phase)itisimportant to control thephasedistributionandtheevolutionoftemperatureandpressureinside thereservoirs.Theevolutionofthese parameters stronglydependsonheatandmasstransfers.Duringtheballisticphaseofthemission,thetankwallisheated by solarradiation andthermal dissipation due to engine andelectrical devices. Since there isno thermal convection in microgravity, the heat transfer between the heated wall and the liquid is mainly dueto heat conduction, andthe wall temperature canbecome greaterthan therequired temperaturefortheonset ofnucleateboiling. Thestudyofboiling in microgravityisthusofparticularinterestinthissituation.

However, boiling is a complexphenomenon, which combinesheat andmass transfers, hydrodynamics,andinterfacial phenomena. Furthermore,gravityaffectsthefluiddynamicsandmayleadtounpredictableperformancesofthermal man-agementsystems.It isthusnecessarytoperformexperimentsdirectlyin(near) weightlessenvironments.Besidesthe ISS, microgravity conditionscan be simulatedby means ofadrop tower, parabolicflights onboard anaircraft ora sounding rocket.Severalstudiesonpoolboilinginmicrogravitywere performedforthelast 50years.Wewillfocusouranalysison nucleateboiling onflatplatesandalsoonboiling onanisolatednucleation site.Sincethisreview isfartobeexhaustive, additionalinformationcanbefoundinotherpreviousreviewsbyStraub[1],Ohta[2],DiMarco[3,4],Kim[5].

The studies performed by the authors of thispaper have been mainly supported by the French Space Agency CNES (“Centre national d’étudesspatiales”)in thenetwork GDR “Fundamental and AppliedMicrogravity” andin theCOMPERE Programme (Behaviour of propellants in tanks) or Programme of the European Space Agency (MAP Project: Multiscale ANalysisofBOiling).Theﬁrstpartofthispaperisdevotedtothestudiesofpoolboilinginmicrogravityonheatedplated. The second part concernsthe behaviour ofisolated bubbles withthe characterisationof localwall heat transferand the studyofbubblehydrodynamics.

2. Poolboilingonﬂatplatesinmicrogravity 2.1. Boilingregimes

The studyofpool boilingin microgravityhasbegun inthe1960s withtheNASA Spaceprogrammewithexperiments performedduringshorttesttimeindroptowersbyMerteandClark[6]orSiegel[7].Duringthe1980sandthe1990s, exper-imentsonflatheatedplateshavebeencarriedoutduringlongermicrogravityperiodsinparabolicflights,soundingrockets oraboardthespaceshuttlebyZelletal.[8],Leeetal.[9],Ohta[10]andOkaetal.[11],Straub[1].Theseexperimentshave showntheexistence ofstableboilingregimesinmicrogravityoverlongperiods. Inareview oftheseexperiments,Straub [1]remarkedthatgravityhasarelativelyweakinfluenceonheattransferinnucleateboiling,butitstronglyaffectsthedry outoftheheatedplate,reducingsignificantlythecriticalheatfluxinmicrogravity.Theseexperimentswereperformedwith different fluids, mainly refrigerantsR11, R12, R113,R123, on flatplates ofdifferentsizes, at differentreducedpressures, liquidsubcoolings.

First,theonsetofnucleateboilingappearsforalowerwallsuperheatinmicrogravitycomparedto1-g withanupward facing plate. Innormalgravity, thermalconvectioncools downthe heatedplateanddelaystheonsetof nucleateboiling. Thendifferentnucleateboilingregimesareobserved.IntheexperimentsofLeeetal.[9],Ohta[10]andOkaetal.[11],one largebubbleislevitatingovertheheatedsurface.Itisseparatedfromthewallbyaliquidlayerinwhichmanyverysmall bubbles arenucleated,growandcoalescewiththelargebubble,whichoscillatesup anddownduetocoalescenceevents, butnevertouchesthewall.

Inotherexperiments[8],thelargebubbleisincontactwiththewallandcoversasignificantpartoftheheatedsurface. Its size iscontrolled by heater size,wall superheat,andliquidsubcooling. Other smallbubbles arenucleated aroundthe largeroneandcoalescewithit.Thisboilingregimeisratherobservedonsmallheatedplatesorathighheatflux.Ifliquid subcooling issufficientlyhigh, the bubblemaykeep aconstant size onthe wall,balanced by evaporationatits footand condensation atits top. Ifsubcoolingis too low,thelarge bubble expandsover theheated surface anda dry-outofthe surface occurs. Alarge bubbleis observed whenthe microgravitylevel is verylow, insounding rocketsorspaceshuttle experiments.Inparabolicflight,duetog-jitter,smallerbubblesareobservedneartheheatedsurface.Theyaredetachedor swept by g-jitter. Alargebubble isnotobservedforlow heatfluxeswhenliquidsubcoolingishigh. Bubblesnucleate on thewall,quicklycoalesceaftertheirdetachment,andsometimesevenbefore.

TheinfluenceofpressurewasstudiedbyStraub[1],whoclearlyshowedthatinearthgravityconditions,anincreaseof pressure causesan increase of heattransfer. The effectofliquidsubcoolingon heat transferhasbeen studiedby several authorslikeLeeetal.[9],Ohta[10]orOkaetal.[11].Unfortunately,inmostoftheseexperiments,subcoolingwaschanged byvaryingthepressure.Itisthereforedifficulttodistinguishseparatelytheeffectofsubcoolingandpressureonthechange inheattransfer.RecentexperimentsofKannengieseretal.[12] withacontrolledpressureshowedthatsubcoolinghadno influenceinthefullydevelopedboilingregime,whenthebubblescoverthewholeheatedplate.Forlowheatfluxes, heat transfer is enhanced inmicrogravity comparedto thecaseof a1-g upward facing plateanddecreasedcompared to the caseofa1-g downwardfacingplate.

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Theeffectofanelectricﬁeldonbubbledynamicsandheattransferwasalsostudied.DiMarcoandGrassi[13]conducted pool-boilingexperimentswithFC72ona2cm

×

2 cm heatedsurfaceaboardtheFotonM2satellite atapressureof1bar. Without electricfield, theyfound that heat transferis lower inmicrogravity comparedto 1-g conditions. Asthe electric fieldisappliedinmicrogravity,thebubblesdetachfromthewall,thenthevoidfractiondecreases,coalescencediminishes andheattransferisimproved.Forthehighestelectricfieldcloseto10kV,similarheattransferisobservedinmicrogravity andin1-g,andtheboilingcrisis isdelayed.Morerecently,experimentswereperformedona smallmicroheaterarray in parabolicflights[14].

Since the 2000s, severalexperiments were performed withlocal measurements of the heat flux orwall temperature forabetterunderstandingofthephysicsofboiling.Ohta [10]performedpoolboilingexperimentonatransparentheater inparabolic flights.It allows boththe observationofthemacrolayer ormicrolayer behaviour fromunderneathandthe measurements of local surface temperatures andof the layer thickness. Tiny bubbles were observed inthe macro layer underneath the large bubble formed at high heat flux. Evaporation at the foot of the primary bubbles dominates heat transferinmicrogravity.Nodrypatchwasobservedintheseexperiments.

ExperimentsonpoolboilingofFC72andn-perﬂuorohexaneat1barwere performedatasurface withcontrolled wall temperaturethanks tomicro heaterarraysin parabolicﬂights byKim etal.[15],ChristopherandKim [16], RajandKim [17],Rajet al.[18–20] andmore recentlyaboard theInternationalSpace Station[21].The heater size was varying from 2 mm

×

2 mm to7 mm

×

7 mm andcontainedupto10

×

10 microheaterskeptatconstanttemperature.Thismeasurement techniqueallowedmeasuringthedistributionofwallheatfluxbelowthebubblesgrowingonthewall.Sincetheheaterwas transparent, video recordings takenfrombelow the heater allowed usto visualisethe contactlines at the bubblefoots. The micro heaters hada very shortresponse time, thus it was possible to investigatethe variation of heat flux during the transient phase from 1.8-g to 0-g and 0-g to 1.8-g in parabolic flights. The effects of residual gravity, heater size, liquidsubcooling,concentrationofnon-condensablegaswerecarefullychecked,andscalinglawsfortheinfluenceofthese parametersontheheattransfercoefficientareprovided[17,20,22].

Zhaoetal.[23]studiedpoolboilingofFC72onanalloysurfaceof15 mm

×

15 mm witharoughnessofafew microme-tresaboard theChinese satellite SJ-8.The durationof themicrogravity periodwas about100s, andquasisteadyboiling was observedforsubcooledregimes.Inmicrogravity, smallbubbles coalesceontheheater,leading toa largebubble,and thewallheattransferwasfoundtobesmallerthanonearthgravityforwallheatfluxeslargerthan6W/cm2.Thecritical heatfluxwasalsolowerinmicrogravitythanonearth.Theamountofdissolvedgaswasnotquantifiedinthisexperiment. RecentexperimentsonpoolboilingonaflatplatewerealsoconductedintheBoilingeXperimentalFacility(BFX)aboard theInternationalSpaceStationbyWarrieretal.[24]onanaluminiumwaferwithfiveartificialnucleationcavities.Several local heatersand thermistors were located on the backside ofthe wafer. Experiments were performed withisolated or multiplebubbles withperfluoro-n-hexaneatdifferentpressures andwitha dissolvedgas concentrationaround 260ppm. Afterboilingincipience,thebubblesstayedonthewallandcoalescedtoalargebubbleinthecentreoftheheater.Asthe heat fluxincreased, new bubbleswere nucleated andcoalescedwiththe large bubble.Thelarge bubble mightcoverthe wall heatersurface andsometimeslifted-offbutlevitated justabovethe heaterandsmallbubbles nucleatedonthe wall continuedtomergewiththelargebubble.Thebehaviourofthebubbleswassimilartowhathadbeenpreviouslyobserved byLeeetal.[9]andStraub[1].Theheatfluxmeasuredinnaturalconvectionandpartialnucleateboilingismuchsmaller inmicrogravitythaninnormalgravity.

Mostof theseexperiments were performedonvery smooth surfaces. Kannengieseret al.[25,12] andSagan [26] per-formed experimentson poolboiling on aheatedcopper plateof1 cm2,withHFE7000 astheworkingfluid inparabolic flightsandalsointwo soundingrocketsMaser11andMaser12forlow-heatfluxes. Inthesoundingrockets, thetestcell wasasmallcylindricaltank60mmindiameterand217 mminlength,withsphericalpartsatbothends.Thelateralsurface ofthetank wasmadeofquartzforflowvisualisations.Thetankwas connectedatitslowerpartwithareservoirofliquid HFE7000.Atitstop,itwas connectedtoagaseousnitrogentankinMaser11ortoapressurisedvapour HFE7000tankin Maser12.Aheatedplateforthestudyofnucleateboilingwaslocatedatthebottomofthereservoir(Fig. 1).Itconsistsofan electricalresistanceheatedbyJouleeffectincontactwithafluxmeterandaroughcopperplatewithathicknessof40μm (Fig. 2).The fluxmeterwas equippedwithtwothermocouples.Itwasthenpossibletomeasureatthesametime theheat fluxtransmittedto theliquidandthe walltemperature. Fivemicrothermocouples100μmindiameterwere placedabove theheatedplatetomeasuretheliquidorvapourtemperature.Beforethelaunchoftherocket,thetankwasvacuumedand the metallic partat thetop of the test cell was heated. Then the lateralquartz cylinderwas also heatedby conduction fromthemetallicpart.Aftertherockettake-offwhenthemicrogravityphasewasreached,thetank waspartlyfilledwith liquidHFE7000 andpressurisedeitherby nitrogenorby itsown vapour (Fig. 1). Thefree surface took asphericalshape. Theliquidwas subcooledandevaporatedincontactwiththeheatedquartz wall.The evaporationratewas highnearthe contactline.Astrong masstransferoccurredatthe interfacebetweenthesubcooledliquidandtheoverheated vapourin Maser12(Fig. 1,right).InMaser11,duetopresenceofanon-condensablegas,astrongMarangoniconvectiontookplace atthefreesurface.Itstabilisedthefreesurfaceandledtoentrainmentanddissolutionofnitrogenintotheliquidbulk.The concentrationofnitrogenintheliquidwasestimatedtobecloseto0.01 mol N2/molHFE7000[25].

Different bubblebehaviourswere observed inthe experimentson the ground, inparabolicﬂights, in soundingrocket without andwithnon-condensablegas(Fig. 3). Inparabolic ﬂights, thebubblesize was smaller thanin soundingrocket experiments. Due to g-jitter, the small bubbles lifted-off or slided on the heated wall, before having time to coalesce andform one large bubble.Inthe sounding rocket Maser12, experimentswere performed withpure liquid/vapour.The

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Fig. 1. Test cell: liquid HFE7000 pressurised by nitrogen in Maser11 (left) and vapour HFE7000 (right) in Maser12.

Fig. 2. Heater.

Fig. 3. FlowboilingregimeswithHFE7000(pureliquid–vapoursystemorwithgaseousnitrogen)ona1-cm2_heated_plate_in_{microgravity:}

_q

₌_{3 W}_/_cm2_, P=1 to1.5 bar,Tsub=5 to10K.

runs consistedofthreemeasurements at1.5barwitha liquidsubcooling

Tsub of10◦Candthree wallheat ﬂuxesq of

1.88 W/cm2_,_2.92_W/cm2_,_and_3.6_W/cm2_._One_experiment_was_also_conducted_at_1.3_bar_at_saturation_with_a_heat_wall_ﬂux

of1.7W/cm2.Afterboilingincipience,severalvapourbubblesnucleatedonthewall.Ittookabout20sforthebubblesto coalesce andformone largebubble,whichremainedinthewall vicinityduring alltheﬂight. Thebubblewas sometimes attached to thewall, sometime detached fromthe wall.Different behaviours were observedin subcooledboiling and in saturatedboiling.Thewall temperaturewas measuredbytwothermocouplesintheﬂuxmeterandtheliquidtemperature was measuredbyan arrayofthermocouplesabovetheheater (Fig. 1).ThermocouplesTC24 andTC23arelocated0.48mm

and0.78mmfromtheheatedwall,respectively.Forsubcooledboiling,thevaluesofTC24

−

Tsat andTC23

−

Tsat,Tsatbeing

thesaturationtemperature,aredisplayed inFig. 4.Thelargebubbleradiusandthedistanceofthebubblefoottothewall are determined by image processingand alsoplotted in Fig. 4.The large bubblehad a strong oscillating motion in the

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Fig. 4. Wall temperature and bubble size and height evolutions for q=2.92 W/cm2_{, P}₌₁_._{5 bar,}_T

sub=10◦C[26].

Fig. 5. Wall temperature and bubble size and height evolutions for q=2.92 W/cm2_{, P}₌₁

.3 bar,Tsub=0◦C[26].

vertical direction.The bubblesize was controlled by thevaporisation on thewall and/or coalescence withsmallbubbles nucleated on the wall andthe condensation at the bubble top. The thermocouplesTC24 and TC23 can be eitherin the

liquidphaseorinthevapourphase sincemanybubblesnucleatedonthewall.TC24measured aliquidtemperatureclose

toTsat,whichwascharacteristicofthevapourorliquidtemperatureatsaturation.Theoverheatedliquidlayerwassmaller

than0.5 mm.ThemeasurementsofTC23variesfromthevapourtemperatureclosetoTsat totheliquidtemperaturelocally

subcooledfrom2to5◦C.Theevolutionofthebubbleradiusseemstobelinkedtotheevolutionoftheliquidtemperature inthewallvicinity.Asthesubcoolingdecreases,thebubbleradiusincreases.Theevolutionofthebubbleverticalmotionis notdirectlycorrelatedwiththevariationofitsradius.

In saturated boiling conditions (Fig. 5), the bubble also had a vertical motion, but with a lower frequency than in subcooledboiling. Itstayed longerperiods onthe wall(85

<

t

<

87 s and 90

<

t

<

92

.

5 s) andlevitated abovethe other bubbles also fora long period (87

<

t

<

90 s). The meanbubble radius increasesfrom 12.5to 13.5 mm.As the bubble levitated,liquidrewettedthewallandnewbubblenucleationtookplace,leadingtoanincreaseintheheattransfer,which canbeseenthroughthedecreaseinthewalltemperature.

The vertical motion of the large bubble was not observed in the Maser 11 experiment with the presence of non-condensablegas.Thelarge bubbleremainedstableonthe heatedplate. Smallbubblesnucleatedonthe wallwere driven bytheMarangoniconvectiontowardthelargebubbleandcoalescedwithit[25].Thedifferentbubblebehavioursobserved intheexperimentsare responsibleforthedifferentevolutions oftheboilingcurvesobservedin normalandmicrogravity conditions.

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Fig. 6. BoilingcurvesinearthgravityandmicrogravityfromZelletal.[8]andLeeetal.[9]withR113,fromKimandRaj[27]with

n-perﬂuorohexane

and fromKannengieseretal.[12]andSagan[26]withHFE7000,atapressureofabout1barto1.5barandasubcoolingvalueofabout10K.

2.2. Inﬂuenceofgravityonheattransferinnucleateboiling

Severalcorrelationsexisttopredictheattransferinnucleatepoolboilinginearthgravity.Thesecorrelationsoftendepend on gravity since the capillarylength istaken as a characteristic length scale forthe bubble size at detachment. Critical reviews ofthe applicationofthese correlationsto microgravityconditions were performedby Di MarcoandGrassi[28], Straub [1]andKannengieser et al.[12]. From these correlations, we can write the dependency of thewall heat ﬂux on gravityas: q q0

∼

a g0

n

(1)

whereq0 isareferenceheatﬂuxtakenatgravity g0,generallyterrestrialgravity.n isaconstant,whichvariesfrom0.3to

1.5formostoftheusualcorrelations[29,30].ExceptforthecorrelationofCooper[31],wheren

=

0,theusualcorrelations anticipateaverylowheatfluxinmicrogravity,whichisnotinagreementwiththeexperimental results.So,these correla-tions arenotadequatetoestimatetheheatfluxinmicrogravityanditismorerelevanttouse g asaconstantequaltoits value onearth,assuggestedbyDhir[32].Infullydevelopednucleateboilingregimeinmicrogravity,themeasurementsof liquidtemperatureprofileinthewallvicinitybyKannengieseretal.[12] showsthat thethermalboundarylayerisofthe orderoftensofmicrometres,whichismuchsmallerthat thecapillarylength.Thislength scaleisthereforemorerelevant thanthecapillarylengthtopredictthewallheattransferinmicrogravityconditions.

From theprevious worksitappearsthat theinfluenceofdifferentparameterslikegravity orsubcoolingorheatersize remains unclear.Forexample,Fig. 6showsthecomparisonoftwoboiling curveson earthandinmicrogravityconditions obtainedbyLeeetal.[9]andZelletal.[8]bothforR113onaflatplatewithagoldcoatedheater.Theseexperimentsdo notdisplaythesametrends:Leeetal.[9]pointedoutanimprovementofheattransferinmicrogravity,whereasZelletal. observed theopposite trend.Forthislast experiment, itwas reportedthatthe footofthelargebubblewas increasing in sizeduring theexperimentandthatastablewalltemperaturehasneverbeenreached.Ataheatfluxofabout40kW/m2_,

theboilingcurveinmicrogravityofZelletal.[8]takesalowslopeand,atthesamevalueofheatflux,theboilingcurveof Leeetal.[9]takesalsoa lowslopeandcrossestheboilingcurve inearthgravitycondition.Atthisvalueoftheheatflux, thetwo authorshavereachedthe‘dry-out’ heatflux.IntheexperimentofZelletal.[8],theboilingcurveinmicrogravity condition wasundertheboilingcurveinearthgravityconditionbecausemostofthemeasurements havebeenperformed abovethe‘dryout’heatflux.Forthislastexperiment,nucleateboilinginearthgravityconditioniscomparedtoboilingin microgravityinaregimeequivalenttofilmboiling.

Some data recentlyobtained by Kim and Raj[22] forthe boiling of n-perﬂuorohexane on micro waterheater arrays onboardtheInternationalSpaceStationarealsoplottedinFig. 6.Forthisdataset,thesizeoftheheaterwas7 mm

×

7 mm, andthesubcoolingwasabout10K.Ataverylowheatﬂux,heattransferwaslargerinmicrogravitythaninnormalgravity. Forwallsuperheatlargerof24 K,thebubblesizereachedtheheatersizeinmicrogravityandthewallheatﬂuxwaslimited to45kW/m2,whereasitcontinuedtoincreaseinnormalgravityupto130kW/m2.

InFig. 6theexperimentalresultsobtainedbyKannengieseretal.[12] inparabolicflights andSagan [26]intheMaser 12soundingrocketwithHFE7000arealsoplotted.Intheseexperiments,thewallheatfluxeswere lowerthan40kW/m2. Heattransferwas largerinmicrogravityconditionsthanonearth andnodry-outwasobservedintheseexperiments.The heat flux was toolow toobservethe dryout andadeterioration ofthe heattransfer inmicrogravitycompared toearth

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gravity.Thewallheatﬂuxwasalittlebitlargerinthesoundingrocketexperimentthatinparabolicﬂights.Thisisprobably duetotheoscillatingmotionofthelargebubbleabovetheheater,enhancingheattransfer.

Severalauthors tried topredict thewall heat transferin pool boilingin microgravity.Raj etal.[18–20,22] performed systematic experimentson microheater arrays to point out theeffect of gravity, heater size, liquid subcoolingand non-condensablegasconcentration.Theyidentiﬁedtwoheat transfermodesversusthegravitylevel.Forthehighestvaluesof

a

/

g (g being theterrestrialgravity), the BuoyancyDominated Boiling regime(BDB) was observed.Bubbles grewand de-tachedfromthewallbythebuoyancyforce.Forthelowestvaluesofa

/

g,boilingregimewascontrolledbysurfacetension effects(SurfacetensionDominatedBoiling regime(SDB)).Alargebubbleappeared ontheheaterandsmallerbubbles nu-cleatedontheheaterandcoalesced withthelargeone.The transitionbetweenthesetwo regimesdependsontheheater widthLhcomparedtothecapillarylength Lc.IfLh

>

2

.

1Lc,BDBregimewasobserved.FortheBDBregimethevalueofthe

wallheatﬂuxatanygravitylevel,iscorrelatedtoareferencevalueoftheheatﬂuxqref foranaccelerationofaref:

qBDB qref

=

a aref

n

BDB nBDB

=

0

.

65T∗ 1

+

1

.

6T∗ T ∗

₌

Tw

−

TONB TCHF

−

TONB (2)

The exponent nBDB is expressedversus thedimensionless wall temperature T∗,which is function ofthe temperature at

theonset ofnucleateboiling, TONB,andofthetemperaturemeasured forthecriticalheat ﬂux TCHF.Thisexpression was

establishedforvariousheatersizes,liquidsubcoolings,wallheatﬂuxes,andnon-condensablegasconcentrations.IntheSDB regime, thedependency onthe gravity levelislower, withnSDB

=

0

.

025 from parabolicﬂight experimentsandn

=

0 for

ISS experiments.Atthe transitionbetweenBDBandSDBregimes,a jumpinthe wallheat ﬂuxis observed.Thisjumpis linkedto theconcentrationofnon-condensablegasandisfunctionofaMarangoni numberMa. Thentheheatﬂux inthe SDBregimeisexpressedasfollows:

qSDB

=

qBDB

atrans aBDB

n

BDB

1

−

eCMa

₍₃₎

whereatrans istheaccelerationforwhichLh

=

2

.

1Lc andC

=

8

.

3

·

10−6 forFC72.Equations(2)and(3)areabletopredict

the wall heat ﬂux in BDB and SDB regimes for a wide range of heater sizes, liquid subcoolings, non-condensable gas concentrations.Theinterestofthiscorrelationisthatitusesa referencevalue ofthewallheatﬂuxatareferencegravity level.Thisvalueofqrefisafunctionofthewallpropertiesandespeciallyofthenucleationsitedensity,whichisaparameter

difficulttoquantifyandcontrol.ThentheuseofEquations(2)and(3)takesintoaccountdefactothesurfaceproperties. Nevertheless,thesecorrelationsalwayspredictalowervalueofthewallheatfluxinmicrogravitythanonearthgravity, whichis notalways thetrendexperimentally observed,especially atlow heat fluxforlarger heatedplates.In the exper-iments performedby Kannengieseret al.[12] in microgravity, the nucleate boiling regime was fullydeveloped. Bubbles covered thewholeheatedsurface andthe temperatureprofilesmeasured inthe liquidshowedthat theoverheated layer closetothewallhadathicknessoftensofmicrometres,indicatingthatheattransferwascontrolledbymechanisms occur-ringinthenear-wallregion,asbubblenucleation,coalescence,motionofcontactlines.ThenKannengieser[33]determined themorerelevantdimensionlessnumberstocharacteriseheattransfer.Therelevantphysicalpropertiesandparametersare liquidandvapour densities

ρ

L and

ρ

V,liquidviscosity

μ

L,thermalconductivity

λ

L,heat capacityCPL, surfacetension

σ

,

latent heat of vaporisation hLV, wall heat ﬂux q, wall superheat

Tsat

=

Tw

−

Tsat, acceleration a, expansion coeﬃcient

β

; thethermal conductivity,the heat capacityofthe vapour are supposed to havenegligible inﬂuence. Consideringthat theheatﬂuxcanbeexpressedversus11parameters,expressedversusfourdimensions(length,mass,time,energy),seven independentdimensionlessnumberscanbebuilt:

Ja

=

CPL

(

Tw

−

Tsat

)

hLV Pr

=

μ

LCPL

λ

L R

=

ρ

L

ρ

V Ca

=

μ

LV

σ

Fr

=

V2 aL Ri

=

V 2 aL

β(

Tw

−

Tsat

)

Ec

=

V 2 CPL

(

Tw

−

Tsat

)

(4)

The EckhertnumberEc isvery smallinall the experiments.Inmicrogravity, the inverseofthe RicharsonandFroude numbers1

/

Fr and1

/

Ri areverysmall,thenonlyfourdimensionlessnumberswillbeconsideredtopredictheattransferin microgravity:theJacobnumberJa,thePrandtlnumberPr,thedensityratio R andthecapillarynumberCa.Arelevant ve-locityscaleV fortheliquidmotionintheoverheatedlayerisduetovaporisation:V

=

q

/(

ρ

VhLV

)

.Thefollowingcorrelation

wasderivedtopredictthewallheatﬂuxatdifferentpressuresforHFE7000inmicrogravityconditions(Fig. 7): Ca

=

μ

Lq

σρ

VhLV

=

4

.

5

·

10−3R0.85Pr−1.5Ja1.8 (5)

This work was performed in the frame of the COMPERE programme of CNES to predict heat and mass transfer in the launchertanks.ExperimentsofboilingliquidoxygeninmagneticcompensationwerealsoperformedbyAirLiquideatCEA Grenoble[33].DespitethewallheatﬂuxwasﬁvetimeslargerwithliquidoxygenthanwithHFE7000,Eq.(5)isalsoableto predicttheheattransferofboilingliquidoxygenatdifferentpressures (Fig. 8).Itcan beexplainedbythefactthat inthe liquidoxygenexperiment,thenucleateboilingregimewasalsofullydeveloped.Inordertoincludetheeffectofgravityon

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Fig. 7. Wall heat ﬂux in microgravity for HFE7000, comparison with correlation(5).

Fig. 8. Wall heat ﬂux for liquid oxygen in microgravity (magnetic compensation), comparison with correlation(5).

heat transfer,theRi andFr numbersshouldbetakenintoaccount.Itisimportanttonoticethatthelength scaleinvolved in thesenumbersisa lengthscale characteristicoftheoverheated layernearthewall (aconductionlength)andnotthe capillarylengthusuallytakenintoaccountinthecorrelations[33].

Despitethenumberofexperimentsonpoolboilinginmicrogravityperformedoverthepast50years,thepredictionof boilingheattransferremainsadiﬃculttask.Correlationsareveryuseful,especiallyforindustrialapplications,evenifthey havealimitedrangeofvalidity.The diﬃcultytopredictheattransferinnucleateboilingboth innormalandmicrogravity conditionscan beeasily understood,regardingthenumerousinvolvedmechanisms:bubblenucleation,evaporationatthe contactline,re-condensation,coalescence,bubbledetachment,slidingonthewall...

3. Boilingonisolatednucleationsite

Inthe2000s,thankstotheimprovementofmeasurementtechniquesandnumericalmethods,severalstudieswere per-formed atthe bubble scale for a better understanding of the mass, momentum andheat transfers involved in nucleate boiling. Bubbledetachmentunder anelectricﬁeld orina shearﬂow was alsoinvestigated.Inthissession,we will sum-marisesomepreviousandrecentresultsandgivesomeprospects.

3.1. Heatandmasstransfersaroundasinglebubble

SeveralexperimentshavebeenfocusedonboilingonanisolatednucleationsitebyQuiandDhir[34],Quietal.[35]and Sodtke etal.[36],SchweizerandStephan[37].Theheatandmasstransferarounda singlebubblewas investigatedusing high-resolution measurementtechniquestocharacterisethelocalheatﬂuxatthewall andinthethermalboundarylayer.

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Severalexperiments were performedin parabolicflight to visualise theperiodically moving local temperature minimum atthewallunderneaththe3-phasecontactline, eitherusingthermochromicliquidcrystals[36,38]orhigh-speedinfrared thermometry [39,37].Some experimentswere also carried out infree-floating during the parabolicflights to reduce the effectof g-jitter. Thanks totheselocalmeasurements andthoseperformedby Kim etal.[15],ChristopherandKim [16], Ohta [10] the different mode ofheat transfer fromthe wall to bubbleand the liquidbulk were highlighted. The strong decreaseinthewalltemperatureclosetothethree-phasecontactlineisduetoastrongevaporationinthisregion.Several authors[40–42]developedtheoreticalanalysesoftheevaporationnearamovingcontactlinetopredictthelocalheatflux andtheevolution ofthe apparentcontactangle.The experiments inmicrogravityprovide goodvalidationtestsfor these theoreticalmodels,becausethebubblesizeandgrowthtimearemuchlargerthanonearth.Itisthereforeeasiertoobtain timeandspaceresolvedmeasurementsoftemperatureandheatfluxatthebubblefoot.

Direct NumericalSimulationwere developedinparallel totheseexperiments.Balance equationsformass,momentum andenergyaresolved inthetwo-ﬂuiddomains usingthe‘volumeofﬂuid’or‘levelset’methods.The resolutionofthese equationsiscoupled withmicrolayerevaporationmodelsnearthe contactline[43,35,44–46,26].Theresults ofthese nu-mericalsimulationsareingoodqualitativeagreementwiththeexperimentalresults.

3.2. Effectofanelectricﬁeldonbubbledetachment

Electricﬁeldshavebeenshowntomitigatetheeffectsofreducedgravityonboilingheat transferbyprovidinga body force on thebubbles. Theseforces maybe conceivedto pressbubbles against thesurface, increasing heat transfer, orto removethemaway,delayingboilingcrisis.Inrecentexperimentsinparabolicﬂights[14],boilingofFC72wasgeneratedon amicroheaterarrayof7 mm

×

7 mm.Anexternalelectricfieldupto10kVwasimposedovertheboilingsurfacebymeans ofagridoffourrodsparalleltotheheater.Theeffectofelectricfieldledtoareductioninthebubbledetachmentdiameter andanincreaseintheheattransfercomparedtomicrogravitywithoutelectricfield.Thiseffectislessimportantastheheat fluxincreases.

DiMarco[47] writesabalanceoftheforcesactingonan isolatedbubbleunderan electricfield.Duringa quasi-static bubblegrowth,theseforcesare thebuoyancy,internal overpressure,surfacetensionatthebubblefootandelectricforces. Theevaluationoftheelectricforceisnottrivialduetothefactthattheelectricfieldconfigurationiscontinuouslymodified bybubblegrowth.Cattideetal.[48]usedexperimentalbubbleshapestocomputetheelectricfieldandtheMaxwellstress tensor,withtheaidofthecodeCOMSOLMultiphysics.Theyassumedanon-conductivefluidwithnochargeattheinterface. Theresultingforcetendstoelongatethebubble.Experimentalresultswerealsocomparedwithdirectnumericalsimulations [49].Thisphenomenon iseven morecomplexin boilingandwork hasstill tobe done fora betterunderstanding ofthe effectofanelectricfieldonthedetachmentofavapourbubble.

Eveniftheseexperimentalactivities havewidelyclarifiedtherole ofelectricforcesinboilingandinbubblegrowthin microgravity[50],furtherinvestigationisneededtounderstandtheroleplayedbythefluidproperties,theactionofelectric forcesatthebubblefoot,intheregionaroundthethree-phaseline,andtooptimisetheelectricfieldconfigurationinorder toreducetheappliedvoltagetotheminimumpossible.

3.3. Bubbledetachmentinashearﬂow

The growth anddetachment of vapour bubbles in a shear flow on a heatedsurface is one of the basicmechanisms thathavetobeunderstoodtoimprovethemodellingofheattransferinconvectivenucleateboiling.Bubblevaporisationat theheatedwall ofaliquidflowhasbeenexperimentallyinvestigatedundernormalgravityconditions.Thorncroftetal.[51] publishedareviewofexperimentalresultsonbubbledetachmentinpoolandconvectiveboilingonhorizontalandvertical surfaces.Theyshowedthatatlowliquidflowrate,onhorizontalsurfaces,buoyancydetachesthebubbleperpendicularlyto thewall.Astheliquidflowrateishigh, thebubbledoesnotliftoff,butslidesontheheatedwallundertheeffectofthe liquiddragforce.Ontheverticalwall,thebubbleslidesonthewallforalongtimebeforelifting-off.Pointforcemodelsare oftenusedtopredictthebubbledetachmentdiameter[52].Themaindifficultyistomodeltheforcesactingonthebubble during itsgrowth.Dragandliftcoefficientsare unfortunatelyunknown forabubblegrowing onthe wallatintermediate bubbleReynoldsnumbers(10to200).Someexperimentsonground[53]anddirectnumericalsimulationsforhemispherical bubbles[54]providedsomeexpressionsofthedragandliftforces.Theevaluationofthecapillaryforcerequiresthevalues ofthecontactanglesatthebubblefootandthewidthofthebubblefootincontactwiththeheatedplate.

Inmicrogravity, bubble vaporisation on heated surfaces has been mainly studied in pool boiling. Ma andChung [55] howeverreportedexperimentsinwhichasingle vapour bubblewasnucleatedandgrown inflowfield ofFC-72onaflat surfaceinterrestrialgravityandmicrogravity.Theyshowedthattheincreaseintheliquidflowrateslowsdownthegrowth ofthe bubble.Forcedconvectionalso enhancesthedeparture ofbubbles fromtheir nucleation sites.Inmicrogravity, the bubblediameterislargerthanat1-g.Theinfluenceofbuoyancydisappears,astheliquidflowrateishigh.Anotherstudy was performedin2D configurationinaHele–Shawcell(Serret etal.[56]).The bubblewascreatedona heatedflow and two cameras were used on both sidesof the channel: one visible camera to record the bubble shape and one infrared cameratomeasurethetemperaturefieldintheliquidsurroundingthebubble.

Another set of experiments was performeda in rectangular channel of cross section 40 mm in width, 5.69 mm in height and650mm inlength[57–59].ArefrigerantHFE7000 wascirculatedinthechannel withvelocitiesupto 0.3m/s

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Fig. 9. Growthanddetachmentofavapourbubbleinashearﬂowonground1-g (left)andinamicro-gravityconditionsμ-g (right).Theliquidﬂow velocityof0.113m/sisfromlefttoright.

Fig. 10. Geometrical parameters deduced from image processing.

correspondingtoaﬂowReynoldsnumberof9000.Fortheexperiments,liquidsubcoolingwasabout 10 K.Isolatedbubbles were nucleatedonabubblegenerator,whichisathinlayer(

∼

200nm)ofgoldsputteredonaglasssubstrate.Acavityof mouthsizearound50μmonthelayerprovidedanucleationsiteandmadepossiblethesinglevapourbubblegeneration.

Experimentswereperformedongroundwiththehorizontalchannelandinmicrogravityduringparabolicﬂight experi-ments.Thedynamic ofthebubblegrowthanddetachmentwas recordedwithahigh-speedvideocameraPCO 1200HSat frequencies of500imagesper secondwith1200

×

1024 pixels (theﬁeldofview was2

.

4 mm

×

2 mm andtheresolution was 508pixels/mm).All the acquisitionswere performedby shadowgraphy.By means ofa light source,a parallel beam illuminates the bubble so that it appears inblack on a white background (Fig. 9). From image processing usingMatlab software,thebubblecontourwasidentiﬁedandthegeometricalparameters(Fig. 10)canbeevaluated:equivalentradius R,

apparentfootradiusRf,contactangles

α

and

β

,coordinatesofthecentreofgravityinthehorizontalandverticaldirections

xGand yG,bubblevolume Vb.

ThenucleatedbubblegrewwithtimeuntilitsdeparturefromthenucleationsiteasshowninFig. 11forbothnormaland microgravity conditions.Aﬁttingcurve ofthebubbleradius R duringthegrowthhasatimedependency R

∼

t1/3_,_which

is slowerthan thediffusioncontrol growthproportional tot1/2_._This_might _be_a _result_of_the _{re-condensation}_of_vapour

atthe bubbletop,wherealuminous plumeindicatingheat transferwas typicallyobservedin liquidﬂow(Fig. 9).In1-g, the bubblesliftedoff,whereas thebubblesslidedparallelto thewallin

μ

-g.Afterits departure,thebubbledecreasedin sizethroughre-condensationinthesubcooledliquid.Thebubbleradiusatdetachmentvariedbetween0.12and0.2mmon thegroundandbetween0.18and0.25mmin

μ

-g.InFig. 11,thebubblefootradius Rf isalsoplotted.Innormalgravity,

the time of lift-off (tdet 1-g

=

0

.

11 s) was three timeslessthan the one inmicrogravity (tdetμ-g

=

0

.

3 s). The bubblefoot

extendedmuchmorein

μ

-g thanin1-g.InFig. 11,theevolutionofthecontactangles

α

and

β

arealsoplottedfor1-g.At thebeginning,thetwocontactangles

α

and

β

wereapproximatelyequaltoeachother.Thebubblewasverysmallandhad a symmetricalshape.Thissuggeststhathydrodynamic effectsoftheﬂowwere notsuﬃcient toovercomethedominancy of thecapillaryeffect.Then,the hydrodynamic effectsbecamemoreandmoreimportantandthe symmetrywas broken, leadingtobubbledetachment.Theupstreamanddownstreamcontactanglesincreasedanddecreased,respectively.

From theevolution of thegeometrical parameters of the bubblesduring their growth,the staticandhydrodynamical forcesareevaluated.Usingaclassicalpoint-forceapproach[51,53,52],abalanceontheforcesactingonthebubblecanbe writtenas:

FB

+

FC

+

FCP

+

FD

+

FL

+

FAM

=

0 (6)

where FB istheArchimedean force, FC thecapillaryforce, FCP thecontactpressureforce, FD thedrag force, FL thelift

force,andFAM theaddedmassforce.Thebuoyancyforce FB vanishesundermicrogravity.Thecontactpressureforce FCPis

duetothefactthatthebubbleisincontactwiththewall,insteadofbeingcompletelysurroundedbytheliquid,andthen tothepressuredifferenceinsideandoutsidethebubble.Thepressuredifferenceacrossthebubbleinterface atitsfootcan

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Fig. 11. Evolutionofthebubbleradius

R and

bubblefootradius

R

fin1-g andμ-g (left).Evolutionoftheupstreamanddownstreamcontactanglesin1-g. be expressedversus theequivalent radius atthebubbletop R (Laplaceequation)andthe hydrostaticpressuredifference betweenthebubbletop( y

=

h)andthebubblefoot

(

y

=

0

)

[60]:

FCP

=

2

σ

R

+ (ρ

L

−

ρ

V

)

gh

π

R2_fey (7)

Thecapillaryforce FC keepsthebubblefootincontactwiththewall.Itappearsatthetripleline(solid–liquid–gas).The

ex-pressionproposedbyKlausneretal.[61]foracircularbubblefoot,assumingalinearevolutionofthecontactanglebetween

α

and

β

,hasbeenextendedbyLebon etal.[59] tothecaseofanon-circularbubblefoot,usingtheapproachdeveloped byDussanetal.[62].Sincethebubblegrowwasquasistatic,theaddedmassforcewasnegligibleintheexperiments.The dragforceactingintheflowdirectionandtheliftforceactingperpendiculartotheflowdirectioncanbeexpressedversus hydrodynamicdragcoefficientCDandliftcoefficientCL:

FD

=

1 2

ρ

lCD

π

R 2

₍

_U L

−

UB

)

|

UL

−

UB

|

ex and FL

=

1 2

ρ

lCL

π

R 2

₍

_U L

−

UB

)

2ey (8) ULandUB beingtheliquidvelocityatthecentreofgravityofthebubbleandthebubblevelocity,respectively.

The expressions for the drag andlift coeﬃcientsfor a bubble growing on a ﬂow are unknown. Expressions exist for sphericalbubblesinthewallvicinity inthelimitofsmallbubbleReynoldsnumberReb orlargebubbleReynoldsnumber.

Forboiling bubbles, therange ofbubbleReynolds numbers isvery large, from1to 200, in ourexperiments. Performing severalexperimentsonbubble injection,it wasnevertheless possiblefromEqs.(6) to(8)andfromtheexpressionofthe capillaryforce,todetermineanapproximatedexpressionforthedragcoeﬃcient:

CD

=

27Re−_b0.65 (9)

Usingtheseexpressionsforthedrag-and-liftcoefficient,thedifferentforcesactingonagrowingbubbleinashearflow in microgravity are plottedin Fig. 12.In the flow direction, thedrag force balances thecapillary force. In the direction perpendiculartothewall,thecontactpressureforcebalancesthecapillaryforce.Theliftforceissmallbutnotnegligiblein microgravity.

Additionalexperimentsanddirectnumericalsimulations[63] arestillneededtounderstandtheeffectofashearonthe bubblehydrodynamicsandtobe abletopredictbubbledetachment.Theeffectofashearﬂowonbubbledetachmentwill be alsoinvestigatedin theRUBIexperimentdescribed belowthanks to aconvection loop,which willcreatea controlled shearﬂowabovetheheater.

3.4. RUBIexperiment

Inordertoevaluatethemodelsquantitatively,genericexperimentshavetobedevelopedandrefined.Thisisthe objec-tiveoftheRUBI(ReferencemUltiscaleBoilingInvestigation)experimentfortheFluidScienceLaboratoryontheInternational SpaceStation, developedbyESAandseveralEuropeanteams[64,65].RUBIwillprovidemeasurementsofwalltemperature andheatfluxdistributionunderneathvapour bubbleswithhighspatial andtemporal resolutionbymeansofIR thermog-raphy.These datawillbe synchronised withthe bubbleshape observationby ahigh-speed video.Furthermore,the fluid temperatureinthevicinityandinsideofthebubbleswillbemeasuredbyanarrayoffourthermocouples.Inordertostudy bubbledetachmentorslidingontheheatingsurface,anelectricalfieldcanbealsoappliedandashearflowwillbecreated byaforcedconvectionloop.

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Fig. 12. Ul=0.113 m/s: Force balance parallel to the wall (left) and perpendicular to the wall (right) inμ-g.

In parallel to RUBI, theoretical models for the prediction ofheat andmass transfers in the microlayer region at the bubblefootareunderdevelopmentandabenchmarkondirectnumericalsimulationofthebubblegrowthanddetachment wouldbe alsointeresting tobe performed. Thenthe complementaryexpertiseon experiments,theoreticalmodellingand numericalsimulationshouldallowmakingsigniﬁcantprogressintheunderstandingandpredictionofthelocalmechanisms ofboiling.

4. Conclusion

Despitethenumberofexperimentsonpoolboilinginmicrogravityperformedoverthepast50years,thepredictionof boilingheattransferremainsadifficulttask,whichisunderstandableregardingthenumerousinvolvedmechanisms:bubble nucleation, evaporation atthe contactline, re-condensation, coalescence,bubble detachment,sliding on the wall.Recent studieshavehighlightedtheroleoftheheatersize,gravitylevel,subcoolingandconcentrationofdissolvedgasonthewall heat flux. At low heat flux,the heat transfer islower in normalgravity than inmicrogravity. In normalgravity, bubbles nucleateonthewallandlift-offveryquickly.Theheatedsurfaceisonlypartiallycoveredbybubbles.Inmicrogravity,even atlowheatflux,bubblesnucleateonthewall,coalesceandformalargeprimarybubble,whichhasan oscillatingmotion above thewall. The nucleateboilingregime isfullydeveloped andheattransferis moreefficientthan innormalgravity. When theheat fluxincreases, thetendencyisopposite. Apartialdry-outofthe wallisobserved inmicrogravityandthe heattransferismuchlessimportantthaninnormalgravity.Theeffectofgravityonheattransferiswell predictedathigh wall heatflux.Nevertheless,thereisstillsomework tobedonetopredict theevolutionsobservedatlowheatflux.Thus correlationsarestillusefulforindustrialapplicationsdespitetheirlimitedrangeofvalidity.

From the 2000s,thanks tothe developmentof advancedmeasurement techniquesandtheimprovement ofnumerical methods, severalanalyseswere performedatthebubblescale toinvestigatethe localheatandmasstransfer aroundthe bubble andat its foot and the bubble detachment mechanisms under an electric field or a shear flow. Severalstudies havebeenperformedbydifferentteams,mainly inparabolicflights andhelpedustodesignthe RUBIexperimentforthe InternationalSpaceStation.Theseobjectivesshouldbeachievedthankstocomplementaryapproachesbasedonnormaland microgravityexperiments,developmentoftheoreticalmodelsanddirectnumericalsimulations.

Acknowledgements

Theauthorswouldliketothankthe“centrenationald’étudesspatiales”(GDR“Microgravitéfondamentaleetappliquée” and the COMPERE Programme, CT-2540000-1404-CNES-01) andthe European SpaceAgency (MAP Multiscale ANalysis of BOilingContract4200020289)fortheﬁnancialsupportoftheirstudies,thePhDthesisandpost-docgrants,theorganisation oftheparabolicﬂightscampaigns,andthesoundingrocketcampaigns.

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